ZA200901803B

ZA200901803B - Method and apparatus for converting waste to energy

Info

Publication number: ZA200901803B
Application number: ZA200901803A
Authority: ZA
Inventors: Mitchell Andrew Gregory
Original assignee: Energetix Llc
Priority date: 2009-01-01
Filing date: 2009-01-01
Publication date: 2009-09-30

Description

2 I] —

METHOD AND APPARATUS FOR CONVERTING WASTE TO'ENERGY

Field of the invention

[0001] The invention is a process that facilitates the generation of methane from solid wastes, purification of the methane, generation of electricity by combustion of solid wastes, capture of a portion of the carbon dioxide produced, and conversion of the captured carbon dioxide into algae.

Background

[0002] The use of renewable resources and in particular animal and human waste for the generation of useful energy has been investigated extensively and is currently pursued as a commercial activity in many countries of the world. Various patents have been issued for processes that enable these objectives. Lutz (US2008/0299634) recently published an application describing a process and method of operation for a dry fermentation process based on a previously described process (EP 0 934998, WO 02/06439, and EP1301583B). The process employs the use of a prefabricated garage and methods incorporating the use of exhaust gases for purging combustible gases from the chamber. The process employs the use of re-circulated digestate liquors sprayed over the biomass in order to keep the biomass moist and the temperature is controlled by heating of the walls and floors of the chamber so as to keep the fermentation in the mesophilic optimal range of 34°C to 37°C range. As a result of the : process, biogas is made available for combustion and useful generation of electricity.

[0003] The sequestration of carbon has received much recent attention, and papers discussing algae aquaculture as a viable method have been published extensively. So has the treatment of wastewater using aerobic and anaerobic photobioreactors. Patents and other papers on both topics have been summarized by Elefritz et al (US Patent 7,455,765).

[0004] Lewnard et al (US Patent 2008/0178739) provide a comprehensive review of both open and closed system designs as well as a hybrid method for cultivating algae in large closed spaces. The main issues cited by most authors are the propensity for contamination in open systems as well as a fairly low yield in terms of algal growth per unit land area compared to closed systems, which have the associated comparatively high capital cost per unit of land area.

Closed systems have the advantage of increased carbon dioxide availability. Freeman (US

Patent 2008/0254529) describes a process whereby liquid culture mediums are exposed to closed carbon dioxide/air mixtures. Whitton (US Patent 286,851) describes a flexible integrated closed system constructed of thin plastics which can potentially be folded up and transported to different sites or mounted on earthen bearms. The inclusion of gas spargers is discussed.

Howard et al (US Patent 2008/0299643) disclose a variant on the hybrid open/closed system with plastic pond covers and the introduction of diffused CO..

Summary of the invention

[0005] In the process described, modified gas-tight steel shipping containers are used to house fabricated cages (made of steel or other suitable material) with a sprinkler system and a mesh screen floor above a liquid sump. The cages contain and support above the mesh either digestate and biomass (hereinafter referred to as “fermenters”) or gas contacting media such as plastic bottle caps (hereinafter referred to as “scrubbers”). The sprinkler system and sump are both connected to external inlet and outlet connections on the container wall by flexible hose. A third outlet connection on the container wall allows the release of biogas (a gas produced by the ~ biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel). Digestate liquors collected in the sump are pumped and sprayed back over the digestate and biomass after passing through a heat exchanger that heats the liquor to the desired temperature for mesophilic or thermophilic methane generation.

The use of the heated digestate liquor, the use of the fabricated cages and the use of the containers for the purpose described are incorporated in this patent application.

[0006] Biogas emanating from the process is delivered to the bottom of the scrubber and water is sprayed from the top. The use of the fabricated cages and the use of the containers for the purpose described are incorporated in this patent application. The gas flows upward countercurrent to the water flow and escapes through the gas outlet connection. Carbon dioxide enriched water collects in the contactor sump and is pumped to the algae generation units. The algae generation units have already been described by the author in patent application SA2009/00499.

[0007] Fermented solid waste is collected and dried using exhaust gas from the furnace. The dried solid is fed to the furnace either pneumatically or mechanically where it is burnt for the generation of steam and electricity. The combustion exhaust gas is blown through a heat exchanger prior to entering the gas scrubber where a portion of the carbon dioxide is removed

Ee. . by dissolution in water. The exhaust gas flows back through the heat exchanger, is heated and used for drying solid waste, heating digestate liquor and / or exhausted to atmosphere. The carbon dioxide enriched water is pumped to the algae generation units already described in patent application SA2009/00499.

[0008] Steam from the turbine is injected directly into the water being conveyed to the gas scrubber. This assists temperature elevation of the water being sent to the algae generation units and captures the residual “waste heat” associated with electricity generation. The direct injection of turbine exit steam into the algae generation feed water is incorporated in this patent application.

Brief description of the drawings

[0009] Further features, benefits and advantages of the invention will become evident from the following description of exemplary embodiments with reference to the drawings, in which:

[0010] FIGURE 1 shows a process flow diagram for the generation of methane from solid;

[0011] FIGURE 2 shows a process flow diagram for the transport of dry solid fuel to a furnace, combustion of the fuel in a furnace, the generation of steam in the furnace, the generation of } electricity from the steam, direct injection of the spent steam into a water stream, forced convection of the exhaust through a heat exchanger to a gas liquid contacting chamber, removal of the exhaust gas from the contacting chamber through the heat exchanger to a drying chamber, a heat exchanger or to atmosphere and removal of the carbon dioxide enriched water to the algae generation facility; and,

[0012] FIGURE 3 shows a diagram of a fermenter or scrubber.

Detailed description

[0013] Within this application, “fermentation” means “an enzymatically controlled anaerobic breakdown of an energy-rich compound (as a carbohydrate to carbon dioxide and alcohol or to an organic acid and ultimately to production of methane).” “Fermenter” means “an apparatus for carrying out fermentation.” FIGURE 1 shows one embodiment of a plant layout which generates methane for commercial use and provides a carbon dioxide enriched water stream for an algae generation facility. Solid wastes stored in a cage are enclosed in a fermenter (described in FIGURE 3) that is gas tight for operating pressures of approximately 1 to approximately 2 bar. Digestate liquor and water are pumped from the fermenter, through a heat exchanger and sprayed over the digestate and biomass. The temperature in the fermenter is maintained at the mesophilic or thermophilic optimum temperatures for methane generation.

Exhaust gas from a furnace (described in FIGURE 2) is used to purge the fermenter of air once a new cage of solid waste has been installed. At the: end of the gas generation cycle, exhaust gas is used to purge the fermenter of combustible gases. During the gas generation cycle, digestate gas passes from the fermenter to the scrubber (described in FIGURE 3). In the scrubber, gas flows from the bottom of the contactor to the top countercurrent to the water sprayed from the top, collected in the sump and pumped to the algae generation units. The enriched gas is further purified with a purification device, such as a molecular sieve or device suitable for the removal of water and other contaminants before being admitted to the gas distribution network.

[0014] FIGURE 2 shows one embodiment of a plant layout which conveys solid fuel from a hopper (pneumatically or mechanically) to a combustion furnace. The combustion exhaust gases are extracted from the furnace and forced through the shell side of a heat exchanger to a scrubber (FIGURE 3), back through the tube side of the heat exchanger and distributed for drying solid fuel, heating digestate liquor, or exhausted to atmosphere. Water is pumped to a delivery pressure in excess of approximately 8 bar and passes through the boiler and furnace heat exchangers before being admitted to the turbine to produce electricity. A secondary high pressure stream of water is sprayed directly into the combustion chamber above the flame to reduce the adiabatic flame temperature and improve the emissivity of the exhaust gas. Excess air requirements normally associated with conventional solid fuel burners are reduced thereby increasing the concentration of carbon dioxide in the exhaust. The scrubber operating pressure is typically above ambient. The processes of increasing the water content of the exhaust gases and subsequently recapturing all the water in the gas scrubber is incorporated in this application. From the scrubber, the carbon dioxide enriched water is delivered to the algae generation facility described in patent application SA2009/0499.

[0015] FIGURE 3 shows a schematic of a fermenter / scrubber.

[0016] The above examples have been depicted solely for the purpose of exemplification and are not intended to restrict the scope or embodiments of the invention. The invention is further illustrated with reference to the claims that follow thereto.

[0017] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about."

Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0018] Notwithstanding that the numerical ranges and parameters setting forth the broad scope : of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0019] The invention has been described with reference to several embodiments. Obviously, modifications and alterations will occur to others upon a reading and understanding of the specification. It is intended by applicant to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

[0020] Having thus described the invention, it is now claimed:

Claims

What is claimed is: \ ee

1. A method for the generation of converting waste to energy, the method comprising the steps of: : adding solid waste to a fermenter; maintaining the temperature in the fermenter at either a mesophilic optimum temperature for methane generation or a thermophilic optimum temperature for methane generation; pumping digestate liquor and water from the fermenter through a heat exchanger; spraying the liquor and water over the solid waste; : pumping biogas to the bottom of a scrubber; spraying water from the top of the scrubber, wherein the biogas flows countercurrent to the flow of water; and, ‘collecting carbon dioxide enriched water in a sump and producing a purified biogas.

2. The method of claim 1, wherein the operating pressure of the fermenter is ambient pressure. -

3. The method of claim 1 or 2, wherein the method further comprises the ; step of: purifying the biogas and pumping the carbon dioxide enriched water to an algae generation unit.

4. The method of any one of claims 1-3, wherein the biogas is further purified with a purification device.

5. The method of claim 4, wherein the purification device is a molecular sieve.

6. A method for converting waste to energy, the method comprising the steps of: conveying solid fuel from a hopper to a combustion furnace; : pumping water at a pressure in excess of approximately eight bar through a boiler and furnace heat exchangers; and,

~~ ) conveying the water to a turbine to produce electricity.

7. The method of claim 6, wherein the method further comprises the step of: spraying a second stream of water into the combustion furnace to reduce adiabatic flame temperature and improve emissivity of the exhaust gas.

8. The method of claim 6 or 7, wherein the exhaust gases from the furnace are forced through the shell side of a heat exchanger to a scrubber where a portion of the carbon dioxide is dissolved in the water, and the carbon dioxide enriched water is pumped to an algae generation facility.

9. The method of any one of claims 6-8, wherein the dried solid fuel is fed to the furnace either pneumatically or mechanically.

10. The method of any one of claims 6-9 wherein the scrubbed exhaust gas is forced back through the tube side of the heat exchanger and distributed for drying solid fuel, heating digestate liquor, or exhausting to atmosphere.

11. The method of any one of claims 6-10, wherein the exhaust gas is blown through a heat exchanger prior to entering the scrubber where a portion of the carbon dioxide is removed by dissolution in water.

12. The method of any one of claims 6-11, wherein steam from the turbine is injected into the water being conveyed to the scrubber.

13. An apparatus for converting waste to energy, the apparatus comprising: a fermenter; ] a pump operatively connected to the fermenter; a heat exchanger, the heat exchanger operatively connected to the pump; a furnace, the furnace having an exhaust tube; a scrubber, the scrubber operatively attached to the fermenter by a valve and at least one tube; and, a sump, the sump operatively connected to the scrubber.

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14. The apparatus of claim 13, wherein a sprinkler system and the sump are both connected to an external inlet and outlet connections on the scrubber by a hose, and a second outlet connection on the scrubber allows the release of biogas.

15. A method for converting waste to energy substantially as herein described with reference to FIGURE 1. :

16. A method for converting waste to energy substantially as herein described with reference to FIGURE 2.

17. An apparatus for converting waste to energy substantially as herein described with reference to FIGURES 1 and 3.

18. An apparatus for converting waste to energy substantially as herein described with reference to FIGURES 2 and 3. Dated this 9th day of March 2009 VON SEIDELS For the applicant