Abstract:
An environmentally controlled greenhouse is located near a landfill site and uses the landfill gas to both heat the greenhouse and furnish a clean carbon dioxide enriched product gas to enhance plant growth. The clean CO 2  system product gas is used to regenerate adsorber beds which remove undesirable constituents from the landfill gas. A portion of the landfill gas may also power an extraction steam turbine which heats the greenhouse with extraction steam. The system is controlled to enhance plant growth by adjusting the temperature and CO 2  levels.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of prior filed co-pending provisional application Ser. No. 60/077,233 filed Mar. 9, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to a method and apparatus for enhancement of artificially cultured agricultural products within an environmentally controlled greenhouse environment. More particularly, the invention relates to utilization of waste gas from a landfill to provide enhanced crop growth within a greenhouse in an environmentally safe and controlled system. 
     Yields from food crops and lengthening of the growing season can be achieved by growing the crops under environmentally controlled conditions in large greenhouses. For example, yields of tomatoes grown in a hydroponic greenhouse with a computer controlled growing environment may be many times higher than the yield of tomatoes grown outdoors which are subject to a shorter growing season and uncontrolled environment. Such a greenhouse is described in an article in the Business section of  The Dallas Morning News  dated Sunday, Jan. 12, 1997 entitled “High-Tech Tomatoes”. The tomatoes are grown in a hydroponic system, supported not in soil but in a porous artificial “rock wool” made from volcanic material. Correct amounts of water and nutrients are supplied to the plants and the plants are kept warm by a circulating hot water system heated by burning natural gas (methane). Controlled amounts of exhaust gas from the burners containing carbon dioxide (CO 2 ) is recycled back into the greenhouse. 
     It is well known that carbon dioxide (CO 2 ) is an essential ingredient of the process of photosynthesis, and will enhance plant growth. 
     Photosynthesis is a metabolic process, occurring in green plants, and characterized by the utilization of light (usually sunlight) to convert carbon dioxide and water into carbohydrates and oxygen. The chemical formula for the photosynthetic reaction is: 
     
       
         6CO 2 +6H 2 O+sunlight=C 6 H 12 O 6 +6O 2    
       
     
     Most green plants of the Temperate Zone photosynthesize at temperatures between 10° C. and 35° C. (50° F.-95° F.). The most important external factors are: intensity and duration of light; concentration of available carbon dioxide; temperature; and water supply. Sunlight of great intensity and long duration increases the amount of photosynthesis performed. About 0.03 of one percent of the total volume of the atmosphere is made up of carbon dioxide; hence, one of the limiting factors on the extent of photosynthesis is the amount of available carbon dioxide. 
     When plants are artificially cultured these external factors can sometimes be controlled. While artificial cultivation of crops in a controlled greenhouse  20  environment increases the yield, it is also a more expensive process because of the requirement to burn natural gas or other fuel to warm the crops and also to enhance the ambient CO 2  levels in the greenhouse which can be accomplished by injection of expensive purchased CO 2 , or, utilization and injection of the CO 2  rich flue gas from the combustion of the natural gas fired heat generators. On the other hand, a potential energy source is available from municipal solid waste landfills, which are generally conceded to be an environmental liability and source of atmospheric pollution. 
     Each person in the United States generates about 4.5 pounds of waste per day, or almost one ton per year, most of which is deposited in municipal solid waste landfills. As this land filled waste decomposes, it produces landfill gas. Uncollected landfill gas contributes to the formation of smog and poses an explosion hazard if uncontrolled. Collected and controlled landfill gas is a viable source of energy. Landfill gas is typically a medium Btu gas that has a number of energy applications. The most prevalent use is production of electricity for sale to a local utility via turbines or engines. Other options include direct use of the gas as boiler fuel, production of compressed natural gas for vehicle fuel, or upgrade to pipeline quality gas. 
     On the negative side, landfill gas is harmful to the environment because it is primarily made up of so-called “greenhouse gases” which contribute to global warming. The “greenhouse gases” capture heat from the sun-warmed earth, which would otherwise be rechanneled back into space, contributing to the so-called “greenhouse effect”. The “greenhouse gases” are carbon dioxide, methane, nitrogen oxide, chlorofluorocarbons and ozone. Landfill gas typically contains 30%-55% methane and 25%-45% carbon dioxide and trace amounts of non-methane hydrocarbons (NMO), chloro-hydrocarbons, and reduced sulphur compounds (TRS)/(H 2 S). 
     Regulations already require many landfills to collect their landfill gas emissions. Once the gas is collected, landfill owner/operators have two choices: (1) flare the gas; or (2) produce energy for sale or on-site use. Both options address local air quality and safety concerns, but only energy recovery capitalizes on the energy value of landfill gas and displaces the use of fossil fuels. 
     Various proposals have been made for utilization of landfill gas. A process for producing essentially pure carbon dioxide from landfill gas is described in U.S. Pat. No. 5,059,405 issued Oct. 22, 1991 to Watson et al. This patent describes a process and apparatus for removing the impurities from a gas stream produced from a landfill such that essentially pure carbon dioxide and methane is recovered. After the landfill gas is mechanically dewatered, the gas is filtered of particulate solids and aerosols and purified by removing sulfur compounds using zinc oxide columns, removing halogens using activated alumina columns, removing hydrocarbons using activated charcoal columns, and oxidizing remaining impurities using potassium permanganate impregnated activated alumina columns. Lastly the gas is incinerated in a boiler/incinerator combustion furnace to produce an exit stream containing essentially pure carbon dioxide and air, which is further treated in a conventional carbon dioxide treatment process. 
     The CO 2  stream resulting from the process is compressed for storage in a liquified condition in a storage tank. According to the patent, the starting landfill gas stream typically contains 40%-60% CH 4 , 30%-45% CO 2 , 1%-3% O 2 , 3% to 10% N 2 , by volume and various trace impurities. 
     Another process for utilizing landfill gas is described in U.S. Pat. No. 5,451,249 issued Sep. 19, 1995 to Spiegel et al. The gas stream which emanates from landfills is treated to produce a purified gas which is essentially a 20 hydrocarbon such as methane which can be used as the fuel source in a fuel cell power plant, or can be used in other power plants which use natural gas as a fuel. The landfill gas passes through a system which removed essentially all of the hydrogen sulfide; water; organic sulfur and halogen compounds; and solid contaminants from the gas stream. The resultant purified gas stream can be cleanly flared; used to power an energy plant; or put to other useful purposes. The resulting fuel is used in a fuel cell operated on landfill gas at a landfill site. 
     Where landfill gas is emitted into the atmosphere without recovery and use, the carbon dioxide and methane have a global warming potential. Furthermore some of the non-methane constituents of landfill gas such as hydrogen sulfide (H 2 S), are odoriferous and also potentially harmful to the environment. In the fuel cell process, moisture and volatile organic compounds (VOCs) including sulfides and halogenated compounds are removed. Dual clean up trains are employed in parallel absobers, so that when one train is in service cleaning the gas, the other is being regenerated with a portion of the cleaned gas. 
     In the foregoing patented systems, the total flow stream of landfill gas is processed, either to produce pure CO 2  for industrial purposes, or to produce a purified hydrocarbon gas to be used as a fuel source in a fuel cell or power plant. 
     It would be desirable to integrate the needs of an environmentally controlled greenhouse for growing plants so as to utilize the environmentally damaging landfill gas emanating from a municipal solid waste landfill in a manner which is both economically beneficial and environmentally safe, so as to increase the yield of growing plants while reducing the emission of potentially polluting gas to the atmosphere. 
     Accordingly, the primary object of the invention is to provide an improved method and apparatus for utilizing landfill gas to enhance plant growth in a greenhouse. 
     SUMMARY OF THE INVENTION 
     Briefly stated the invention comprises a method and apparatus for an environmentally controlled greenhouse located near a landfill site, comprising collecting landfill gas (LFG) from the landfill, transporting the landfill gas, distributing a first LFG portion of the landfill gas to a combustion system, heating a heat exchange fluid with the products of combustion, and controlling the temperature of growing plants in the greenhouse with the heat exchange fluid, distributing a second LFG portion of the landfill gas to a CO 2  system, where the non-methane hydrocarbons (NMO) and reduced sulphur compounds (TRS) are adsorbed from the LFG prior to a controlled combustion which removes methane and generates a clean CO 2  system product gas (CSPG), conducting the CSPG to the greenhouse enclosure, and selectively controlling the temperature and the carbon dioxide level in the greenhouse. In a preferred embodiment, a portion of the CO 2  system product is mixed with air and used to regenerate dual-connected adsorber beds used to remove the NMO&#39;s from the second LFG portion. A modified system interposes a turbine generator to recover additional energy from the combustion of the first LFG portion and uses steam extracted from the turbine to heat the greenhouse. 
    
    
     DRAWING 
     FIG. 1 is a simplified schematic drawing of the landfill and greenhouse system, and 
     FIG. 2 is a schematic drawing of portions of a modified system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 of the drawing, a municipal solid waste landfill  10  is located near an environmentally controlled greenhouse  12 , the latter being serviced by one or more heating systems  14 . Landfill  10  typically will have enough LFG generation to meet the peak winter thermal load of the greenhouse, and includes a landfill gas collection header  18  supplied by lateral collection feed lines  18   a ,  18   b  etc. distributed throughout the landfill. 
     Greenhouse  12  is a completely enclosed structure with special glass panes  20  to admit the sunlight and air via electrically controlled vents  22 . Food crop plants  24 , such as tomatoes, are arranged in hydroponic trays  26  carrying water, nutrients and having frames supporting the plants in rock wool. The greenhouse temperature is maintained by means of a heat exchanger  28  supplied by a hot water circulation system. Temperature sensors  30  detect the temperature of both the hot water piping and the ambient temperature in the greenhouse. 
     An injection system  32  is supplied by means of a CO 2  system to be described, and the CO 2  level inside greenhouse  12  is sensed and measured by a CO 2  sensor  34 . The heating system  14  includes one or more steam or hot water boilers  36  heated by a combustion burner  38 , utilizing fuel supplied through a line  40  and supplied with combustion air by a blower  42 . The waste combustion products are emitted through stack(s)  44 . Boiler  36  supplies heat exchange fluid, hot water in this case, via supply line  46  to greenhouse heat exchanger  28  and then to return line  48 . A pump  49  circulates the heated water to regulate the temperature in response to appropriate control signals. 
     Referring now to the collection and distribution of landfill gas (LFG), gas from the header  18  is conducted through a low pressure supply line  50  to a pair of gas compressors  52 ,  54  which compress the LFG to a pressure as may be required to transport the LFG to the greenhouse site and provide the pressure at the boiler plant as may be required for proper burner operation—typically 3-20 psig. From the compressors  52 ,  54 , the LFG flows to an electronically controlled flow distribution valve  56 . A first portion of LFG exits distribution valve  56  to the fuel line  40  supplying burner  38  in the boiler power plant  14 . A second portion of the LFG exits the distribution valve  56  to a line  58  which supplies a carbon dioxide system shown within the box indicated by dashed line  60 . 
     CO 2  system  60  includes an adsorption filter  62 , consisting of activated carbon with a replaceable insert for removing hydrogen sulfide (H 2 S), a pair of parallel-connected adsorbers  64 ,  66  to absorb halogen and long chain hydrocarbons, and a thermal oxidizer (or controlled combustion system)  68  for removal of residual combustible hydrocarbons. The treated CO 2  system product gas (CSPG) exits CO 2  system  60  via a line  70 , is cooled in a heat exchanger  72  and conducted to the previously mentioned CO 2  distribution line  32  inside the greenhouse. 
     The details of the CO 2  system  60  may vary according to the make up of the landfill gas and according to the degree of CO 2  treatment desired. The landfill gas typically contains 30-55% methane, 20-45% carbon dioxide, and the balance of other gases, some of which are harmful to plant growth. The CO 2  system  60  is designed to provide CO 2  system product gas (CSPG) which is enriched with CO 2  to enhance plant growth, and from which the methane and other gases harmful to plant growth have been removed. 
     In the system shown in FIG. 1, the parallel connected adsorbers  64 ,  66  are regenerated in alternate cycles by the system described as follows. Regenerating gas as required is drawn from CSPG line  70  through regenerator supply line  74 , as controlled by valve  76 , mixed with ambient air entering at  78  and transported via fan  80 . Adsorber  66  has admission valves  66   a ,  66   b , exit valves  66   c ,  66   d . Similarly adsorber  64  has valves  64   a - 64   d . When the valves are in the position shown in the drawing, with valves  64   a ,  64   c ,  66   b ,  66   d  closed, and valves  64   b ,  64   d ,  66   a ,  66   c  open, the regenerating gas, which is primarily air, flows through adsorber  66  to regenerate the adsorber. The regenerating gas exits via a line  82 , whence it joins with combustion air supplying burner  38 . Alternatively, the spent regenerating gas may be drawn off through a line  84  to support combustion in an on-site flare  86 . The flare combustion fuel is LFG supplied to flare  86  via a line  88  connected to the compressed LFG supply line  58 . 
     While adsorber  66  is being regenerated, the primary flow of the second LFG portion from the distribution valve  56  is flowing through adsorber  64  where most of the non-methane hydrocarbons (NMO) are removed. The residual hydrocarbon is oxidized in thermal oxidizer  68 . 
     Lastly, a control system  90  is illustrated which provides output signals over lines  92 ,  94  to increase or decrease the total LFG flow, and over a control line  96  to distribution valve  56  to vary the distribution of LFG between the burner  38  in the boiler plant  14  and the carbon dioxide system  60  supplying CSPG to the greenhouse. Control system  90  controls the heat energy supplied to the plants via a control signal line  98  connected to the heat exchange pump  49  and also controls the ventilation and air temperature of the greenhouse through a control signal line  100  to the electrically controlled ventilation flaps. Inputs to the control system  90  include a temperature signal over line  102  from the temperature sensor  30  and CO 2  level signal over a line  104  from the CO 2  sensor  34 . Many other input signals may be provided to control system  90  to monitor the environmental conditions in the greenhouse, the status of the CO 2  system and the LFG supply, the sensors shown being only exemplary. 
     Referring to FIG. 2 of the drawing, a modified system interposes a turbine/generator to produce electricity and utilize energy present in any excess landfill gas not needed in the greenhouse plant growth process. FIG. 2 omits the parts of the FIG. 1 system which are the same as previously described and repeats the same reference numbers when the elements are identical to those in FIG.  1 . 
     Instead of generating hot water in heating plant  14 , a steam generator  36   a  is fired by the combustion burner  38  to generate steam, using a first LFG portion of the landfill gas. The second LFG portion is treated in the CO 2  system and supplied to the greenhouse as before. 
     Steam from steam boiler  36   a  is conducted via line  110  to steam turbine  112  coupled to generator  114 . Exhaust steam is lead to an air-cooled condenser  116  through a control valve  118 . A part of the exhaust steam from turbine  112  is extracted through an extraction control valve  120  and fed through a steam-hot water heat exchanger  122  to join with condensate from condenser  116 . The condensate is returned to steam generator  36   a  by pump  124  via condensate return line  126 . 
     Extraction steam flowing through heat exchanger  122  is passed in heat exchange relationship with a hot water heating coil  126 . This is fed by line  48  and used to supply the greenhouse hot water heating line  46  as before. 
     OPERATION 
     The system is operated so as to maximize the growing cycle of plants in the greenhouse and to minimize the emissions of harmful gas to the atmosphere during conditions of favorable growth, i.e., in the sunlight during the daytime (or alternatively under artificial lighting at night). The CO 2  level in the greenhouse is controlled to be as high as possible but not to exceed 1,000 ppm for safety reasons. However, it is recognized that some plants, such as tomatoes, require resting periods when photosynthesis is not taking place and at such times, the generation of carbon dioxide is not required. Typically at such times, which are during night time, or on sunless days, the ambient temperature is cooler and more heat is required from the boiler system to keep the plants warm. 
     By suitable operation of the control system  90 , the flow rate of LFG supplied to the overall system and the distribution of LFG between that used to heat the plants and that used to supply CSPG to the plants can be operated to achieve the most economical operation. 
     The regeneration of the adsorbers takes place in the following manner. The landfill gas (LFG) to the CO 2  system  60  is processed prior to the thermal oxidizer  68 . The reduced sulfur compounds (TRS) are adsorbed in replaceable reactors  62 . The non-methane hydrocarbons (NMO) are adsorbed over the period of operation of the CO 2  system—typically 8-10 hours during daylight hours. There will be two trains of NMO adsorption—one train in operation for CO 2  generation with the second train being regenerated. The regeneration medium will be essentially air entering at  78 . The regeneration cycle will be approximately 8 hours, the initial 1-2 hours for heat-up (to 500-600° F.), the third and fourth hours for driving off the adsorbed NMOs, the last four hours for cooling with unheated air. It is anticipated that the regenerated gas will not have any NMO during the first and last four hours of the cycle. The average NMO (VOC) emissions over the 8 hour cycle will be about the same as for the hot water boilers. The regenerating gas (essentially air) will normally be routed to the inlet air of operating hot water boiler plant  14 . It is anticipated that the boilers will be in operation during the day for 7-8 months. For the late spring and summer months (May through September) this regenerating gas will be re-routed to the on-site enclosed type combustor/flare  86 . 
     EXEMPLARY SYSTEM 
     A system comprising a 20 acre hydroponic greenhouse, consisting of two ten-acre glass greenhouse banks connected by a packing house is proposed near a landfill operation to raise beefsteak tomatoes. To support 20 acres of growing tomatoes, the landfill should be able to supply sufficient LFG for the peak heat load requirement. LFG produced by the landfill and typically flared, will be burned to produce hot water to heat the greenhouse and produce carbon dioxide system product gas that will be recycled back to the greenhouse plants to accelerate growth. Rain water will be collected and recycled. 
     The greenhouse is a hydroponic facility with computers controlling many aspects of the glass enclosed growing environment. Temperature, feeding and watering will be monitored and controlled constantly, with harmful pests controlled by the introduction of natural predators. The 200,000 tomato plants will produce approximately 50 pounds of tomatoes each, or about 10 million pounds per year. The landfill/greenhouse combination is located in an area favorable (i.e., temperature, humidity, sunlight, etc.) to greenhouse operation utilizing heating of the greenhouse with a first portion of the LFG and enhancing tomato plant growth with a second portion of the LFG after removal and/or burn-out of methane and other gases potentially harmful to plant growth. The system is designed for a hot water boiler plant  14  capable of 90 million BTU/H. The flow of CO 2  system product gas (CSPG) to the greenhouse is capable of 10,000 cubic feet per minute (cfm) with impurities of SOX&lt;1 ppm, NOX&lt;15 ppm, CO&lt;5 ppm and VOC&lt;1 ppm. The CO 2  content of the CSPG is approximately 5.5% CO 2 , but can range between 5-13% CO 2 . 
     As an alternative to using flare  86  for unused LFG, it is of course possible to maximize on-stream utilization of the LFG via steam generators/steam turbines as described in FIG. 2 or other energy utilization devices such as engines. 
     The system may be further optimized by configuring the thermal oxidizer  68  as a hot water heater to further supplement the heat supplied to greenhouse. 
     While there is described what is considered to be the preferred embodiment of the invention, other modifications will occur to those skilled in the art without departing from the inventive concept.