Patent Publication Number: US-2023160080-A1

Title: Methods of forming aqueous urea utilizing carbon dioxide captured from exhaust gas at wellsite

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     TECHNICAL FIELD 
     The present disclosure relates generally to systems and methods of sequestering carbon dioxide (CO 2 ). More specifically, this disclosure relates to collecting exhaust gas comprising CO 2  and forming urea utilizing at least a portion of the CO 2  in the collected exhaust gas. Still more specifically, this disclosure relates to collecting exhaust gas comprising CO 2 , separating high purity CO 2  from the collected exhaust gas, and forming urea utilizing at least a portion of the high purity CO 2 . 
     BACKGROUND 
     Natural resources (e.g., oil or gas) residing in a subterranean formation can be recovered by driving resources from the formation into a wellbore using, for example, a pressure gradient that exists between the formation and the wellbore, the force of gravity, displacement of the resources from the formation using a pump or the force of another fluid injected into the well or an adjacent well. A number of wellbore servicing fluids can be utilized during the formation and production from such wellbores. For example, in embodiments, the production of fluid in the formation can be increased by hydraulically fracturing the formation. That is, a treatment fluid (e.g., a fracturing fluid) can be pumped down the wellbore to the formation at a rate and a pressure sufficient to form fractures that extend into the formation, providing additional pathways through which the oil or gas can flow to the well. Subsequently, oil or gas residing in the subterranean formation can be recovered or “produced” from the well by driving the fluid into the well. During production of the oil or gas, substantial quantities of produced water, which can contain high levels of total dissolved solids (TDS), and produced gas can also be produced from the well, and a variety of exhaust gases and flare gases conventionally sent to flare can be formed. For example, oil and gas wells produce oil, gas, and/or byproducts from subterranean formation hydrocarbon reservoirs. A variety of subterranean formation operations are utilized to obtain such hydrocarbons, such as drilling operations, completion operations, stimulation operations, production operations, enhanced recovery operations, and the like. Such subterranean formation operations typically use a large number of vehicles, heavy equipment, and other apparatus (collectively referred to as “machinery” herein) in order to achieve certain job requirements, such as treatment fluid pump rates. Such equipment may include, for example, pump trucks, sand trucks, cranes, conveyance equipment, mixing machinery, and the like. Many of these operations and machinery utilize combustion engines that produce exhaust gases (e.g., including carbon dioxide/greenhouse gas emissions) that are emitted into the atmosphere. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG.  1    is a schematic flow diagram of a method, according to embodiments of this disclosure; 
         FIG.  2    is a schematic of a system, according to one or more embodiments of the present disclosure; 
         FIG.  3    is a schematic of a system, according to one or more embodiments of this disclosure; 
         FIG.  4    is a schematic of a urea production apparatus, according to one or more embodiments of the present disclosure; 
         FIG.  5    is a schematic of an electrocatalyst suitable for use in the urea production apparatus of  FIG.  4   , according to one or more embodiments of the present disclosure; and 
         FIG.  6    is a schematic of a plurality of machinery that may be located and operated a wellsite for performing a subterranean formation operation and may produce exhaust gas comprising CO 2 , according to one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods can be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques below, including the exemplary designs and implementations illustrated and described herein, but can be modified within the scope of the appended claims along with their full scope of equivalents. 
     A method of this disclosure will now be described with reference to  FIG.  1   , which is a schematic flow diagram of a method I according to one or more embodiments of this disclosure. As seen in  FIG.  1   , method I includes collecting exhaust gas comprising carbon dioxide (CO 2 ) at a wellsite to provide a collected exhaust gas at  10 , separating CO 2  from the collected exhaust gas to provide a separated CO 2  at  20 , and forming urea utilizing at least a portion of the separated CO 2  at  30 . A method I of this disclosure can further comprise: at  40 , separating one or more components from the urea; at  50 , utilizing heat from the collected exhaust gas collected at  10  and/or the forming of the urea at  30  in the separating CO 2  at  20 , the forming of urea at  30  and/or the separating components at  40 ; and/or, at  60 , forming diesel exhaust fluid (DEF). Although depicted in a certain order in  FIG.  1   , in embodiments, one or more of steps  10  to  60  can be absent, and/or the one or more of steps  10  to  60  can be performed more than once and/or in a different order than described herein or depicted in the embodiment of  FIG.  1   . 
     The method of this disclosure will now be detailed and a system for carrying out the method according to embodiments of this disclosure described with reference to  FIG.  2   , which is a schematic of a system  100  according to one or more embodiments of this disclosure,  FIG.  3   , which is a schematic of a system,  200  according to one or more embodiments of this disclosure,  FIG.  4   , which is a schematic of a urea production apparatus  130 B disparate from urea production apparatus  130 A of system  200  of  FIG.  3   , and  FIG.  5   , which is a schematic of an electrocatalyst  131  suitable for use in the urea production apparatus  130 B of  FIG.  4   , according to one or more embodiments of the present disclosure. 
     With reference now to  FIG.  2   , system  100  comprises: an exhaust gas collection system  110  configured for collecting exhaust gas  115  comprising carbon dioxide (CO 2 ) at a wellsite  111  ( FIG.  3   ) to provide a collected exhaust gas (e.g., step  10  of  FIG.  1   ); a CO 2  separation apparatus  120  configured for separating CO 2  from the collected exhaust gas  115  to provide a separated CO 2    125  (e.g., step  20  of  FIG.  1   ); and a urea production and/or purification apparatus  130  (referred to hereinafter simply as “urea production apparatus  130 ”) configured for forming urea (and optionally separating one or more components therefrom) to provide a urea product  135  comprising urea utilizing at least a portion of the separated CO 2    125  (e.g., step  30  of  FIG.  1   ). As depicted in  FIG.  2   , system  100  can further comprise a diesel exhaust fluid (DEF) production apparatus  140  configured to form DEF (e.g., by diluting the urea product  135  with dilution water  144  to form DEF  145 ). 
     As noted above with reference to  FIG.  1   , method I includes collecting exhaust gas comprising carbon dioxide (CO 2 ) at a wellsite to provide a collected exhaust gas  115  at  10 . Collecting exhaust gas at  10  can be effected via exhaust gas collection apparatus  110 . Exhaust gas collection apparatus  110  can include and/or obtain the collected exhaust gas  115  from field operating equipment  112  ( FIG.  3   ) at a wellsite  111 . The field operating equipment  112  can comprise one or more vehicles (e.g., diesel trucks, cars, etc.), pumps (e.g., hydraulic pumps, fracturing pumps, etc.), or other equipment at a wellsite  111  that produces an exhaust gas comprising CO 2  from which collected exhaust gas  115  is obtained. Exhaust gas collection apparatus  110  can further comprise piping configured to combine the exhaust gas from a plurality of the field operating equipment  112  and introduce it to CO 2  separation apparatus  120 , storage apparatus to store the collected exhaust gas  115  prior to introduction into CO 2  separation apparatus  120 , or a combination thereof. Collecting the collected exhaust gas  115  comprising CO 2  at  10  can be performed by piping exhaust gas from one or more pieces of field operating equipment or machinery  112  at a wellsite  111  to provide the collected exhaust gas  115 . 
     The exhaust gas comprising CO 2  collected at step  10  can include greater than or equal to about 0.04, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, or 100 volume percent (vol. %) CO 2 . By way of examples, the collected exhaust gas comprising CO 2    115  can include a waste gas, or one or more components thereof, produced at the wellsite  111  or another jobsite, such as, without limitation, one or more wellsites or industrial plants. The one or more industrial plants can include, without limitation, a cement plant, a chemical processing plant, a mechanical processing plant, a refinery, a steel plant, a power plant (e.g., a gas power plant, a coal power plant, etc.), or a combination and/or a plurality thereof. In embodiments, the exhaust gas comprising CO 2    115  comprises a waste gas that is a product of fuel combustion, for example, the product of an internal combustion engine, or a gas fired turbine engine, such as, for example, from a microgrid having electric pumps. In embodiments, the internal combustion engine includes an engine fueled by diesel, natural gas (e.g., methane), gasoline, or a combination thereof (e.g., a diesel engine, or a hybrid engine that is fueled by diesel and natural gas). The collected exhaust gas comprising CO 2    115  can be produced at the wellsite  111  and/or another jobsite. A plurality of machinery  112  can be located and operated at a wellsite  111  for performing a subterranean formation operation, according to one or more embodiments of the present disclosure, and the collected exhaust gas comprising CO 2    115  can, in embodiments, be obtained therefrom. For example, the exhaust gas comprising CO 2  from which the collected exhaust gas  115  can be produced at the wellsite  111  from machinery  112  used to perform a wellbore servicing operation. The machinery may include one or more internal combustion or other suitable engines that consume fuel to perform work at the wellsite  111  and produce exhaust gas comprising CO 2  from which collected exhaust gas  115  is collected. 
     The wellbore  101  at wellsite  111  may be a hydrocarbon-producing wellbore (e.g., oil, natural gas, and the like) or another type of wellbore for producing other resources (e.g., mineral exploration, mining, and the like). Machinery  112  typically associated with a subterranean formation operation related to a hydrocarbon producing wellbore, and from which the exhaust gas comprising CO 2  can be produced, can be utilized to perform such operations as, for example, a cementing operation, a fracturing operation, or other suitable operation where equipment is used to drill, complete, produce, enhance production, and/or work over the wellbore. Other surface operations may include, for example, operating or construction of a facility. 
     As depicted in  FIG.  6   , which is a schematic of a plurality of machinery  112  that may be located and operated a wellsite  111  for performing a subterranean formation operation and may produce exhaust gas comprising CO 2  from which collected exhaust gas  115  is collected, according to one or more embodiments of the present disclosure, the machinery  112  from which the exhaust gas comprising CO 2  can be produced, in embodiments, can include sand machinery  112 A, gel machinery  112 B, blender machinery  112 C, pump machinery  112 D, generator machinery  112 E, positioning machinery  112 F, control machinery  112 G, and other machinery  112 H. The machinery  112  may be, for example, truck, skid or rig-mounted, or otherwise present at the wellsite  111 , without departing from the scope of the present disclosure. The sand machinery  112 A may include transport trucks or other vehicles for hauling to and storing at the wellsite  111  sand for use in an operation. The gel machinery  112 B may include transport trucks or other vehicles for hauling to and storing at the wellsite  111  materials used to make a gelled treatment fluid for use in an operation. The blender machinery  112 C may include blenders, or mixers, for blending materials at the wellsite  111  for an operation. The pump machinery  112 D may include pump trucks or other vehicles or conveyance equipment for pumping materials down the wellbore  101  for an operation. The generator machinery  112 E may include generator trucks or other vehicles or equipment for generating electric power at the wellsite  111  for an operation. The electric power may be used by sensors, control machinery, and other machinery. The positioning equipment  112 F may include earth movers, cranes, rigs or other equipment to move, locate or position equipment or materials at the wellsite  111  or in the wellbore  101 . 
     The control machinery  112 G may include an instrument truck coupled to some, all, or substantially all of the other equipment at the wellsite  111  and/or to remote systems or equipment. The control machinery  112 G may be connected by wireline or wirelessly to other equipment to receive data for or during an operation. The data may be received in real-time or otherwise. In another embodiment, data from or for equipment may be keyed into the control machinery. 
     The control machinery  112 G may include a computer system for planning, monitoring, performing or analyzing the job. Such a computer system may be part of a distributed computing system with data sensed, collected, stored, processed and used from, at or by different equipment or locations. The other machinery  112 H may include equipment also used at the wellsite  111  to perform an operation. 
     In other examples, the other machinery  112 H may include personal or other vehicles used to transport workers to the wellsite  111  but not directly used at the wellsite  111  for performing an operation. 
     Many if not most of these various machinery  112  at the wellsite  11  accordingly utilize a diesel or other fuel types to perform their functionality. Such fuel is expended and exhausted as exhaust gas, such as exhaust gas including CO 2 . The embodiments described herein provide a system and method for collecting, converting to urea, and, thus, sequestering CO 2  from such machinery  112  located and operated at a wellsite  111 , thus reducing atmospheric CO 2  emissions, while reducing material and time costs. It is to be appreciated that other configurations of the wellsite  111 , including other machinery  112  at the wellsite  111  or another jobsite, may be employed, without departing from the scope of the present disclosure. Although a number of various machinery  112  at a jobsite (e.g., a wellsite  111 ) have been mentioned, many other machinery may utilize diesel or other fuel that creates exhaust gas including CO 2  that may conventionally be exhausted into the atmosphere, but herein utilized to form urea as described herein. 
     In some embodiments, the present disclosure provides capturing exhaust gas comprising CO 2    115  from such machinery located and operated at a wellsite  111  and utilizing such exhaust gas to form urea as detailed herein. 
     Although described hereinabove with reference to a wellsite  111 , the source of the collected exhaust gas comprising CO 2    115  that is collected at step  10  of the method I can be any convenient CO 2  source. The CO 2  source can be a gaseous CO 2  source. This gaseous CO 2  may vary widely, ranging from air, industrial waste streams, etc. As noted above, the gaseous CO 2  can, in certain instances, comprise an exhaust waste product from an industrial plant. The nature of the industrial plant may vary in these embodiments, where industrial plants of interest include power plants, chemical processing plants, and other industrial plants that produce exhaust gas comprising CO 2  as a byproduct. By waste stream is meant a stream of gas (or analogous stream) that is produced as a byproduct of an active process of the industrial plant, e.g., an exhaust gas. The gaseous stream may be substantially pure CO 2  or a multi-component gaseous stream that includes CO 2  and one or more additional gases. Multi-component gaseous streams (containing CO 2 ) that may be employed as a CO 2  source in embodiments of the subject methods include both reducing, e.g., syngas, shifted syngas, natural gas, and hydrogen and the like, and oxidizing condition streams, e.g., flue gases from combustion. Particular multi-component gaseous streams of interest that may be treated according to the subject invention include: oxygen containing combustion power plant flue gas, turbo charged boiler product gas, coal gasification product gas, shifted coal gasification product gas, anaerobic digester product gas, wellhead natural gas stream, reformed natural gas or methane hydrates, and the like. 
     As noted above, in embodiments, the collected exhaust gas comprising CO 2    115  comprises greater than or equal to about 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 95, 96, 97, 98, 99, or 100 volume percent (vol %) CO 2 . In embodiments, the exhaust gas comprising CO 2    115  comprises primarily CO 2  (e.g., greater than or equal to about 50, 60, 70, 80, 90, 95, 96, 97, 98, 99, or 100 volume percent (vol %) CO 2 ). For example, when the exhaust gas comprising CO 2    115  is obtained from a waste gas produced at a different jobsite (e.g., at an another jobsite) than the wellsite  111 , CO 2  can be separated from the waste gas in order to reduce a volume of gas to be transported to the wellsite  111 . For example, when the exhaust gas includes a flue gas from a power plant, which typically contains from about 7 to about 10 vol. % CO 2 , the method I can further include transporting the exhaust gas comprising CO 2  (or a waste gas from which the exhaust gas comprising CO 2    115  is obtained) from the another jobsite at which the waste gas is obtained to the wellsite  111 . In embodiments, the method I can further include separating the exhaust gas from the waste gas comprising CO 2 , to reduce a volume of gas, e.g., for transport. Although the separating of the exhaust gas comprising CO 2  from the waste gas can be performed at the wellsite  111  (e.g., after transport of the waste gas from the another jobsite at which the waste gas is obtained and/or produced to the wellsite  111 ), to facilitate transportation, the separating of the collected exhaust gas comprising CO 2    115  from the waste gas can be performed at the another jobsite at which the waste gas is produced and/or obtained and, subsequently, the collected exhaust gas comprising CO 2    115  can be transported to the wellsite  111 . 
     As noted above, method I comprises, at  20 , separating CO 2  from the collected exhaust gas to provide a separated CO 2    125 . Separating the CO 2  from the collected exhaust gas  115  at  20  can comprise separating substantially pure CO 2  from the collected exhaust gas  115 . That is, in embodiments, the separated CO 2    125  is substantially pure CO 2 . The substantially pure CO 2  (and the substantially pure separated CO 2    125 ) can include greater than or equal to about 90, 95, 96, 97, 98, 99, 99.5, 99.8, 99.9, or 100 vol % CO 2 . Separating CO 2  from the collected exhaust gas  115  to provide a separated CO 2    125  at  20  can be effected by CO 2  separation apparatus  120 . CO 2  separation apparatus  120  can comprise any apparatus operable to provide high purity (e.g., greater than or equal to 50, 60, 70, 80, 90, 95, 96, 97, 98, 98.5, 99, 99.5, 99.9, 99.99, or substantially 100 volume percent (vol %) CO 2  from the collected exhaust gas  115 . In embodiments, CO 2  separation apparatus  120  can operate by separating via amine absorption, calcium oxide (CaO) absorption, filtration, packed bed, another technique, or a combination thereof. In embodiments, CO 2  separation apparatus  120  comprises batch reactor, continuous reactor, packed-bed column, fluidized bed column, or a combination thereof. 
     As noted above, method  1  comprises, at  30 , forming urea utilizing at least a portion of the separated CO 2    125 . With reference now to  FIG.  3   , forming urea at step  30  can comprise reacting the separated CO 2    125  with ammonia (NH 3 )  141  to form ammonium carbamate (e.g., in ammonium carbamate solution  165 ) and decomposing the ammonium carbamate in ammonium carbamate solution  165  to form the urea (e.g., in aqueous urea solution  175 ); or, with reference to  FIG.  4   , forming urea at step  30  can comprise reacting the separated CO 2    125  directly with nitrogen (N 2 ) gas  142  to form the urea  136 . 
     Forming urea at  30  can be effected via urea production apparatus  130 . Urea production apparatus  130  can comprise any apparatus operable to produce a urea product  135  comprising urea from at least a portion of the separated CO 2    125 . For example, and with reference to  FIG.  3   , in embodiments, urea production apparatus  130  comprises an NH 3 —CO 2  reactor  160  configured for reacting the separated CO 2    125  with ammonia (NH 3 )  141  to form an ammonium carbamate solution  165  comprising ammonium carbamate and an ammonium carbamate decomposition reactor  170  configured for decomposing the ammonium carbamate to form the urea product  135  comprising urea. By way of further example, and with reference to  FIG.  4   , in embodiments, urea production apparatus  130  comprises an electrocatalytic reactor  130  configured for forming the urea product  135  by reacting the separated CO 2    125  directly with nitrogen gas  142 . Utilization of urea production apparatus  130  other than that depicted in  FIG.  3   ,  FIG.  4   , and  FIG.  5    to form the urea at step  30  is intended to be within the scope of this disclosure. 
     In embodiments, described now with reference to system  200  of  FIG.  3   , the substantially pure CO 2    125  separated from the collected exhaust gas  115  is compressed to react with liquid ammonia  141  in NH 3 —CO 2  reactor  160 , wherein an exothermic reaction occurs between CO 2  and NH 3  to form ammonium carbamate (NH 2 COONH 4 ) as shown in Equation (1): 
       2NH 3 +CO 2 →NH 2 COONH 4   Eq. (1)
 
     The heat generated from the exothermic reaction of Equation (1) (e.g., about 380° F. and 150 atm) can be optionally captured by a heat exchanger (e.g., heat exchanger  150 ) to produce steam  155  and provide a heat source for heating the ammonium decomposition reactor  170  which can function as a distillation column to decompose the ammonium carbamate in ammonium carbamate solution  165  into an aqueous urea solution  175  via Equation (2) as the temperature and pressure in the ammonium carbamate decomposition reactor  170  are decreased (e.g., to about 275° F. and 35 atm). 
       NH 2 COONH 4 →NH 2 CONH 2 +H 2 O  Eq. (2)
 
     The distillation process in ammonium carbamate decomposition reactor  170  can also purify the produced aqueous urea solution  175  by removing a stream  176  comprising water, unreacted NH 3 , CO 2 , and ammonium carbamate (e.g., at separating component(s) step  40  of method I of  FIG.  1   ). Stream  176  can be recycled to NH 3 —CO 2  reactor  160 . 
     The aqueous urea solution  175  can be transferred to a unreacted component removal reactor  180  (also referred to herein as an “unwanted component removal apparatus  180 ”, as reactor  180  removes non-urea components of aqueous solution  170 , not just unreacted reactants NH 3  and CO 2 ), which can act as a flash chamber/condenser in embodiments, allowing the unreacted component removal apparatus  180  to be depressurized to, for example, ambient pressure while heating to further decompose any unreacted ammonium carbamate to NH 3  and CO 2 , as shown in Equation (3). 
       NH 2 COONH 4 →2NH 3 +CO 2   Eq. (3).
 
     Like stream  176  from ammonium carbamate decomposition reactor  170 , a stream  186  from unreacted component removal reactor  180  comprising water and unreacted NH 3  and CO 2  can also be recycled to NH 3 —CO 2  reactor  160 , thus increasing the concentration of urea in the concentrated aqueous urea  185  relative to a concentration of urea in the aqueous urea solution  175 . In embodiments, the concentrated aqueous urea solution  185  comprises greater than or equal to about 80, 85, or 90, or from about 80 to 100 weight percent (wt %) urea. In embodiments, the concentrated urea solution  185  can be transformed into semi-solid, dry solids, or granulated urea  191  by additional drying of concentrated aqueous urea solution  185  in a drying apparatus  190 . In embodiments, drying apparatus  190  can comprise a vacuum evaporator operated under vacuum with temperatures of greater than or equal to about 250° F. 
     In embodiments, the heat of the collected exhaust gas  115  collected from the equipment  110  operating at wellsite  111  or heat provided by the reaction of Eq. (1) in NH 3 —CO 2  reactor  160  can be utilized to attain operating temperatures in the ammonium carbamate decomposition reactor  170 , unreacted component removal reactor  180 , and/or drying apparatus  190 , as described further hereinbelow. 
     In embodiments such as depicted in  FIG.  3   , the urea production apparatus  130  ( FIG.  2   ) comprises urea production apparatus  130 A comprising NH 3 —CO 2  reactor  160  and ammonium carbamate decomposition reactor  170  configured for decomposing the ammonium carbamate to provide urea. 
     With reference now to  FIG.  1    and  FIG.  3   , in embodiments, forming urea at step  30  comprises reacting the at least the portion of the separated CO 2    125  with ammonia  141  to form ammonium carbamate and decomposing the ammonium carbamate to form the urea. In such embodiments, forming urea at step  30  can further comprise: compressing the at least a portion of the separated CO 2    125  to provide a compressed CO 2    125 A; reacting the compressed CO 2    125 A with the ammonia  141  to form an ammonium carbamate solution  165  comprising the ammonium carbamate; and decomposing the ammonium carbamate to provide an aqueous urea solution  175  comprising the urea. 
     Thus, as depicted in  FIG.  3   , system  200  can further comprise a compressor  156  configured for compressing the at least a portion of the separated CO 2    125  to provide a compressed CO 2 ,  125 A. In such embodiments, the NH 3 —CO 2  reactor  160  can be configured for reacting the compressed CO 2    125 A with the liquid ammonia  141  via an exothermic reaction to form the ammonium carbamate. In embodiments, compressor  156  is operable to compress the at least the portion of the separated CO 2    125  to a pressure of greater than or equal to about 130, 140, or 150 atmospheres (13.2, 14.2, or 15.2 MPa). Method I can comprise, at forming urea step  30 , compressing the at least the portion of the separated CO 2    125  (e.g., in compressor  156 ) to provide compressed CO 2    125 A having a pressure of greater than or equal to about 130, 140, or 150 atmospheres (13.2, 14.2, or 15.2 MPa). Reacting the compressed CO 2    125 A and ammonia  141  to form the ammonium carbamate solution  165  can comprise introducing the compressed CO 2    125 A and liquid ammonia  141  into NH 3 —CO 2  reactor  160 , whereby the compressed CO 2    125 A and the liquid NH 3    141  react via the exothermic reaction of Eq. (1) to produce the ammonium carbamate. Ammonium carbamate solution  165  comprising the ammonium carbamate can be removed from NH 3 —CO 2  reactor  160  and introduced into ammonium carbamate decomposition reactor  170 . 
     NH 3 —CO 2  reactor  160  is configured for reacting the separated CO 2    125  with liquid ammonia comprising the NH 3    141  to form the ammonium carbamate solution  165  comprising ammonium carbamate. NH 3 —CO 2  reactor  160  can have an operating temperature in a range of from about 350° F. to about 390° F. (from about 176.7° C. to about 198.9° C.), from about 360° F. to about 390° F. (from about 182.2° C. to about 198.9° C.), from about 370° F. to about 390° F. (from about 176.7° C. to about 198.9° C.), or greater than or equal to about 350° F., 360° F., 370° F., 380° F., or 390° F. (greater than or equal to about 176.7° C., 182.2° C., 187.8° C., 193.3° C., or 198.8° C.), and/or a pressure in a range of from about 130 atmospheres (atm) to about 160 atm (from about 13.2 MPa to about 16.2 MPa), from about 140 atm to about 160 atm (from about 14.2 MPa to about 16.2 MPa), from about 150 atm to about 160 atm (from about 15.2 MPa to about 16.2 MPa), or greater than or equal to about 130, 140, 150, 160, or 170 atm (greater than or equal to about 13.2, 14.2, 15.2, 16.2, or 17.2 MPa). 
     As noted above and detailed further hereinbelow, method I can further comprise using heat produced by the exothermic reaction in the NH 3 —CO 2  reactor  160  to produce steam  155  (e.g., via heat exchanger  150  or another heat exchanger). The steam can be utilized as a heat source for decomposing the ammonium carbamate utilized in forming urea at step  30 , or elsewhere throughout system  200 . 
     Decomposing the ammonium carbamate during the forming of urea at  30  utilizing system  200  of  FIG.  3    can comprise introducing the ammonium carbamate solution  165  comprising ammonium carbamate into ammonium carbamate decomposition reactor  170 . As noted above, in ammonium carbamate decomposition reactor  170 , the ammonium carbamate in ammonium carbamate solution  165  decomposes to provide the urea of the aqueous urea solution  175  via a reduction in the temperature and the pressure of the ammonium carbamate (e.g., in ammonium carbamate decomposition reactor  170 ). Decomposing the ammonium carbamate during forming of the urea at step  30  can further comprise reducing the temperature and pressure of the ammonium carbamate in the ammonium carbonate decomposition reactor  170  to a temperature in a range of from about 250° F. to about 280° F. (from about 121.1° C. to about 137.8° C.), from about 260° F. to about 280° F. (from about 126.7° C. to about 137.8° C.), from about 270° F. to about 280° F. (from about 132.2° C. to about 137.8° C.), or less than or equal to about 250° F., 260° F., 270° F., 280° F., or 290° F. (less than or equal to about 121.1° C., 126.7° C., 132.2° C., 137.8° C., or 143.3° C.), and/or a pressure in a range of from about 20 atmospheres (atm) to about 50 atm (from about 2.1 MPa to about 5.1 MPa), from about 25 atm to about 45 atm (from about 2.5 MPa to about 4.6 MPa), from about 30 atm to about 50 atm (from about 3.0 MPa to about 5.1 MPa), or less than or equal to about 50, 45, 40, 35, or 30 atm (less than or equal to about 5.1, 4.6, 4.1, 3.5, or 3.0 MPa). 
     Accordingly, urea production apparatus  130 A can further comprise ammonium carbamate decomposition reactor  170 . Ammonium carbamate solution  165  comprising ammonium carbamate, water, unreacted NH 3  and unreacted CO 2  can be introduced into ammonium carbamate decomposition reactor  170 . The ammonium carbamate decomposition reactor  170  is configured for decomposing the ammonium carbamate to form an aqueous urea solution  175  comprising the urea. An ammonium carbamate decomposition reactor outlet stream  176  comprising water and unreacted CO 2  and NH 3  can be removed from ammonium carbamate reactor decomposition reactor  170 . Ammonium carbamate decomposition reactor  170  can be operable to decompose the ammonium carbamate to provide the aqueous urea solution  175  via a reduction in the temperature and pressure of the ammonium carbamate in the ammonium carbamate decomposition reactor  170 . In embodiments, the ammonium carbamate decomposition reactor  170  comprises and/or is operable as a distillation column. 
     In embodiments, the ammonium carbamate decomposition reactor  170  effects the decomposing of the ammonium carbamate in ammonium carbamate solution  175  via a reduction of the temperature and pressure of the ammonium carbamate therein to a temperature in a range of from about 250° F. to about 280° F. (from about 121.1° C. to about 137.8° C.), from about 260° F. to about 280° F. (from about 126.7° C. to about 137.8° C.), from about 270° F. to about 280° F. (from about 132.2° C. to about 137.8° C.), or less than or equal to about 250° F., 260° F., 270° F., 280° F., or 290° F. (less than or equal to about 121.1° C., 126.7° C., 132.2° C., 137.8° C., or 143.3° C.), and/or a pressure in a range of from about 20 atmospheres (atm) to about 50 atm (from about 2.1 MPa to about 5.1 MPa), from about 25 atm to about 45 atm (from about 2.5 MPa to about 4.6 MPa), from about 30 atm to about 50 atm (from about 3.0 MPa to about 5.1 MPa), or less than or equal to about 50, 45, 40, 35, or 30 atm (less than or equal to about 5.1, 4.6, 4.1, 3.5, or 3.0 MPa). The ammonium carbamate decomposition reactor  170  can further comprise an outlet for a stream  176  comprising water, unreacted NH 3  and CO 2 , and (e.g., undecomposed) ammonium carbamate. The ammonium carbamate decomposition reactor  170  can be fluidly connected with the NH 3 —CO 2  reactor  160 , whereby the stream  176  comprising water, unreacted NH 3  and CO 2 , and undecomposed ammonium carbamate can be recycled to the NH 3 —CO 2  reactor  160 , to increase a conversion to urea by the system  200 . 
     In embodiments such as depicted in  FIG.  4    and  FIG.  5   , the urea production apparatus  130  ( FIG.  2   ) comprises an electrocatalytic reactor comprising a flow reactor cell  130 B. A system of this disclosure can thus be as depicted in  FIG.  3   , with urea production apparatus  130 B of  FIG.  4    utilized in place of urea production apparatus  130 A of  FIG.  3   . The electrocatalytic reactor of urea production apparatus  130 B is configured for forming urea  136  by reacting the separated CO 2    125  directly with nitrogen gas  142  in water  143  to form the urea  136 . Electrocatalytic reactor  130 B can be configured for forming the urea  136  by converting nitrogen gas  142  and the at least the portion of the separated CO 2    125  directly into urea  136  in water  143  via an electrocatalytic reaction. In embodiments, electrocatalytic reactor  130 B is configured to effect the electrocatalytic reaction at about ambient temperature and pressure (e.g., a temperature of about 77° F. (21° C.) and atmospheric pressure (101 kPa)). 
     As depicted in  FIG.  5   , electrocatalytic reactor  130 B can comprise an electrocatalyst  131  comprising palladium-copper (Pd—Cu) nanoparticles  132  on titanium dioxide (TiO 2 ) nanosheets  134 , having oxygen vacancies  133 . Electrocatalytic reactor  130 B can comprise a flow reactor cell comprising a cathode C of carbon paper  137  loaded with the catalyst  131  and a nickel based anode A, separated by a membrane  139  in a chamber  138  filled with an aqueous (e.g., potassium bicarbonate) electrolyte E. In such embodiments, urea  136  is formed by pumping the N 2  gas  142  and the at least the portion of the separated CO 2    125  gas through the flow reactor cell  130 B so both the nitrogen gas  142  and the CO 2  gas  125  are adsorbed on the catalyst  131  and react to produce the urea  136 . 
     Thus, in embodiments, forming urea at  30  can be effected directly at ambient conditions. For example, with reference to electrocatalytic urea production apparatus  130 B of  FIG.  4    and electrocatalyst  131  of  FIG.  5   , forming urea at step  30  can comprise coupling N 2  gas  142  and separated CO 2    125  in H 2 O  143  to directly synthesize urea  136  under ambient conditions. In such embodiments, electrocatalytic reaction can be applied to convert nitrogen gas  142  and substantially pure separated CO 2    125  directly into urea  136  aqueous solution (e.g., the presence of water  143 ) via electrocatalytic reaction. The electrocatalytic reaction can occur at ambient temperature and pressure (e.g., a temperature of about 77° F. (21° C.) and a pressure of about atmospheric pressure (101 kPa)). 
     The electrocatalytic reaction can employ an electrocatalyst  131  ( FIG.  5   ) comprising or consisting of palladium-copper nanoparticles  132  on titanium dioxide nanosheets  134 , or another electrocatalyst. The electrocatalytic reaction can be effected in a flow reactor cell  130 B containing a cathode C made of carbon paper  137  or another cathode material loaded with the electrocatalyst  131  (or other electrocatalyst) and a nickel-based anode A or another anode, separated by a membrane  139  in a chamber  138  filled with an aqueous potassium bicarbonate electrolyte E or another electrolyte. The electrodes, separated by a membrane  139 , are positioned in a chamber  138  filled with an electrolyte E (e.g., an aqueous potassium bicarbonate electrolyte). The N 2    142  and CO 2    125  gases are pumped through the flow reactor cell  130 B so that both gases are adsorbed on the electrocatalyst  131  and react to produce urea  136 . Forming the urea at step  30  can include pumping the N 2  gas  142  and the at least the portion of the separated CO 2  gas  125  through the flow reactor cell  130 B so both the nitrogen gas  142  and the CO 2  gas  125  are adsorbed on the catalyst  131  and react to produce the urea  136 . 
     Without being limited by theory, nitrogen  142  can promote the reduction of CO 2    125  on the catalyst  131  surface to produce carbon monoxide (CO). The CO can then react with N 2    142  to generate intermediate species. Further interactions between CO and these intermediate species hydrogenate N 2    142  and create C-N bonds, thereby forming urea  136 . The titanium dioxide support  134  can play a key role in the urea  136  synthesis by stabilizing the intermediates. Increasing the electrolyte E concentration and flow rate can be utilized to improves the efficiency of the electrocatalytic reaction system  130 B. 
     As noted above, method I can further comprise, at  40 , separating one or more components from the urea. For example, water, unreacted NH 3 , unreacted CO 2 , ammonium carbamate (e.g., undecomposed ammonium carbamate), or a combination thereof can be removed from aqueous urea solution  175  at  40 . 
     With reference back to system  200  of  FIG.  3   , in embodiments, separating components from the urea at  40  can further comprise removing water, and unreacted NH 3  and CO 2  stream  186  from the aqueous urea solution  175  to provide a concentrated aqueous urea  185 . In embodiments, urea production apparatus  130  can include unwanted component removal apparatus configured to remove one or more unwanted components (e.g., water, unreacted reactants (e.g., NH 3 , CO 2 , N 2 ) from the urea produced in the urea production apparatus  130 . For example, urea production apparatus  130 A of system  200  can further comprise an unreacted component removal apparatus  180  configured for removing a stream  186  comprising water and unreacted NH 3  and CO 2  from the aqueous urea solution  175  to provide a concentrated aqueous urea  185 . 
     As noted above, removing the stream  186  comprising additional water and unreacted NH 3  and CO 2  can be effected via unreacted component removal reactor  180  (e.g., a flash chamber). Aqueous urea solution  175  provided in the ammonium carbamate decomposition reactor  170  can thus be introduced into unreacted component removal reactor  180 . Unreacted component removal reactor  180  is operable to provide concentrated aqueous urea  185 . Concentrated aqueous urea  185  has a higher concentration of urea than aqueous urea solution  175  introduced thereto. In embodiments, the unreacted component removal apparatus  180  comprises a flash chamber. Stream  186  comprising water, unreacted NH 3  and/or unreacted CO 2  can be removed from unreacted component removal apparatus  180 . Unreacted component removal apparatus  186  can be fluidly connected with NH 3 —CO 2  reactor  160 , such that stream  186  comprising water and unreacted components removed from unreacted component removal apparatus  180  can be introduced into NH 3 —CO 2  reactor  160 , to increase a conversion to urea by the system  200 . Step  40  of method I can thus comprise removing a stream  176  comprising water and unreacted NH 3  and CO 2 , and undecomposed ammonium carbamate from the ammonium carbamate decomposition reactor  170  and/or a stream  186  comprising water and unreacted NH 3  and CO 2  from unwanted component removal apparatus  180 . Method I can include recycling the stream  176  comprising the water, the unreacted NH 3  and CO 2 , and the undecomposed ammonium carbamate and/or the stream  186  comprising water, unreacted NH 3  and unreacted CO 2  to the NH 3 —CO 2  reactor  160 , to increase a conversion to urea provided by the method. 
     Method I can further comprise removing additional water  192  from the concentrated aqueous urea  185 , or a portion  189  thereof, to provide a semi-solid, molten, and/or solid urea  191 . Removing additional water  192  can comprise drying the concentrated aqueous urea  185  or the portion  189  thereof. Accordingly, in embodiments, urea production apparatus  130 A of system  200  can further include a drying apparatus  190 . Drying apparatus  190  is configured for removing additional water  192  from the concentrated aqueous urea  185  introduced thereto to provide a semi-solid, molten, and/or solid urea  191 . The drying apparatus  190  can comprise a vacuum evaporator. Step  40  of method I can thus comprise removing water  192  via drying apparatus  192 . 
     As noted above, method I can further include, at  50 , utilizing heat from the collected exhaust gas  115  (e.g., high temperature exhaust gas  115 A of  FIG.  3   ) collected at  10  and/or produced during the forming of the urea at  30  in the separating CO 2  from the collected exhaust gas  115  at  20 , the forming urea from at least a portion of the separated CO 2    125  at  30 , and/or the separating of one or more components from the urea at  40 . For example, system  200  can further include steam production apparatus (e.g., heat exchanger  150  or another heat exchanger) operable to utilize heat produced by the exothermic reaction (Eq. (1)) in the NH 3 —CO 2  reactor  160  to produce steam  155 . At least a portion of the steam  155 A can be utilized as a heat source for decomposing the ammonium carbamate in ammonium carbamate decomposition reactor  170 , at least a portion of the steam  155 B can be utilized as a heat source for effecting unreacted component removal in unwanted component removal apparatus  180 , and/or at least a portion of the steam  155 C can be utilized as a heat source in drying apparatus  190 . 
     Accordingly, as depicted in  FIG.  3   , urea production apparatus  130 A of system  200  can further comprise a heat exchanger  150  configured to transfer heat from a high temperature collected exhaust gas  115 A (e.g., a collected exhaust gas  115 A having a temperature of greater than or equal to about 350, 500, or 700° C.) to produce steam  155  from water  114  introduced into heat exchanger  150  and provide a low temperature exhaust gas  115 B (e.g., a collected exhaust gas having a temperature of less than or equal to about 75, 100, or 150° C.) as the collected exhaust gas  115  ( FIG.  2   ). The steam  155  can be utilized in the urea production apparatus  130  (e.g., in ammonium carbamate decomposition reactor  170 , in unreacted component removal reactor  180 , in drying apparatus  190 , or a combination thereof.) In embodiments, heat from the exothermic reaction (e.g., of Eq. (1)) in the urea production apparatus  130  (e.g., produced in NH 3 —CO 2  reactor  160 ) can be utilized in the urea production apparatus  130  (e.g., in ammonium carbamate decomposition reactor  170 , in unreacted component removal reactor  180 , in drying apparatus  190 , or a combination thereof. (For example, the heat exchanger  150  or another heat exchanger) can be utilized to produce steam  155  that can be utilized to heat ammonium carbamate decomposition reactor  170 , unreacted component removal reactor  180 , and/or drying apparatus  190 .) 
     System  100 / 200  can thus further include heating apparatus (e.g., heat exchanger  150 , etc.) configured for utilizing heat of the collected exhaust gas  115  elsewhere in the system  100 / 200 . For example, system  200  of  FIG.  3    can further include heat exchanger  150  configured for utilizing heat of the high temperature collected exhaust gas  115 A in the ammonium carbamate decomposition reactor  170 , the unreacted component removal apparatus  180 , and/or the drying apparatus  190 . 
     In embodiments, method I can further comprise utilizing heat of the collected exhaust gas  115  (e.g., high temperature exhaust gas  115 A of  FIG.  3   ) and/or heat of ammonium carbamate solution  165  in the decomposing of the ammonium carbamate, the separating CO 2  from the collected exhaust gas  115 , and/or the removing water, unreacted NH 3  and CO 2  stream  186  from the aqueous urea solution  175  to provide the concentrated aqueous urea  185 , and/or the drying of all or a portion  189  of the concentrated aqueous urea  185  in drying apparatus  190 . Utilizing heat can comprise forming steam  155  and utilizing the steam  155 A in the decomposing of the ammonium carbamate, utilizing steam  155  in the separating CO 2  from the collected exhaust gas in separation apparatus  120 , utilizing steam  155 B in the removing water, unreacted NH 3  and CO 2  stream  186  from the aqueous urea solution  175  to provide the concentrated aqueous urea  185 , and/or utilizing steam  155 C in the drying in drying apparatus  190 . 
     With reference to  FIG.  1   , as depicted at step  50 , method I can include utilizing heat obtained from the collected exhaust gas  115  obtained at step  10  and/or the forming of the urea at step  30  in the CO 2  separation at step  20 , the forming of urea at step  30 , and/or the separating of components at step  40 . Heat of the collected exhaust gas  115  can be utilized in CO 2  separating step  20 , urea forming at step  30  and/or in component removal at step  40  of method I. For example, heat of the collected exhaust gas  115  can be utilized the separating CO 2    125  from the collected exhaust gas  115  in CO 2  separation apparatus  120  (in CO 2  separating step  20 ), in the decomposing of the ammonium carbamate in ammonium carbamate decomposition reactor  170  (in urea forming step  30 ), in component removal in unreacted component removal reactor  180  (in component removal step  40 ), and/or in drying in drying apparatus  190  (also in component removal step  40 ). Utilizing heat can comprise forming steam  155  and utilizing the steam  155  for the heating of the various aforementioned processes. 
     As noted above, method I can further comprise forming diesel exhaust fluid (DEF) at  60 . Forming DEF can comprise diluting the urea with water  144  at the wellsite  111  or another wellsite to form diesel exhaust fluid (DEF)  145  for converting toxic nitrogen oxides (NOx) in a diesel combustion exhaust gas of diesel combustion into inert nitrogen gas. The method I can further utilizing the DEF onsite in a diesel engine  113  (e.g., a diesel engine  113  of a diesel truck  114 ). Forming DEF utilizing at least a portion of the urea produced in urea production apparatus  130  (e.g., urea production apparatus  130 A of  FIG.  3    and/or urea production apparatus  130 B of  FIG.  4   ) at  60  can comprise utilizing the urea product  135  (e.g., aqueous urea solution  175 , concentrated aqueous urea  185 , or semi-solid, molten, or solid urea  191 ) as source of urea  136  for preparing DEF  145  to be used in converting toxic nitrogen oxides (NOx) in exhaust gas of diesel combustion into inert N 2  gas. 
     Accordingly, as depicted in  FIG.  2   , a system  100  of this disclosure can comprise a DEF production apparatus  140  configured to form the DEF  145  at  60 . DEF production apparatus  140  is configured for diluting the urea  165  (e.g., in urea product  135 , which can be or comprise aqueous urea solution  175 , concentrated aqueous urea  185 , and/or semi-solid, molten, or dry urea  191 ) with water  144  at the wellsite  111  to form diesel exhaust fluid (DEF)  145  for converting toxic nitrogen oxides (NOx) in a diesel combustion exhaust gas of diesel combustion into inert nitrogen gas. The system  100 / 200  can further comprise one or more onsite diesel engines  113  (e.g., a diesel engine  113  of a diesel truck  114 ) comprising some of the DEF  145 . 
     Alternatively or additionally, the urea product  135  (e.g., aqueous urea solution  175 , concentrated aqueous urea  185 , or semi-solid, molten, or solid urea  191 ) can be utilized as source of liquid or dry fertilizers that can potentially be sold or donated to local farmers. 
     As depicted in  FIG.  2   , in embodiments, a system, such as system  100  of  FIG.  2    or system  200  of  FIG.  3    (or a system  100  or  200  including an electrocatalytic reaction apparatus, such as electrocatalytic reaction apparatus  130 B described with regard to  FIG.  4    and  FIG.  5   , as urea production apparatus  130 ) of this disclosure, or one or more components thereof (e.g., exhaust gas collection apparatus  110 , CO 2  separation apparatus  120 , urea production apparatus  130 , DEF production apparatus  140 , or a combination thereof) can be provided on one or more skids  150  (e.g., a trailer skid), whereby at least a portion the separated CO 2    125  and liquid ammonia  141  can be converted to urea product  135  comprising urea  136  (e.g., aqueous urea solution  175 , concentrated aqueous urea  185 , semi-solid, molten, or solid urea  191 ) at the wellsite  111 . For example, in embodiments, such as depicted in  FIG.  2   , urea production apparatus  130 , DEF production apparatus  140 , or both can be located on one or more skids  150 . 
     Also provided herein is a method comprising: forming urea using, as a reactant, carbon dioxide separated from exhaust gas produced at a wellsite. 
     The collecting of exhaust gas at  10 , the separating of CO 2  at  20 , the forming of urea at  30 , or a combination thereof can be performed substantially continuously or intermittently. 
     The system and method of this disclosure can provide for continuous, semi-continuous, or intermittent collecting of exhaust gas  115  from field operating equipment  112  at a wellsite  111  and utilization of the collected exhaust gas  115  to produce urea. The urea can be utilized to benefit at the wellsite  111 , for example, as a component of DEF and/or can be sold for profit (e.g., as a fertilizer to farms local or remote from wellsite  111 ). 
     Rather than or in addition to injecting downhole separated CO 2    125  separated from collected exhaust gas  115  for sequestration purposes, separated CO 2    125  can be converted by the system and disclosure provided herein to convert it, at wellsite  111 , to a useful urea product  135  comprising urea  136 , such as an aqueous urea solution  175 , a concentrated aqueous urea  185 , or a semi-solid, molten, or solid urea  191 . 
     Rather than relying on commercial diesel exhaust fluid (DEF) or a commercial urea source for making DEF, an aqueous urea solution can be manufactured at wellsite  111  via the system and method of this disclosure to provide ample supply of DEF for diesel engines of diesel trucks and other apparatus/machinery at the wellsite  111 . Aqueous urea solution  175  produced at wellsite  111  can be diluted with dilution water  144  to form DEF  145  for converting toxic nitrogen oxides (NOx) in the exhaust gas of diesel combustion into inert N 2  gas. The DEF can be utilized at wellsite  111  and/or off-site. 
     In embodiments, the system of this disclosure can be provided on a skid (e.g., a trailer skid), whereby at least a portion the separated CO 2    125  and liquid ammonia  141  can be converted to urea product  135  comprising urea  136  (e.g., aqueous urea solution  175 , concentrated aqueous urea  185 , semi-solid, molten, or solid urea  191 ) at the wellsite  111 . 
     In embodiments, the heat of collected exhaust gas  115  can be utilized in providing a heat source utilized during the separating of one or more components from the urea at step  40  of method I. 
     In embodiments, at least a portion of the system  100  (e.g., urea production apparatus  130 , DEF production apparatus  140 ) is provided as a small-scale urea plant (e.g., on one or more skids  150 ) at wellsite  111 , whereby urea product  135  (e.g., aqueous urea solution  175 , concentrated aqueous urea  185 , or dry urea  191 ) can be produced on location, DEF can be produced on location (e.g., and utilized in diesel trucks or other diesel engines), and/or a source of liquid urea fertilizer (or dry fertilizer) can be provided and sold or donated to farmers (e.g., local farmers). 
     Additional Disclosure 
     The following are non-limiting, specific embodiments in accordance with the present disclosure: 
     In a first embodiment, a method comprises collecting exhaust gas comprising carbon dioxide (CO 2 ) at a wellsite to provide a collected exhaust gas; separating CO 2  from the collected exhaust gas to provide a separated CO 2 ; and forming urea utilizing at least a portion of the separated CO 2 . 
     A second embodiment can include the method of the first embodiment, wherein forming urea comprises reacting the separated CO 2  with ammonia (NH 3 ) to form ammonium carbamate and decomposing the ammonium carbamate to form the urea; or wherein forming urea comprises reacting the separated CO 2  directly with nitrogen (N 2 ) gas to form the urea. 
     A third embodiment can include the method of the first or the second embodiment, wherein forming urea comprises reacting the at least the portion of the separated CO 2  with ammonia to form ammonium carbamate and decomposing the ammonium carbamate to form the urea. 
     A fourth embodiment can include the method of the third embodiment, wherein forming urea further comprises: compressing the at least a portion of the separated CO 2  to provide a compressed CO 2 ; reacting the compressed CO 2  with the ammonia to form an ammonium carbamate solution comprising the ammonium carbamate; and decomposing the ammonium carbamate to provide an aqueous urea solution comprising the urea. 
     A fifth embodiment can include the method of the fourth embodiment further comprising removing water, and unreacted NH 3  and CO 2  from the aqueous urea solution to provide a concentrated aqueous urea. 
     A sixth embodiment can include the method of the fifth embodiment, further comprising utilizing heat of the collected exhaust gas in the decomposing of the ammonium carbamate, the separating CO 2  from the collected exhaust gas, and/or the removing water, unreacted NH 3  and CO 2  from the aqueous urea solution to provide the concentrated aqueous urea. 
     A seventh embodiment can include the method of the sixth embodiment, wherein utilizing heat further comprises forming steam and utilizing the steam in the decomposing of the ammonium carbamate, the separating CO 2  from the collected exhaust gas, and/or the removing water, unreacted NH 3  and CO 2  from the aqueous urea solution to provide the concentrated aqueous urea. 
     An eighth embodiment can include the method of any one of the fifth to seventh embodiments, wherein removing the water and unreacted NH 3  and CO 2  is effected via a flash chamber. 
     A ninth embodiment can include the method of any one of the fifth to eighth embodiments further comprising removing additional water from the concentrated aqueous urea to provide a semi-solid, molten, and/or solid urea. 
     A tenth embodiment can include the method of the ninth embodiment, wherein removing additional water comprises drying the concentrated aqueous urea in a vacuum evaporator. 
     An eleventh embodiment can include the method of any one of the fourth to tenth embodiments, wherein compressing comprises compressing to a pressure of greater than or equal to about 130, 140, or 150 atmospheres (13.2, 14.2, or 15.2 MPa), and/or in a range of from about 130 to about 150 atm (from about 13.2 to about 15.2 MPa). 
     A twelfth embodiment can include the method of any one of the fourth to eleventh embodiments, wherein reacting the compressed CO 2  and ammonia to form the ammonium carbamate solution comprises introducing the compressed CO 2  and liquid ammonia into an NH 3 —CO 2  reactor, whereby the compressed CO 2  and the liquid NH 3  react via an exothermic reaction to produce the ammonium carbamate. 
     A thirteenth embodiment can include the method of the twelfth embodiment, wherein the NH 3 —CO 2  reactor has an operating temperature in a range of from about 350° F. to about 390° F. (from about 176.7° C. to about 198.9° C.), from about 360° F. to about 390° F. (from about 182.2° C. to about 198.9° C.), from about 370° F. to about 390° F. (from about 176.7° C. to about 198.9° C.), or greater than or equal to about 350° F., 360° F., 370° F., 380° F., or 390° F. (greater than or equal to about 176.7° C., 182.2° C., 187.8° C., 193.3° C., or 198.8° C.), and/or a pressure in a range of from about 130 atmospheres (atm) to about 160 atm (from about 13.2 MPa to about 16.2MPa), from about 140 atm to about 160 atm (from about 14.2 MPa to about 16.2MPa), from about 150 atm to about 160 atm (from about 15.2 MPa to about 16.2MPa), or greater than or equal to about 130, 140, 150, 160, or 170 atm (greater than or equal to about 13.2, 14.2, 15.2, 16.2, or 17.2 MPa). 
     A fourteenth embodiment can include the method of the thirteenth embodiment further comprising using heat produced by the exothermic reaction in the NH 3 —CO 2  reactor to produce steam. 
     A fifteenth embodiment can include the method of the fourteenth embodiment further comprising using the steam as a heat source for decomposing the ammonium carbamate. 
     A sixteenth embodiment can include the method of any one of the fourth to fifteenth embodiments, wherein decomposing the ammonium carbamate further comprises introducing the ammonium carbamate into an ammonium carbamate decomposition reactor, whereby the ammonium carbamate decomposes to provide the urea of the aqueous urea solution via a reduction in the temperature and the pressure of the ammonium carbamate in the ammonium carbamate decomposition reactor. 
     A seventeenth embodiment can include the method of the sixteenth embodiment, wherein the ammonium carbamate decomposition reactor is and/or is operated as a distillation column. 
     An eighteenth embodiment can include the method of any one of the sixteenth to seventeenth embodiments, wherein decomposing further comprises reducing the temperature and pressure of the ammonium carbamate in the ammonium carbonate decomposition reactor to a temperature in a range of from about 250° F. to about 280° F. (from about 121.1° C. to about 137.8° C.), from about 260° F. to about 280° F. (from about 126.7° C. to about 137.8° C.), from about 270° F. to about 280° F. (from about 132.2° C. to about 137.8° C.), or less than or equal to about 250° F., 260° F., 270° F., 280° F., or 290° F. (less than or equal to about 121.1° C., 126.7° C., 132.2° C., 137.8° C., or 143.3° C.), and/or a pressure in a range of from about 20 atmospheres (atm) to about 50 atm (from about 2.1 MPa to about 5.1 MPa), from about 25 atm to about 45 atm (from about 2.5 MPa to about 4.6 MPa), from about 30 atm to about 50 atm (from about 3.0 MPa to about 5.1 MPa), or less than or equal to about 50, 45, 40, 35, or 30 atm (less than or equal to about 5.1, 4.6, 4.1, 3.5, or 3.0 MPa). 
     A nineteenth embodiment can include the method of any one of the sixteenth to eighteenth embodiments further comprising removing a stream comprising water and unreacted NH 3  and CO 2 , and undecomposed ammonium carbamate from the ammonium carbamate decomposition reactor. 
     A twentieth embodiment can include the method of the nineteenth embodiment, further comprising recycling the stream comprising the water, the unreacted NH 3  and CO 2 , and the undecomposed ammonium carbamate to the NH 3 —CO 2  reactor, to increase a conversion to urea. 
     A twenty first embodiment can include the method of any one of the fourth to twentieth embodiments further comprising utilizing heat of the collected exhaust gas in the decomposing of the ammonium carbamate and/or the separating CO 2  from the collected exhaust gas. 
     A twenty second embodiment can include the method of the twenty first embodiment, wherein utilizing heat further comprises forming steam and utilizing the steam in the decomposing of the ammonium carbamate and/or the separating CO 2  from the collected exhaust gas. 
     A twenty third embodiment can include the method of any one of the second to twenty second embodiments, comprising forming urea by reacting the separated CO 2  directly with nitrogen gas to form the urea. 
     A twenty fourth embodiment can include the method of the twenty third embodiment further comprising forming the urea by converting nitrogen gas and the at least the portion of the separated CO 2  directly into the urea in aqueous solution via an electrocatalytic reaction. 
     A twenty fifth embodiment can include the method of the twenty fourth embodiment, wherein the electrocatalytic reaction occurs at ambient temperature and pressure (e.g., a temperature of about 77° F. (21° C.) and a pressure of about atmospheric pressure (101 kPa). 
     A twenty sixth embodiment can include the method of any one of the twenty fourth to twenty fifth embodiments, wherein the electrocatalytic reaction is effected in the presence of a catalyst comprising palladium-copper (Pd—Cu) nanoparticles on titanium dioxide (TiO 2 ) nanosheets. 
     A twenty seventh embodiment can include the method of the twenty sixth embodiment, wherein the electrocatalytic reaction is carried out in a flow reactor cell comprising a cathode of carbon paper loaded with the catalyst and a nickel based anode, separated by a membrane in a chamber filled with an aqueous potassium bicarbonate electrolyte. 
     A twenty eighth embodiment can include the method of the twenty seventh embodiment further comprising pumping the N 2  gas and the at least the portion of the separated CO 2  gas through the flow reactor cell so both the nitrogen gas and the CO 2  gas are adsorbed on the catalyst and react to produce the urea. 
     A twenty ninth embodiment can include the method of any one of the first to twenty eighth embodiments further comprising diluting the urea with water at the wellsite to form diesel exhaust fluid (DEF) for converting toxic nitrogen oxides (NOx) in a diesel combustion exhaust gas of diesel combustion into inert nitrogen gas. 
     A thirtieth embodiment can include the method of the twenty ninth embodiment further comprising utilizing the DEF onsite in a diesel engine (e.g., a diesel engine of a diesel truck). 
     In a thirty first embodiment, a system comprises: an exhaust gas collection system configured for collecting exhaust gas comprising carbon dioxide (CO 2 ) at a wellsite to provide a collected exhaust gas; a CO 2  separation apparatus configured for separating CO 2  from the collected exhaust gas to provide a separated CO 2 ; and a urea production apparatus configured for forming urea utilizing at least a portion of the separated CO 2 . 
     A thirty second embodiment can include the system of the thirty first embodiment, wherein the urea production apparatus comprises a NH 3 —CO 2  reactor configured for reacting the separated CO 2  with ammonia (NH 3 ) to form ammonium carbamate and an ammonium carbamate decomposition reactor configured for decomposing the ammonium carbamate to form the urea, or wherein the urea production apparatus comprises an electrocatalytic reactor configured for forming the urea by reacting the separated CO 2  directly with nitrogen gas. 
     A thirty third embodiment can include the system of the thirty second embodiment, wherein the urea production apparatus comprises the NH 3 —CO 2  reactor and the ammonium carbamate decomposition reactor configured for decomposing the ammonium carbamate, wherein the NH 3 —CO 2  reactor is configured for reacting the separated CO 2  with liquid ammonia comprising the NH 3  to form the ammonium carbamate and wherein the ammonium carbamate decomposition reactor is configured for decomposing the ammonium carbamate to form an aqueous urea solution comprising the urea. 
     A thirty fourth embodiment can include the system of the thirty third embodiment further comprising: a compressor configured for compressing the at least a portion of the separated CO 2  to provide a compressed CO 2 , wherein the NH 3 —CO 2  reactor is configured for reacting the compressed CO 2  with the liquid ammonia via an exothermic reaction to form the ammonium carbamate; and an unreacted component removal apparatus (e.g., a flash chamber) configured for separating water and unreacted CO 2  and NH 3  from the aqueous urea solution provided in the ammonium carbamate reactor to provide a concentrated aqueous urea having a higher concentration of urea than the aqueous urea solution. 
     A thirty fifth embodiment can include the system of the thirty fourth embodiment further comprising a drying apparatus configured for removing additional water from the concentrated aqueous urea to provide a semi-solid, molten, and/or solid urea. 
     A thirty sixth embodiment can include the system of the thirty fifth embodiment, wherein the drying apparatus comprises a vacuum evaporator. 
     A thirty seventh embodiment can include the method of any one of the thirty fourth to thirty sixth embodiments, wherein the compressor is operable to compress the at least the portion of the separated CO 2  to a pressure of greater than or equal to about 130, 140, or 150 atmospheres (13.2, 14.2, or 15.2 MPa), and/or in a range of from about 130 to about 150 atm (from about 13.2 to about 15.2 MPa). 
     A thirty eighth embodiment can include the system of any one of the thirty fourth to thirty seventh embodiments, wherein reacting the compressed CO 2  and the liquid ammonia to form the ammonium carbamate comprises introducing the compressed CO 2  and liquid ammonia into the NH 3 —CO 2  reactor, whereby the compressed CO 2  and the liquid NH 3  react via an exothermic reaction to form the ammonium carbamate. 
     A thirty ninth embodiment can include the system of the thirty eighth embodiment, wherein the NH 3 —CO 2  reactor has an operating temperature in a range of from about 350° F. to about 390° F. (from about 176.7° C. to about 198.9° C.), from about 360° F. to about 390° F. (from about 182.2° C. to about 198.9° C.), from about 370° F. to about 390° F. (from about 176.7° C. to about 198.9° C.), or greater than or equal to about 350° F., 360° F., 370° F., 380° F., or 390° F. (greater than or equal to about 176.7° C., 182.2° C., 187.8° C., 193.3° C., or 198.8° C.), and/or a pressure in a range of from about 130 atmospheres (atm) to about 160 atm (from about 13.2 MPa to about 16.2MPa), from about 140 atm to about 160 atm (from about 14.2 MPa to about 16.2MPa), from about 150 atm to about 160 atm (from about 15.2 MPa to about 16.2MPa), or greater than or equal to about 130, 140, 150, 160, or 170 atm (greater than or equal to about 13.2, 14.2, 15.2, 16.2, or 17.2 MPa). 
     A fortieth embodiment can include the system of any one of the thirty eighth to thirty ninth embodiments further comprising steam production apparatus operable to utilize heat produced by the exothermic reaction in the NH 3 —CO 2  reactor to produce steam. 
     A forty first embodiment can include the system of the fortieth embodiment further comprising a steam line from the NH 3 —CO 2  reactor to the ammonium carbamate decomposition reactor, whereby the at least a portion of the steam is utilized as a heat source for decomposing the ammonium carbamate. 
     A forty second embodiment can include the system of any one of the thirty second to forty first embodiments, wherein the ammonium carbamate decomposition reactor is operable to decompose the ammonium carbamate to provide the aqueous urea solution via a reduction in the temperature and pressure of the ammonium carbamate in the ammonium carbamate decomposition reactor. 
     A forty third embodiment can include the system of the forty second embodiment, wherein the ammonium carbamate decomposition reactor comprises a distillation column. 
     A forty fourth embodiment can include the system of any one of the forty second to forty third embodiments, wherein the ammonium carbamate decomposition reactor effects the decomposing via a reduction of the temperature and pressure of the ammonium carbamate therein to a temperature in a range of from about 250° F. to about 280° F. (from about 121.1° C. to about 137.8° C.), from about 260° F. to about 280° F. (from about 126.7° C. to about 137.8° C.), from about 270° F. to about 280° F. (from about 132.2° C. to about 137.8° C.), or less than or equal to about 250° F., 260° F., 270° F., 280° F., or 290° F. (less than or equal to about 121.1° C., 126.7° C., 132.2° C., 137.8° C., or 143.3° C.), and/or a pressure in a range of from about 20 atmospheres (atm) to about 50 atm (from about 2.1 MPa to about 5.1 MPa), from about 25 atm to about 45 atm (from about 2.5 MPa to about 4.6 MPa), from about 30 atm to about 50 atm (from about 3.0 MPa to about 5.1 MPa), or less than or equal to about 50, 45, 40, 35, or 30 atm (less than or equal to about 5.1, 4.6, 4.1, 3.5, or 3.0 MPa). 
     A forty fifth embodiment can include the system of any one of the thirty second to forty fourth embodiments, wherein the ammonium carbamate decomposition reactor provides a stream comprising water, unreacted NH 3  and CO 2 , and undecomposed ammonium carbamate. 
     A forty sixth embodiment can include the system of the forty fifth embodiment, wherein the ammonium carbamate decomposition reactor is fluidly connected with the NH 3 —CO 2  reactor, such that the stream comprising unreacted NH 3  and CO 2 , and undecomposed ammonium carbamate is recycled to the NH 3 —CO 2  reactor, to increase a conversion to urea by the system. 
     A forty seventh embodiment can include the system of any one of the thirty third to forty sixth embodiments further comprising an unreacted component removal apparatus configured for removing water and unreacted NH 3  and CO 2  from the aqueous urea solution to provide a concentrated aqueous urea. 
     A forty eighth embodiment can include the system of the forty seventh embodiment, wherein the unreacted component removal apparatus comprises a flash chamber. 
     A forty ninth embodiment can include the system of any one of the thirty third to forty eighth embodiments further comprising heating apparatus configured for utilizing heat of the collected exhaust gas in the ammonium carbamate decomposition reactor and/or the unreacted component removal apparatus. 
     A fiftieth embodiment can include the system of the forty ninth embodiment, wherein the heating apparatus comprises steam generating apparatus, and wherein the steam is utilized for heating in the ammonium carbamate decomposition reactor and/or the unreacted component removal apparatus. 
     A fifty first embodiment can include the system of any one of the forty ninth or fiftieth embodiments further comprising a drying apparatus configured for removing additional water from the concentrated aqueous urea to provide a semi-solid, molten, and/or solid urea, and wherein the heating apparatus is configured for utilizing the heat of the collected exhaust gas in the ammonium carbamate decomposition reactor, the unreacted component removal apparatus, the drying apparatus, or a combination thereof. 
     A fifty second embodiment can include the system of any one of the thirty second to fifty first embodiments, wherein the urea production apparatus comprises the electrocatalytic apparatus configured for forming the urea by reacting the separated CO 2  directly with nitrogen gas to form the urea. 
     A fifty third embodiment can include the system of the fifty second embodiment, wherein the electrocatalytic apparatus is configured for forming the urea by converting nitrogen gas and the at least the portion of the separated CO 2  directly into urea in water via an electrocatalytic reaction. 
     A fifty fourth embodiment can include the system of the fifty third embodiment, wherein the electrocatalytic apparatus is configured to effect the electrocatalytic reaction at about ambient temperature and pressure (e.g., a temperature of about 77° F. (21° C.) and atmospheric pressure (101 kPa). 
     A fifty fifth embodiment can include the system of the fifty fourth embodiment, wherein the electrocatalytic reactor comprises a catalyst comprising palladium-copper (Pd—Cu) nanoparticles on titanium dioxide (TiO 2 ) nanosheets. 
     A fifty sixth embodiment can include the system of the fifty fifth embodiment, wherein the electrocatalytic reactor comprises a flow reactor cell comprising a cathode of carbon paper loaded with the catalyst and a nickel based anode, separated by a membrane in a chamber filled with an aqueous potassium bicarbonate electrolyte, wherein the urea is formed by pumping the N 2  gas and the at least the portion of the separated CO 2  gas through the flow reactor cell so both the nitrogen gas and the CO 2  gas are adsorbed on the catalyst and react to produce the urea. 
     A fifty seventh embodiment can include the system of any one of the thirty first to fifty sixth embodiments further comprising DEF production apparatus configured for diluting the urea with water at the wellsite to form diesel exhaust fluid (DEF) for converting toxic nitrogen oxides (NOx) in a diesel combustion exhaust gas of diesel combustion into inert nitrogen gas. 
     A fifty eighth embodiment can include the system of the fifty seventh embodiment further comprising an onsite diesel engine (e.g., a diesel engine of a diesel truck) comprising some of the DEF. 
     In a fifty ninth embodiment, a method comprises: producing urea a detailed herein using, as a reactant, carbon dioxide separated from exhaust gas produced at a wellsite. 
     A sixtieth embodiment can include the method of the fifty ninth embodiment, wherein the producing is performed substantially continuously or intermittently. 
     While embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of this disclosure. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the embodiments disclosed herein are possible and are within the scope of this disclosure. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numerical range with a lower limit, RI, and an upper limit, Ru, is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=R1+k* (Ru−R1), wherein k is a variable ranging from 1 percent to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . 50 percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed. Use of broader terms such as comprises, includes, having, etc. should be understood to provide support for narrower terms such as consisting of, consisting essentially of, comprised substantially of, etc. When a feature is described as “optional,” both embodiments with this feature and embodiments without this feature are disclosed. Similarly, the present disclosure contemplates embodiments where this “optional” feature is required and embodiments where this feature is specifically excluded. 
     Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as embodiments of the present disclosure. Thus, the claims are a further description and are an addition to the embodiments of the present disclosure. The discussion of a reference herein is not an admission that it is prior art, especially any reference that can have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.