Patent Description:
Biogas can be produced by anaerobic digestion (AD) or fermentation of biodegradable materials, such as manure, wastewater and sewage sludge, municipal waste, green waste, plant material, and crops. Biogas consists mainly of methane (CH<NUM>) and carbon dioxide (CO<NUM>), and small amounts of hydrogen sulphide (H<NUM>S), moisture, and siloxanes.

If the anaerobically produced biogas is to be used as a transportation fuel (vehicle grade), it first has to be upgraded to remove impurities and increase its calorific value (heating value). This upgrading step includes drying, desulfurization, and CO<NUM>-removal. The CO<NUM>-separation is normally carried out by water scrubbing, physical or chemical absorption using organic solvents, pressure swing adsorption, or by permeation using membranes, depending on the location and size of the system.

In a presentation made at <NPL>), as also documented in <NPL>, a plant for so-called bio-ZEG methane was presented and discussed, including the use of solid oxide fuel cells and use of CaO as a CO<NUM> scavenger. There was, however, no mention of using raw CO<NUM> containing biogas as a source feed for such a process.

<CIT> teaches an apparatus for producing high-purity gas which includes a column configured for sorption-enhanced reaction (SE-SMR) for removing a by-product through a catalyst reaction. The column is divided into a plurality of sections, the sections having decreasing proportions of catalyst and increasing proportions of an absorbent.

<CIT> and <CIT> are other publications in this technical field. <CIT> discloses a process for hydrogen production by utilizing the reaction of calcium oxide to absorb carbon dioxide and reinforce continuous reforming of methane and water vapor. The starting gas may be a biogas obtained by anaerobic digestion.

However, one of the main drawbacks and challenges with biogas is the requirement for upgrading to bio-methane, with a quality for use as fuel for (bio) gas vehicles, or as a source for hydrogen fuel cell electric vehicles (FCEVs). Biogas from food waste (or other sources such as manure and wastewater) treatment facilities typically consist of <NUM>-<NUM>% CH<NUM> and <NUM>-<NUM>% CO<NUM>. Upgrading (CO<NUM> removal) consumes energy and adds significant costs to the overall system (Luo and Angelidaki, <NUM>). Hence, there is a need to find new, more efficient, and less costly methods for upgrading anaerobically produced biogas for example used directly as fuel in vehicles, and as a source for hydrogen production used in hydrogen fuel cell electric vehicles.

There is still, however, a need for further improvements in this area to make fuel produced by digestion of organic waste competitive as vehicle fuel.

This can to some extent be achieved by reduction of the CO<NUM> content and enhancement of the methane content, by adding hydrogen to the anaerobic digestion (AD) process, thereby increasing the methane content to about <NUM>%.

The main objective of the present invention is thus to develop a new and cost-efficient process that allows production of vehicle grade fuels based on anaerobic digestion of wet organic substrates, with CO<NUM> capture or no negative climate consequence.

The above mentioned objects are achieved by the method according to the invention as defined by claim <NUM>.

According to another aspect the invention concerns a device for performing the method as defined by claim <NUM>.

Preferred embodiments are disclosed by dependent claims.

By "raw biogas" as discussed herein is understood a biogas from which Sulfur have been removed but in which the content of CO<NUM> is as originated from the anaerobic digester reactor, contrary to upgraded biogas which is essentially pure methane.

Addition of H<NUM> to the digestion process increases the ratio between bio-methane and CO<NUM>, with CO<NUM> content potentially lower than <NUM>%.

A main feature of the present invention is the conversion of the raw biogas with enhanced methane: CO<NUM> ratio directly in a sorption enhanced reforming (SER) process without prior separation of CO<NUM>. This is achieved by dimensioning the reformer <NUM> to capture both CO<NUM> from the initial desulfurized raw biogas (CH<NUM> + CO<NUM>) and the CO<NUM> formed in the reforming step (SE-SMR process).

Ca-looping has, to our knowledge, not been suggested as a method for CO<NUM> removal in relation to production of hydrogen from raw biogas or gases with considerable amounts of initial CO<NUM> in addition to methane.

The total SER-process with desulfurized raw biogas as feed gas is illustrated in the chemical reaction (not balanced) given below;.

CH<NUM> + CO<NUM> +H<NUM>O + CaO = CaCO<NUM> + H<NUM>.

Two alternative embodiments of the present invention are illustrated below, namely (<NUM>) Biological conversion of CO<NUM> to CH<NUM> by the addition of H<NUM> from an SER-process in an AD reactor and (<NUM>) Direct conversion of desulfurized raw biogas (CH<NUM> + CO<NUM>) to hydrogen in a SER reactor.

As illustrated and exemplified below the present invention provides in one embodiment a combined system for production of vehicle grade biomethane, and vehicle grade hydrogen, with the option of total CO<NUM> capture, from anaerobic digestion of organic waste.

Cost efficiency and sustainability are keyword and common denominators for the overall process.

Different embodiments of the invention are illustrated below with reference to the enclosed drawings, where:.

Attention is drawn to <FIG>. An anaerobic digester <NUM> for biogas production is charged with a charge material <NUM> based on sewage sludge, domestic organic waste, animal or agricultural waste.

The produced raw biogas <NUM> is desulfurized <NUM>, and the desulfurized, still raw biogas <NUM> (CH<NUM> + initial CO<NUM>) is charged to a reformer <NUM> together with water <NUM> in the form of steam for a sorption enhanced reforming (SER) process. A substantially pure hydrogen <NUM> gas leaves the reformer unit. In the reformer unit, one part 15b of which typically being returned to the digester <NUM> to enhance the methane yield while another and typically larger part 15a is subjected to purification in a hydrogen-purifier <NUM>. The hydrogen purification may typically be performed as a pressure swing adsorption process. The purified hydrogen <NUM> leaving the purifier is of vehicle grade.

While the step of desulfurization <NUM> is a step commonly used in such processes and not inventive as such, it is a step which for practical chemical purposes will rarely or never be omitted.

The process of reforming and CO<NUM> capture in the reformer <NUM> involves a reaction between fuel (CH<NUM>), water (steam), CO<NUM> (both from the original biogas and from the SER process) and CaO as a CO<NUM> absorber, a process in which CaO is converted to CaCO<NUM> in an exothermic reaction known per se.

The off-gas <NUM> from the hydrogen purification unit <NUM> is mixed with raw biogas 13c, which may be a partial flow of biogas flow <NUM>, and charged to a burner <NUM> for production of the necessary heat (<NUM> to <NUM>) for regeneration of CaCO<NUM> to CaO in a CaO regenerator <NUM>, in an endothermic process. The CO<NUM> (<NUM>%) flow <NUM> produced in the reformer <NUM> and released in regenerator <NUM>, may be used or stored (sequestration). The burner <NUM> is also charged with an oxygen containing gas <NUM>, typically air.

The CO<NUM> in the exhaust from the burner <NUM> would have no climatic consequence since the fuel source is of biogenic origin. In addition, the CO<NUM> flow captured as flow <NUM> has a "negative" CO<NUM> climate impact, if this flow of CO<NUM> is stored or used.

Flow <NUM> is a flow from the reformer <NUM> to the regenerator <NUM> of solid CaCO<NUM>, resulting from CaO having absorbed CO<NUM>, while flow <NUM> is a flow of solid CaO, converted back from CaCO<NUM>, from the regenerator <NUM> back to the reformer <NUM>. This Ca-looping process is well known as such, but not in the context here presented.

Attention is now directed to <FIG>. Most of the components and flows of <FIG> are the same as the ones in <FIG> and are numbered equally. The process according to <FIG>, however, has the additional ability of producing vehicle grade biogas. A first part 13a of the desulfurized biogas <NUM> is charged to the reformer <NUM> like in <FIG> and treated accordingly. A second part of the biogas 13b from the desulfurization unit <NUM> is charged to a CO<NUM> separation unit <NUM>. The CO<NUM>-separation is normally carried out by water scrubbing, physical or chemical absorption using organic solvents, pressure swing adsorption, or by permeation using membranes, depending on the location and size of the system. The bio methane <NUM> discharged from CO<NUM> separation unit <NUM> may be said to be of natural gas quality or of vehicle grade and hence used for such purposes.

The CO<NUM> <NUM> released from the CO<NUM> separation unit <NUM> may be stored or used, if the method applied makes this economically feasible. This is however usually not the case. Regardless of the method used, the biogenic origin of the fuel source would result in no climatic consequence. The purity of the CO<NUM> <NUM> released from the CO<NUM> separation unit <NUM> depends on the type and nature of this unit.

Attention is now directed to <FIG>. Most of the components of <FIG> are the same as the ones in <FIG> and are equally numbered. The process according to <FIG>, however, has the additional ability of producing electricity and high temperature heat due to the presence of a solid oxide fuel cell (SOFC) integrated in the process and equipment. Thus, the substantially pure hydrogen discharged from the reformer, is typically divided into three substreams, namely substream 15a which is charged to the SOFC, substream 15b which is recycled to the digester <NUM> (like in <FIG>) and substream 15c which, when present, is used as a fuel for a heater <NUM>.

The hydrogen substream 15a is partially used to produce electricity in the SOFC while another part of the hydrogen flow <NUM>' leaves the SOFC for further upgrading in a hydrogen purifier <NUM>' which may or may not be similar to the unit <NUM> shown in <FIG> to obtain vehicle grade hydrogen <NUM>. The electricity may be used internally or externally or both.

The high temperature exhaust gas of the SOFC is used to heat the regenerator <NUM>, but may typically need some assistance since the temperature needed in the regenerator <NUM> is <NUM> to <NUM>. This temperature may be reached (without any assistance) if ceramic interconnects are used in the SOFC system.

In practice, however, the temperature of the exhaust gas (<NUM>, Megel et. al <NUM>) is too low to effectively provide a temperature in the regenerator at which the CaCO<NUM> is converted to CaO for further use. A dedicated system, to elevate the temperature of the exhaust gas in a temperature increasing cell/heating device, would thus be necessary.

The heat integration between the SOFC <NUM> and the Regenerator <NUM>, via the heater <NUM>, is in <FIG> provided by a closed heat loop <NUM>', <NUM>', however other options are possible.

The heat transfers medium of the heat loop <NUM>', <NUM>' in <FIG>, can be different gases, such as for example; Hydrogen, CO<NUM>, air, helium, water vapor, different gas mixtures or fluids such as; mineral oils, hydrocarbons and different types of molten salts. The heat of the heat loop in <FIG>, leaving the SOFC system, is typically about <NUM>. The heat of this heat loop is enhanced in the heater <NUM> to at least <NUM>, more preferable at least <NUM> and most preferred at least <NUM>, in order to meet the temperature regeneration requirement in the regenerator <NUM> where CaCO<NUM> is converted to CaO while releasing CO<NUM>. While the heater <NUM> may be heated in different ways, one convenient way is the use of a partial flow of hydrogen 15c from the reformer. Another option for fuel to <NUM> would be raw biogas 13c. Whether the fuel for the heater has the form of raw biogas 13c according to <FIG>, or <FIG> or hydrogen 15c according to <FIG>, the heat delivered by the burner <NUM> or by the heater <NUM> is at least partially provided, directly or indirectly, by the desulfurized raw bio-gas to be upgraded. The SOFC exhaust air <NUM> provides the oxygen for the heating process in <NUM>. The CO<NUM> in the exhaust outlet <NUM> from the heating device would be climate neutral because of the biogenic origin of the fuel used. If required, additional air (not shown) may be supplied to the heater <NUM>.

Attention is now directed to <FIG>. Most of the components in <FIG> are the same as in <FIG>. The process of <FIG>, however, has the additional ability of allowing production of vehicle grade biogas, in a manner similar to the difference between <FIG>.

Thus, according to <FIG>, the flow of desulfurized biogas <NUM> is split into a first flow 13a which is charged to the reformer <NUM> while the other flow 13b is charged to a CO<NUM> separation unit <NUM> to be separated into vehicle grade biomethane <NUM> and CO<NUM> <NUM>. The treatment of the first flow 13a is as described above in relation to <FIG>, while the other flow 13b is subjected to a treatment as generally described in relation to <FIG>.

With regard to the SOFC and the processes involved therein, there is no difference between the embodiments of <FIG>. The same is to be said about the closed loop heat exchange <NUM>', <NUM>' including the use of the heater <NUM>.

It is to be understood that the processes according to <FIG> and <FIG> allows a split of the desulfurized raw biogas into a first flow 13a and a second flow 13b, covering a ratio between the two from <NUM>:<NUM> to <NUM>:<NUM>. Thus, if demand is high for vehicle grade biomethane and low for vehicle grade hydrogen and electricity, the flow 13a would be reduced so as to basically just cover the need for hydrogen in the digester. On the other hand, if the demand for vehicle grade biomethane is low, the flow 13b could basically be cut off, rendering the embodiment of <FIG> temporarily identical to the embodiment of <FIG> and/ or rendering the embodiment of <FIG> temporarily identical to the embodiment of <FIG>.

The general concept of the present invention is a method for the manufacture of vehicle grade fuels from biological materials in a cost-efficient and sustainable manner, involving a minimum of steps. There is a versatility in the method in the sense that vehicle grade biomethane and vehicle grade hydrogen may be produced at a flexible mutual ratio, as well as flexible amounts of electricity.

While not representing the core of the present invention, a step of desulfurization <NUM> is typically conducted upstream of the step of sorption enhanced reforming <NUM>.

As explained in relation to the drawings, a partial flow of desulfurized biogas <NUM> is according some embodiments subjected to treatment in a CO<NUM> separation unit <NUM> thereby providing one discharge flow of vehicle grade biomethane <NUM> and one discharge flow <NUM> containing CO<NUM>.

The CO<NUM> separation unit <NUM> is typically one using a principle for separation selected among water scrubbing, physical or chemical absorption using organic solvents, pressure swing adsorption, and permeation using membranes.

According to at least some embodiments the heat required for regenerating CaO is provided by burning a gas containing a partial flow of desulfurized biogas. In some embodiments heat for regeneration of CaO may also be provided in part from a solid oxide fuel cell <NUM> charged with hydrogen 15a from the sorption enhanced reforming step <NUM>. Additional heat may in case be provided by a heater <NUM> charged with hydrogen 15c discharged from the sorption enhanced reforming step <NUM>. In other embodiments the fuel cell <NUM> may be charged with raw biogas or a combination of hydrogen and raw biogas.

In some embodiments a heat medium <NUM>', <NUM>' is circulated in a closed loop between at least the solid oxide fuel cell <NUM>, the heater <NUM> and the regenerator <NUM>. The addition of hydrogen to the digestion process may be arranged at least in part as a recycle hydrogen flow 15b from the reforming step <NUM>.

Claim 1:
Method for upgrading biogas from anaerobic fermentation of biological material in a digester (<NUM>) for production of hydrogen (<NUM>, <NUM>) and optionally methane (<NUM>), comprising addition of hydrogen gas to the fermentation step (<NUM>) to enhance the methane:CO<NUM> ratio in the raw biogas (<NUM>) produced and subjecting the raw biogas to a step of desulfurization to the extent required characterized in further comprising subjecting at least a part of the desulfurized raw biogas to a step of sorption enhanced reforming (<NUM>) without prior separation of CO2 using CaO as an absorbent to capture CO<NUM> from the raw biogas as well as CO<NUM> released in the reforming reaction, regenerating CaO in an endothermic reaction (<NUM>) using heat at least partially provided, directly or indirectly, by the bio-gas to be upgraded, thereby producing substantially pure hydrogen (<NUM>) and substantially pure CO<NUM> (<NUM>).