Waste to energy conversion without CO.SUB.2 .emissions

Abstract

The invention provides a method for energy extraction from municipal and mixed waste streams. The method employs a three-stage pyrolysis to produce a hydrogen-rich pyrolysis gas, which maximizes energy extraction without releasing carbon dioxide into the atmosphere.

Claims

1. A process for obtaining energy from waste, which comprises: (a) drying the waste; (b) anaerobic pyrolysis of the waste at 300 C. to 600 C. to produce syngas, char, and bio-oil; (c) anaerobic pyrolysis of the syngas in the presence of a portion of the bio-oil, a portion of the char, additional carbon dioxide, and added water, at 600 C. to 900 C., to produce a gas with increased hydrogen content and a product oil; (d) anaerobic pyrolysis of the gas and product oil produced at (c), in the presence of additional water and additional carbon monoxide, at 800 C. to 1200 C. and about 20 atmospheres pressure, to further increase the hydrogen content of the gas; (e) separating hydrogen, carbon dioxide and carbon monoxide from the gas produced at (d); (f) using the separated carbon monoxide as the additional carbon monoxide in step (d); and (g) using the separated carbon dioxide as the additional carbon dioxide in step (c).

2. The process of claim 1, further comprising: (h) fuelling a steam generator by combusting the hydrogen separated at (e).

3. A process for obtaining energy from waste, comprising: (a) drying the waste: (b) anaerobic pyrolysis of the waste at 300 C. to 600 C. to produce syngas, char, and bio-oil: (c) anaerobic pyrolysis of the syngas in the presence of a portion of the bio-oil, a portion of the char, additional carbon dioxide, and added water, at 600 C. to 900 C., to produce a gas with increased hydrogen content and a product oil; (d) anaerobic pyrolysis of the gas and product oil produced at (c) in the presence of additional water and additional carbon monoxide, at 800 C. to 1200 C. and about 20 atmospheres pressure, to further increase the hydrogen content of the gas; (e) fuelling a fuel cell with the gas produced at (d); (f) driving a steam generator with steam produced by the fuel cell; (g) separating carbon dioxide and carbon monoxide from the effluent gases of the fuel cell; (h) using the carbon monoxide as the additional carbon monoxide in step (c); (i) using a portion of the carbon dioxide as the additional carbon dioxide in step (b).

4. The process of claim 3, wherein the fuel cell is a solid oxide fuel cell (SOFC).

5. The process of claim 4, wherein the SOFC and steam generator are an integrated SOFC/turbine system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram showing the initial shredding and drying of the incoming waste stream.

(2) FIG. 2 is a diagram showing the low-temperature pyrolysis stage of the process.

(3) FIG. 3 is a diagram showing the medium- and high-temperature pyrolysis stages of the process.

(4) FIG. 4 is a diagram showing the fuel cell, turbine, and gas separator of the process.

DETAILED DESCRIPTION OF THE INVENTION

(5) A key element of the present invention is the use of the Boudouard reaction (Equation 1), the conversion of carbon dioxide and solid carbon into carbon monoxide:
C(s)+CO.sub.2(g)2CO(g)(Eqn. 1)

(6) The Boudouard equilibrium favors the formation of CO at high temperatures, shifting to the right at temperatures above ca. 700 C. In the process of the invention, the Boudouard equilibrium is coupled with the water-gas shift reaction (Equation 2) at to 600-900 C., which converts the CO to CO.sub.2 and hydrogen:
CO+H.sub.2OCO.sub.2+H.sub.2(Eqn. 2)

(7) The net result is shown in Equation 3:
C(s)+CO.sub.2(g)+2H.sub.2O2CO2+2H2(Eqn. 3)

(8) The overall process transfers the potential chemical energy present in elemental carbon to the potential chemical energy found in hydrogen. This is accomplished by carrying out an initial low-temperature anaerobic pyrolysis, to produce syngas, bio-oil, and carbon char, and then recycling CO.sub.2, carbon biochar, and water (end products of the overall process) back into the system prior to high-temperature pyrolysis and reforming. The bio-oil and syngas, and recycled char and CO.sub.2, are fed to a medium-temperature pyrolysis unit operating at 600-900 C., at which temperature the gas mixture becomes enriched in CO via the operation of Eqn. 1. A portion of the bio-oil can be removed at this point for use as liquid fuel.

(9) The CO-enriched gas is then compressed and fed, together with the bio-oil and additional water, to a high-pressure (20 atm), high-temperature (up to 1200 C.) reforming unit, where cracking of liquid hydrocarbons takes place, and any remaining methane is oxidized by water to CO and H.sub.2 (Equation 4):
CH.sub.4+H.sub.2OCO+3H.sub.2(Eqn. 4)

(10) The composition of the gas is further shifted toward CO.sub.2 and H.sub.2 via the water-gas shift reaction (Eqn. 2.) Through the above three-stage process, a large fraction of the chemical energy contained in the biomass is converted into the chemical energy contained in elemental hydrogen.

(11) The hydrogen-rich gas is then fed to a fuel cell, along with oxygen or air, for generation of electricity. The exhaust from the fuel cell is high-temperature steam, which can be used to generate additional electricity via a steam turbine. In an alternative embodiment, the steam turbine may be powered by combustion of the hydrogen. This is a less efficient process, but it avoids the capital investment and maintenance costs of the fuel cell.

(12) The unburned gas from the fuel cell, consisting of CO and CO2, is compressed to 40 atm and fed to a separator, where it is cooled to liquefy the CO.sub.2. Gaseous CO is separated and returned to the high temperature reforming unit, while the liquid CO.sub.2 is sent to expansion (evaporation) units. Part of the now-gaseous CO.sub.2 is returned to the medium-temperature pyrolysis unit, and the remainder, still at about 20 atm, constitutes the CO.sub.2 effluent of the overall process. The expansion units serve as heat sinks for the coolant used to cool the separator.

(13) The invention will now be described in greater detail. Turning to FIG. 1, mixed solid waste (municipal garbage, dried sludge, agricultural bagasse, etc.), preferably freed of ferrous metals and aluminium, is fed to a shredder 1. Liquid waste (sewage, concentrated sludge, etc.) is piped in at intake 2, and combined with the output of the shredder at mixer 3, which may be for example an auger for both mixing and propelling the waste stream. A heater 4 raises the temperature of the waste stream to about 140 C. in order to dry the waste. Pressurized steam is preferably used to energize the heater, and in the embodiment shown the steam enters at 5, with the condensate exiting at 6, where it joins the flow of condensate exiting from the steam turbine (FIG. 4, described below). The steam is preferably provided by the hot exhaust from the fuel cell, as described below. Steam produced by the drying waste collects in chamber 7, where it serves to pressurize the waste and ensure that it flows in the proper direction. The pressure is controlled by valve 8; steam released through 8 is condensed, combined with the heater and turbine condensates at 9, and exits at 10 for use elsewhere in the system. The dried waste 11 exits through transport pipe 12 and proceeds to the pyrolyzer units (FIG. 2.)

(14) Referring now to FIG. 2, waste 11 arrives at the distal end of transport pipe 12, and flows upward to the induction-heated low-temperature pyrolyzer unit 13. The dry waste is suspended, throughout the system, in bio-oil 14, which serves as the heat transfer fluid and reaction medium for the pyrolysis reactions. The pyrolyzer 13 is operated anaerobically, at 300 C. to 600 C., preferably at about 500 C. At this temperature, as is known in the art, organic materials thermally break down into char, gases and oils, yielding bio-oil 14 which flows into reservoir 15. Bio-oil is removed at 16 at the same rate it is formed, so as to maintain the waste 11 within the heated zone of the pyrolyzer. A portion of the carbon in the waste is reduced to biochar, or coke, which is carried over with the bio-oil into reservoir 15. A solids separator 17 collects the bio-char, along with inorganic solids (ash, silica, glass and metal fragments, etc.) that are present in or generated from the waste. The solid wastes are collected at 18. Carbon char is separated from the collected solids, and is returned to the medium temperature pyrolyzer (FIG. 3.) The syngas 19 produced by the reaction is largely H.sub.2 and CO, with lesser amounts of CO.sub.2 and CH.sub.4. It is removed via valve 20 and also passes on to the medium-temperature pyrolyzer.

(15) The above description is intended to be an outline of one embodiment of a portion of the invention. Those skilled in the art will appreciate that other methods of processing, drying, transporting, and anaerobically pyrolyzing biomass and organic waste to produce bio-oil, bio-char, and bio-gas are known in the art (see M. I. Jahirul et al., Biofuels Production through Biomass PyrolysisA Technological Review. Energies 2012, 5:4952-5001; doi:10.3390/en5124952), and any of the known methods are contemplated to be adaptable for use at this stage of the presently-described process. Precise operating details, such as operating temperature, residence time, and throughput, will be adjusted for optimum performance as the composition of the waste stream varies overtime. Agricultural wastes, in particular, are likely to vary with the seasons.

(16) Turning to FIG. 3, the medium- and high-temperature pyrolysis units are illustrated. Bio-gas exiting from valve 20 (FIG. 2) enters the medium-temperature pyrolyzer 21 via port 22. Bio-oil 14 exiting the reservoir at 16 (FIG. 2) enters the bottom of the medium-temperature pyrolyzer at 23. Water enters via tube 24, and bio-char (carbon) recovered from the low-temperature pyrolyzer is introduced at port 25. A separate feed of waste oil (from fryers, auto maintenance, etc.) may be separately fed into the system at 26, and carbon dioxide from the CO.sub.2 separator (FIG. 4) is introduced at 27.

(17) The pyrolyzer 21 is operated at a pressure of 1-5 atm, between 600 C. and 900 C. Under these conditions, the Boudouard reaction (Eqn. 1) oxidizes the added carbon to carbon monoxide, with concomitant reduction of the added CO.sub.2 to additional CO. Due to the presence of water, a water-gas shift reaction (Eqn. 2) then takes place, with the net production of additional hydrogen gas. The hydrogen-rich syngas 28 is then compressed by compressor 29 to about 20 atm before being fed to the high-temperature pyrolyzer 30. Bio-oil is removed at 31, pressurized to about 20 atm at 32, and also fed to the high-pressure pyrolyzer 30. Excess bio-oil is drawn off at 33, for use as a fuel or feedstock. Water is introduced to the high-temperature pyrolyzer via 24, and recycled CO from the CO2 separator (FIG. 4) is introduced at 34.

(18) The high-temperature pyrolzyer is operated at a pressure of about 20 atm, at 800 C. to 1200 C., preferably at a temperature of about 900 C. Under these conditions, hydrocarbon cracking and steam reforming (Equation 4) take place, further enriching the gas phase in hydrogen, and the water-gas shift reaction converts the CO thus produced to yet more hydrogen. The net result of these processes is Equation 5:
CH.sub.4+2H.sub.2OCO.sub.2+4H.sub.2(Eqn. 5)

(19) The gases 35, which are at this point principally hydrogen and CO.sub.2, are drawn away through outlet 36, and delivered to the fuel cell (FIG. 4).

(20) Turning now to FIG. 4, the pyrolyzer gas outlet 36 leads to valve 37, which in normal operation passes the gases to fuel cell 38. Fuel cell 38 is preferably a solid oxide fuel cell (SOFC) designed for high-pressure and high-temperature operation; such units are known in the art and are commercially available. Air or oxygen is fed to the cell via inlet 39. Hot steam issues at 40, and is used to drive turbine 41 for generation of additional electricity. A portion of the steam is diverted at 42 to the heater 4 (FIG. 1) that dries the incoming waste stream, and the steam may be used as a thermal energy source elsewhere in the installation as needed.

(21) Hybrid systems combining SOFC fuel cells and turbines powered by the SOFC exit gases are known in the art; see U. Damo et al., Solid oxide fuel cell hybrid system: A detailed review of an environmentally clean and efficient source of energy. Energy 168:235-246 (2019) doi:10.1016/j.energy.2018.11.091. Integrated SOFC/turbine systems have become commercially available; an example is the MEGAMIE series of integrated systems manufactured by Mitsubishi Hitachi Power Systems, Ltd. of Yokohama, Japan. It is contemplated that commercial integrated systems can be readily adapted for use in the process of the present invention.

(22) A mixture of carbon dioxide and carbon monoxide remains after the hydrogen is oxidized in the fuel cell, and these gases are compressed at 43 to about 40 atm and passed to the gas separator 44. The CO.sub.2, still under 40 atm pressure, is cooled to about 4 C., at which point it liquefies, permitting the gaseous CO to be drawn away at 45 and returned to the high-temperature pyrolyzer at 34 (FIG. 3). The liquid CO.sub.2 46 is sent on to expanders 47 and 48. Expander 47 discharges CO.sub.2 at a pressure of about 1 atm; this gas is recycled to the medium-pressure pyrolyzer 21 (FIG. 3). Expander 48 discharges CO.sub.2 at a pressure of about 20 atm. This gas is pipeline-ready, and can be used as a feedstock for chemical processes, for fertilizer production, or for enhanced oil recovery or underground sequestration. The costs and inefficiencies of CO.sub.2 capture are entirely avoided, due to the closed nature of the system of the invention.

(23) The expansion of the CO.sub.2 in expanders 47 and 48 is accompanied by considerable cooling. This is captured by heat exchangers 49 and 50, respectively, which serve to cool and liquefy the compressed CO.sub.2. A third heat exchanger 51, immersed in coolant 52, provides cooling for reactors, condensers, and other equipment as desired.

(24) Of the energy supplied by hydrogen, only about 35% is used by the chemical processes of the invention, and the addition to the pyrolyzers of additional water makes it possible to improve upon this balance. The remainder is available for electricity production.

(25) The CO.sub.2 produced is easily transmitted to agriculture in liquefied form, for dissolution in irrigation water. The use of carbonated water for irrigation is known to increase yields, particularly in greenhouse environments, but the method has not been widely employed to date. The availability of piped-in CO.sub.2 from installations of the system of this invention will make the technology readily available.

(26) The present invention is sufficiently clean and efficient to make mining of landfills for their energy content a viable enterprise, and could make it possible to reclaim land currently given over to the storage of trash.