Multi-reaction process for forming a product gas from solid carbonaceous material

Abstract

A direct carbonaceous material to power generation system integrates one or more solid oxide fuel cells (SOFC) into a fluidized bed gasifier. The fuel cell anode is in direct contact with bed material so that the H.sub.2 and CO generated in the bed are oxidized to H.sub.2O and CO.sub.2 to create a push-pull or source-sink reaction environment. The SOFC is exothermic and supplies heat within a reaction chamber of the gasifier where the fluidized bed conducts an endothermic reaction. The products from the anode are the reactants for the reformer and vice versa. A lower bed in the reaction chamber may comprise engineered multi-function material which may incorporate one or more catalysts and reactant adsorbent sites to facilitate excellent heat and mass transfer and fluidization dynamics in fluidized beds. The catalyst is capable of cracking tars and reforming hydrocarbons.

Claims

1. A method of producing a final product gas from shredded and dried biomass, comprising: (a) introducing shredded and dried biomass into a first stage gasifier having a first fluidized bed, to produce a first stage product gas comprising H.sub.2O, H.sub.2, CO, CO.sub.2 and char; (b) after step (a), introducing at least a portion of the first stage product gas to a first cyclone to separate fine char therefrom; (c) after step (b), introducing the fine char separated from the first stage product gas to a second stage gasifier having a second fluidized bed, to produce a second stage product gas; (d) after step (c), introducing at least a portion of the second stage product gas to a second cyclone to separate ash therefrom; (e) after step (d), combining the first stage product gas from which the char was separated, with the second stage product gas from which ash has been separated, to produce a final product gas; (f) after step (e), introducing the final product gas to a heat recovery steam generator (HRSG) to produce steam; (g) after step (f), removing particulates from the final product gas with a venturi scrubber; (h) after step (g), cooling the final product gas to condense excess steam; and (i) after step (h), capturing CO.sub.2 present in the final product gas; wherein: step (a) comprises operating during startup, pulse combustion heaters which protrude into the first stage gasifier; the first fluidized bed is fluidized with superheated steam, CO2 and 02, and is maintained at a temperature ranging from 600 C. to 1,000 C.; and the second fluidized bed is fluidized with O.sub.2 produced by a VPSA oxygen separation system.

2. The method according to claim 1, wherein: step (a) comprises transferring shredded and dried biomass at atmospheric pressure into the first stage gasifier operating at a pressure between 1 and 15 bar gauge.

3. The method according to claim 2, further comprising: maintaining the temperature of the first fluidized bed with plurality of indirect heater conduits protruding into the first fluidized bed.

4. The method according to claim 3, wherein: the plurality of indirect heater conduits includes pulse combustion heaters.

5. The method according to claim 4, wherein: the pulse combustion heaters are driven by at least a portion of the final product gas.

6. The method according to claim 4, further comprising: providing a plurality of fuel cell elements which protrude into the first fluidized bed within the first stage gasifier.

7. The method according to claim 3, wherein: the plurality of indirect heater conduits includes electrical heaters.

8. The method according to claim 1, wherein step (a) comprises: (a1) introducing, into a drying and devolatilization zone of the first fluidized bed, the shredded and dried biomass and converting the shredded and dried biomass into H.sub.2O, H.sub.2, CO, CO.sub.2 and char, the char comprising carbonaceous material; and (a2) simultaneously, endothermically reacting: (i) superheated steam with some of the carbonaceous material to produce additional H.sub.2 and additional CO; and (ii) CO.sub.2 with some of the carbonaceous material, to produce additional CO.

9. The method according to claim 1, wherein: the shredded and dried biomass has a top size of 1 inch and a moisture content of 10%.

10. The method according to claim 1, wherein the pulse combustion heaters do not play a role in sustaining gasification during steady state operations.

11. A method of producing a final product gas from shredded and dried biomass, comprising: (a) introducing shredded and dried biomass to a first stage gasifier having a first fluidized bed, to produce a first stage product gas comprising H.sub.2O, H.sub.2, CO, CO.sub.2 and char; (b) after step (a), introducing at least a portion of the first stage product gas to a first cyclone to separate fine char therefrom; (c) after step (b), introducing the fine char separated from the first stage product gas to a second stage gasifier having a second fluidized bed, to produce a second stage product gas; (d) after step (c), introducing at least a portion of the second stage product gas to a second cyclone to separate ash therefrom; (e) after step (d), combining the first stage product gas from which the char was separated, with the second stage product gas from which ash has been separated, to produce a final product gas; wherein: step (a) comprises operating during startup, pulse combustion heaters which protrude into the first stage gasifier; the first fluidized bed is fluidized with superheated steam, CO.sub.2 and O.sub.2, and is maintained at a temperature ranging from 600 C. to 1,000 C.; and the second fluidized bed is fluidized with O.sub.2.

12. The method according to claim 11, further comprising: maintaining the temperature of the first fluidized bed with plurality of indirect heater conduits protruding into the first fluidized bed.

13. The method according to claim 12, wherein: the plurality of indirect heater conduits includes pulse combustion heaters.

14. The method according to claim 13, wherein: the pulse combustion heaters are driven by at least a portion of the final product gas.

15. The method according to claim 13, further comprising: providing a plurality of fuel cell elements which protrude into the first fluidized bed within the first stage gasifier.

16. The method according to claim 12, wherein: the plurality of indirect heater conduits includes electrical heaters.

17. The method according to claim 11, wherein step (a) comprises: (a1) introducing, into a drying and devolatilization zone of the first fluidized bed, the shredded and dried biomass and converting the shredded and dried biomass into H.sub.2O, H.sub.2, CO, CO.sub.2 and char, the char comprising carbonaceous material; and (a2) simultaneously, endothermically reacting: (i) superheated steam with some of the carbonaceous material to produce additional H.sub.2 and additional CO; and (ii) CO.sub.2 with some of the carbonaceous material, to produce additional CO.

18. The method according to claim 11, further comprising; (g) after step (f), removing particulates from the final product gas with a venturi scrubber; (h) after step (g), cooling the final product gas to condense excess steam; and (i) after step (h), capturing CO.sub.2 present in the final product gas.

19. The method according to claim 11, wherein: the shredded and dried biomass has a top size of 1 inch and a moisture content of 10%.

20. The method according to claim 11, wherein: the second fluidized bed is fluidized with O.sub.2 produced by a VPSA oxygen separation system.

21. The method according to claim 11, wherein the pulse combustion heaters do not play a role in sustaining gasification during steady state operations.

22. A method of producing a final product gas from shredded and dried biomass, comprising: (a) introducing shredded and dried biomass to a first stage gasifier having a first fluidized bed, to produce a first stage product gas comprising H.sub.2O, H.sub.2, CO, CO.sub.2 and char; (b) after step (a), introducing at least a portion of the first stage product gas to a first cyclone to separate fine char therefrom; (c) after step (b), introducing the fine char separated from the first stage product gas to a second stage gasifier having a second fluidized bed, to produce a second stage product gas; (d) after step (c), introducing at least a portion of the second stage product gas to a second cyclone to separate ash therefrom; and (e) after step (d), combining the first stage product gas from which the char was separated, with the second stage product gas from which ash has been separated, to produce a final product gas; wherein: the first fluidized bed is fluidized with superheated steam and CO.sub.2 and is maintained at a temperature ranging from 600 C. to 1,000 C.; the second fluidized bed is fluidized with O.sub.2; step (a) comprises operating during startup, pulse combustion heaters which protrude into the first stage gasifier; and step (a) further comprises: (a1) introducing, into a drying and devolatilization zone of the first fluidized bed, the shredded and dried biomass and converting the shredded and dried biomass into H.sub.2O, H.sub.2, CO, CO.sub.2 and char, the char comprising carbonaceous material; and (a2) simultaneously, endothermically reacting: (i) superheated steam with some of the carbonaceous material to produce additional H.sub.2 and additional CO; and (ii) CO.sub.2 with some of the carbonaceous material, to produce additional CO.

23. The method according to claim 22, wherein the pulse combustion heaters do not play a role in sustaining gasification during steady state operations.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows an integrated gasifier/fuel cell system in accordance with one embodiment of the present invention.

DETAIL DESCRIPTION

(2) FIG. 1 shows an integrated carbonaceous material to power generation system 100 in accordance with one embodiment of the present invention. The system 100 includes a reaction vessel 105 comprising a gasifier chamber 110 having a longitudinal axis A defining a vertical direction. In some embodiments, the gasifier chamber 110 may be in the form of a steam reformer, of a sort well-known to those skilled in the art.

(3) A distributor 112 is provided at the base 108 of the chamber 110 to facilitate entry of the fluidization medium 109, which may include such items as steam, air, CO.sub.2, O.sub.2, etc. Above the distributor, the gasifier 110 has a plurality of zones including an enhanced char conversion zone 114 proximate the chamber base, a drying and devolatilization zone 116 above the enhanced char conversion zone 114, and a dense fluid bed zone 118 above the drying and devolatilization zone 116. Above the dense fluid bed zone 118 is a freeboard area 120 which occupies the upper portion of the chamber 110. The top of the chamber 110 may be configured to permit the exit of product gas 111. One or more cyclones, filters and other equipment (not shown) may be used to separate and/or recover solids from the product gas, in a manner known to those having ordinary skill in art.

(4) In one embodiment, the gasifier bed particles comprise engineered multi-function particles which are porous and lightweight, have active reactant (H2, CO, H2O, CO2, etc.) sorption sites, have excellent heat and mass transfer properties, and may be either catalytic or non-catalytic. The engineered particles may be a composite of ceramics such as alumina and solid adsorbent materials such as zeolites or molecular sieves for gases including H2, CO, H2O and CO2. Such particles may be manufactured for example via sol-gel processing. Such engineered particles can help reduce the dense bed pressure drop, gas expansion and gas bypassing, lower the fluidization velocity required and facilitate superior gas-solid contact, heat and mass transfer and promote reactions. The engineered particles enable the operation of the chamber with temperature and concentration gradients that are relatively low to very low, and in turn improve fuel or feedstock conversion efficiency. The function of the catalyst is to crack tars and reform hydrocarbons, and catalysts such as those described in the Background section may be suitable. Furthermore, specially formulated sorbents (once through or regenerable) may be injected into the fluid bed to capture contaminants.

(5) A feedstock inlet 148 is provided in a sidewall 126 of the reaction vessel 105 for introducing feedstock 150 into the drying and devolatilization zone 116 of the chamber 110. While only a single feedstock inlet 148 is shown, it is understood that a plurality of such inlets may be provided, each inlet for introducing feedstock. The feedstock 150 is introduced into the drying and devolatilization zone 116, combined with the fluidization medium and is conveyed in an upward direction towards the fuel cell elements 134 (described below), which are exothermic and transfer heat to the dense fluid bed zone 118.

(6) One or more spaced apart fuel cell modules 130a, 130b are attached to the reaction vessel 105. Each fuel module comprises a set 132a, 132b of one or more fuel cell elements 134 which protrude into the chamber 110. Each fuel cell module 130a, 130b comprises an oxidant manifold 136 through which fuel cell inlet gases 137 such as one or more of air, O2 and enriched air are supplied to the fuel cell elements 134. The fuel cell module may also include a spent oxidant plenum 138 for the removal of fuel cell exhaust gases 139 such as N.sub.2, leftover O.sub.2 and the like that were not consumed by the fuel cell elements 134. While two such fuel cell modules 130a, 130b are shown in FIG. 1, each outputting power 160, it is understood that a single fuel cell module, or even a collection of individual fuel cell elements, would suffice.

(7) The fundamental building block for a fuel cell comprises an electrolyte layer in contact with a porous anode on one side and a porous cathode on the other side. In the embodiment shown, each fuel cell element 134 is in the form of an elongated tube 134a comprising an outer anode, an inner cathode and a ceramic electrolytic electrode in-between. In the case of the SOFC, the ceramic electrolyte has an oxygen ion void in the lattice that causes the ceramic to become an oxygen ion-conducting electrolyte. In this regard it has similarities to the electron void found in materials used as semi-conductors. The equilibrium potential developed by the cell can be calculated by using the Nernst equation. The actual potential generated, however, is lower than the equilibrium potential due to irreversible losses. The cell voltage and the power density of the fuel cell will vary with pressure, temperature, reactant (fuel and oxidant) type, composition and utilization, current density and impurities and their concentration. The SOFC elements each comprise two electrodes (outer anode and inner cathode) and an electrolyte in between. The anodes are in direct contact with the reformer bed material and the cathodes are supplied with oxidant (air; O2 or enriched airoptional).

(8) The chamber 110 utilizes superheated steam as the main fluidizing medium (CO2, O2 and/or air are optional) and the heat provided by electrochemical oxidation to convert the fuel or feedstock. The injected fuel or feedstock undergoes one or more of the steps of drying, devolatilization, gasification, tar cracking and hydrocarbon reforming reactions to generate a product gas comprising primarily H2, CO and CO2, and to a limited extent hydrocarbon gases (fixed gases such as CH4, C2+ and condensable vapors or tars); depending upon the fuel or feedstock and the operating conditions (pressure, temperature and the fluidization medium), the additional constituents of the product gas may be HCl, H2S, NH3, HCN, COS, and alkali and trace metal vapors. The H2 and CO generated in the fluidized bed are electrochemically oxidized at the anodes of the SOFC (due to oxygen ion transport from the cathodes) to H2O and CO2 to create a push-pull or source-sink reaction environment.

(9) Meanwhile, the SOFC fuel cell elements 134 operate in exothermic mode and supply heat to the chamber 110 which operates in the endothermic mode and thus requires heat. Thus, the products from the anode of the fuel cell elements 134 are the reactants for the reformer and vice versa. Due to fluid bed mode and coupled operation, the SOFC thermal management in some embodiments of the present invention can become simpler and more efficient.

(10) As mentioned above, the configuration shown in FIG. 1 partitions the chamber 110 into distinct reaction zones, including on zone conducive to drying and devolatilization of the feedstock and another promote char reaction and carbon conversion, as disclosed in US Patent Application Pub. No. US 2007/0245627 A1 dated Oct. 25, 2007. This facilitates improvements in syngas yield and composition, carbon conversion and process thermal efficiency and in turn higher throughput for a given chamber. Thus, the chamber operating temperature will nominally be about 800 C. (1,472 F) and may range from 600 C. to 1,000 C., depending on the fuel or feedstock.

(11) Chamber Reactions. The principal groups of reactions in the chamber are:

(12) Drying and Devolatilization
Fuel or feedstock.fwdarw.H2O,H2,CO,CO2,HC Gases,Tars,Char
Catalytic Cracking/Reforming
HC Gases,Tars+H2O+Heat.fwdarw.H2,CO
Gasification
Superheated steam reacts endothermically (consumes heat) with the carbonaceous components of the fuel or feedstock to produce hydrogen and carbon monoxide fuel gases (synthesis gas or syngas):
H2O+C+Heat.fwdarw.H2+CO(steam reforming)
Carbon dioxide reacts endothermically (consumes heat) with the carbonaceous components of the fuel or feedstock to produce carbon monoxide:
CO2+C+Heat.fwdarw.2CO(dry reforming or Boudouard)
Oxidation
Oxygen transported from the cathode to the anode of the SOFC reacts exothermically (releases heat) with carbon monoxide to produce carbon dioxide and with hydrogen to produce water vapor:
H2+O2.fwdarw.H2O+Heat
CO+O2.fwdarw.CO2+Heat
Shift Reaction
Water-gas shift reaction also occurs simultaneously with the steam reforming reaction to yield additional hydrogen and carbon dioxide:
H2O+CO.fwdarw.H2+CO2+Heat(water-gas shift)
Secondary Reactions
CO+3H2.fwdarw.CH4+H2O+Heat(methanation)
C+2H2.fwdarw.CH4+Heat(hydrogasification)
H2O+C+Heat.fwdarw.CH4+CO2(gasification)

(13) Depending upon other elements or contaminants (S, Cl, N, heavy/hazardous metals) present in the fuel or feedstock, the syngas produced may include H2S, COS, HCl, NH3, HCN, and metal and alkali vapors. Also certain minerals (Na, K, Ca, Mg, Si, Fe) if present in sufficient concentration could form low melting compounds or eutectics and in turn lead to agglomeration, clinker formation, fouling of internals and defluidization. In these cases, additives such as kaolinite, diatomaceous earth, emathlite, magnesium oxide etc. may be added to the chamber on a continuous basis to tie up the minerals and facilitate good fluidization characteristics.

(14) While only the vessel 105 and its chamber 110 is shown, the overall system 100 is envisioned to also include one ore more of the following: fuel or feedstock preparation, handling and feed subsystem, heat recovery and cold gas cleanup subsystem, optional CO.sub.2 capture subsystem, optional gas turbine subsystem, optional bottoming steam cycle subsystem and optional oxygen supply subsystem.

(15) Fuel such as natural gas may be cleaned using sorbent beds etc. to remove sulfur and other impurities, if present. If the feedstock is a solid such as biomass, it is prepared if necessary by shredding (25 mm or 1 inch top size) and drying (10% moisture) typically using process flue gas. The feed subsystem will include an optional compressor and one or more fuel injectors if the fuel is natural gas, pump and fuel injectors in the case of a liquid fuel and a storage bin, a proprietary plug feeder and injection auger for solid feedstock; the auger will transfer the solid feedstock from the atmospheric pressure bin to the chamber operating at low to moderate pressure (1 to 15 bar gauge). The gasifier subsystem may employ a two-step gasification process. The first stage will be the integrated gasifier-SOFC unit described earlier with internal and external cyclones; a second stage termed a char converter or carbon trim cell (CTC) may be included downstream to gasify fine char and ensure high carbon conversion. This converter operates as a trim fluidized bed with oxygen or air in a sub-stoichiometric mode to gasify the residual carbon. The products from the converter pass through a cyclone to drop out the fly ash. The solid products of reaction (mostly inorganic material) are discharged from this cyclone catch; if the feedstock contains K and P compounds, these are likely to report to this catch stream. The ash from the cyclone catch is cooled and conveyed to storage for disposal. The syngas from the first and second stages are blended together and sent to a clean-up subsystem comprising a heat recovery steam generator (HRSG), followed by a venturi scrubber and gas cooler to remove particulates from the syngas, capture the tars and condense excess steam. The tars are collected and separated by a proprietary solvent-based system and routed to the carbon trim cell to crack and reform. The clean syngas is burned in a second HRSG and the flue gas is routed to the feedstock dryer and then the stack. An optional on-site merchant VPSA oxygen separation plant supplies industrial purity oxygen for the CTC. The high pressure superheated steam generated in the HRSGs are sent to a bottoming steam cycle subsystem to generate power. Alternately, if the fuel is natural gas or liquids, the syngas from the gas cleanup train may be sent to a gas turbine combined cycle for power generation. The gas cleanup step may also include means for CO.sub.2 capture and sequestration. The overall system may thus be configured to generate all power or combined heat and power for distributed generation.

(16) Protruding into the chamber 110 from a sidewall 128, are the indirect heater conduits 122A belonging to one or more spaced apart sets 120a, 120B of indirect heater elements 122. While two such sets 120a, 120B are shown, it is understood that only one such set, or simply even a plurality of such indirect heater elements not arranged in any particular manner, may be provided. In some embodiments, the indirect heating elements 122 may comprise pulse combustion heaters driven, for instance, by a portion of the created product gas. In other embodiments, the indirect heating elements 122 may comprise electrical heaters.

(17) The indirect heater elements 122 assist with startup of fluidized bed operations, and also with control of dense fluid bed temperature (temperature trim). However, the indirect heater elements 122 generally play no role in supplying heat to sustain gasification (e.g., carbonaceous material-to-product gas) reactions, during steady state operation of the chamber 110.

(18) In the embodiment of FIG. 1, a lower, first fuel cell module 130a is provided immediately above the enhanced char conversion zone 114 and below the lower first set 120a of indirect heater elements 122. Meanwhile, the upper, second fuel cell module 13BA is provided immediately above the upper, second set 120B of indirect heater elements 122. The feedstock 150 is introduced into the drying and devolatilization zone 116 between the lower, first set 120a and the upper, second set 120B of indirect heaters. The feedstock 150 is combined with the fluidization medium and is conveyed in an upward direction towards the fuel cell elements 134. The fuel cell elements 134 may be controlled to maintain the dense fluid bed zone 118 at a desired temperature during steam reforming process which releases the product gas from the feedstock.

(19) A system in accordance with the present invention may realize certain efficiencies. For example, with biomass as the feedstock, a net plant electrical efficiency on the order of 50% on HHV basis may be realized. Such an efficiency is about twice that of conventional steam based biomass power generation and is a significant improvement over the current state of the art. With natural gas, there is potential to achieve a net electrical efficiency on the order of 75% on HHV basis. This has significant implications for energy utilization, energy security and climate change mitigation.

(20) Although the present invention has been described with reference to certain embodiments, it should be understood that various alterations and modifications could be made without departing from the spirit or scope of the invention as hereinafter claimed.