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Title:
A METHOD FOR PROCESSING SOLID MUNICIPAL WASTE
Document Type and Number:
WIPO Patent Application WO/1999/037738
Kind Code:
A1
Abstract:
The invention pertains to methods to process combustible solid municipal wastes (SMW), primarily highly humid ones, by means of pyrolysis and gasification of their organic part so as to produce liquid hydrocarbon products of pyrolysis and fuel gas, which are used for energy generation. The method can be used for environmentally friendly processing/disposal of poorly combustible wastes. SMW is charged in a gasifier reactor type of a shaft kiln, possibly together with a solid incombustible material, countercurrently to an oxygen-containing gasifying agent and combustible components of SMW are gasified. Smoke gases are introduced in the gasifying agent. The maximum temperature in the reactor is controlled within 800 to 1300 °C through variation of at least one of the following parameters, the mass fraction of oxygen in the gasifying agent $i(a), and the mass fraction of incombustibles in the processed SMW $i(b), and the mass fraction of combustibles in processed SMW $i(c), while the ratio $i(A = ab/c) is maintained within 0.022 to 0.1.

Inventors:
MANELIS GEORGI B (RU)
FOURSOV VICTOR P (RU)
POLIANTCHIK EVGUENI V (RU)
Application Number:
PCT/FI1999/000045
Publication Date:
July 29, 1999
Filing Date:
January 22, 1999
Export Citation:
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Assignee:
FIOTER OY (FI)
MANELIS GEORGI B (RU)
FOURSOV VICTOR P (RU)
POLIANTCHIK EVGUENI V (RU)
International Classes:
F23G5/027; C10J3/16; (IPC1-7): C10J3/02
Domestic Patent References:
WO1996000267A11996-01-04
WO1990007085A11990-06-28
Foreign References:
US4164397A1979-08-14
EP0381254A11990-08-08
Attorney, Agent or Firm:
PAPULA REIN LAHTELA OY (Fredrikinkatu 61A P.O. Box 981 Helsinki, FI)
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Description:
A METHOD FOR PROCESSING SOLID MUNICIPAL WASTE This invention pertains to methods for proc- essing solid municipal wastes (SMW), primarily highly humid ones, by means of pyrolysis and gasification of their organic part so as to produce hydrocarbon prod- ucts of pyrolysis and fuel gas, which are used for en- ergy generation. The method can be used for environ- mentally friendly and energy efficient process- ing/disposal of poorly combustible wastes.

A number of methods for incineration of com- bustible wastes with energy generation is known. Among those, distinguished by their environmental friendli- ness are the methods based on two-stage processing, first gasification and then combustion of the product gas. Generally, the gasification of organic fuels in counterflow of a gasifying agent can be presented as follows.

The gasifying agent containing oxygen and possibly water and/or carbon dioxide enters the com- bustion zone wherein oxygen reacts at 900-1500°C with the carbon of solid fuel in the form of char. The gas- ifying agent is fed to the reactor countercurrently to the fuel so that the oxidant gas at least partially is passed through a layer of solid combustion products (ashes) that already do not contain carbon. In this zone, solid combustion products cool down and the gas- ifying agent correspondingly heats before it enters the combustion zone. In the combustion zone, the oxy- gen of the gasifying agent is totally consumed and hot gaseous combustion products, including carbon dioxide and water, enter the next zone of the charge, which is called reduction zone, where carbon dioxide and steam react with the carbon of fuel yielding combustible gases. The sensible heat of the gases heated in the combustion zone is partially consumed in these reduc- tion reactions. The temperature of the gas flow de- creases as the gas filters through the solid fuel and

lends to the latter its sensible heat. The fuel heated in oxygen-free environment is pyrolyzed yielding char, pyrolysis tars, and combustible gases. The product gas is passed through fresh fuel so as to cool down the gas while fuel is heated and dried. Finally, the prod- uct gas (containing steam, hydrocarbon vapors, and tars) is withdrawn for further use.

A method for pyrolysis and combustion of com- bustible part of SMW is described in patent US-A- 4732091. According to the method the solid fuel is charged in the upper part of a shaft kiln. The fuel charged is supplied with the rate controlled by moving horizontal grates through a succession of zones wherein the fuel pyrolyzes and burns in a counterflow of air-steam gasifying agent. This method is based on loosening the material with gratings; thus gas- permeability of the fuel is secured. This method also proposes way to control fuel supply to respective zones.

A method described in patent RU-2079051 pro- poses gasification of solid municipal waste in a coun- tercurrent of gasifying agent containing oxygen and also water and/or carbon dioxide. The maximum tempera- ture in the combustion zone (i. e., maximum temperature in the reactor) is controlled within 700 to 1400°C (preferably 1000 to 1200°C) while the temperature of the product gas at the reactor outlet is maintained below 400°C (preferably under 250°C). The temperature regime of the process is controlled through variation at least one of the following parameters, mass frac- tion of oxygen in the gasifying agent"a", mass frac- tion of incombustibles in the fuel charge"b", mass fraction of combustibles in the fuel charge"c", while the ratio A = ab/c is maintained within 0.1 to 4.0.

Preferably, A is maintained within 0.15 < A < 1.0. The product gas is directed for combustion in a boiler.

Introduction of water (carbon dioxide) in the gasifying agent provides means to enhance content of hydrogen (carbon monoxide) in the product gas and to reduce the temperature in the combustion zone. On the other hand, supply of steam necessitates introduction of additional equipment in the installation and when steam boiler is a part of the installation increases in-process steam consumption. Besides, a general draw- back of the aforementioned methods when applied to gasification of humid waste is unavoidable addition of steam to the already humid product gas. The latter is diluted with steam, which further increases heat losses with the smoke gas and reduces energy effi- ciency of the boiler and of the process in general.

The objective of this invention is to perform pyrolysis and gasification of SMW without external heat supply, with high energy efficiency and high yield of valuable products (pyrolysis tars and combus- tible gases).

This invention provides a method for process- ing condensed combustibles that includes: -charging in the reactor SMW so as to pyro- lyze and gasify the latter; -establishing gas flow through said charge by means of supply into said reactor, into a zone where solid processing products accumulate, of a gas- ifying agent containing oxygen, steam, and carbon di- oxide, withdrawal of gaseous and liquid processing products, the successive cross-sections of said charge successively enter the zones of heating, pyrolysis, coking, gasification, and cooling; -discharging solid residue of processing from the reactor; -and burning at least part of the combusti- ble gas; -controlling of the maximum temperature in the reactor within 800 to 1300°C through variation of

least one of the following parameters, mass fraction of oxygen in the gasifying agent"a", mass fraction of incombustibles in SMW"b", mass fraction of combusti- bles in SMW charge"c", which is distinguished by that gasifying agent is the smoke gas, preferably in mixture with air, while mass fraction of oxygen in the gasifying agent, mass fraction of incombustibles in SMW, and mass fraction of combustibles in SMW are controlled so that 0.022 < ab/c < 0.1.

Thus it is possible to combine relatively high combustibility of the product gas with high en- ergy efficiency of the process. In order to secure uniform distribution of the gasifying agent over the reactor cross-section one can introduce in the charge pieces of incombustible material predominantly with mesh size less than 200 mm. This also provides possi- bility to compensate dilution of the gasifying agent with the nitrogen of the smoke gas. The heat exchange with solid incombustible material provides a possibil- ity to preheat the gasifying agent and thus to in- crease the temperature in the gasification zone. The regulation limits for the aforementioned parameters can be found experimentally for each particular case, depending on the waste composition. The gasifying agent is supplied to the part of the reactor where solid products of processing accumulate so as to pass gas flow through the layer of these products. The gas- ifying agent or its constituents can be supplied in one stream or in distributed mode. In particular, air and smoke gases can be supplied via their own separate inlets. The product gas can be burnt on its own or as a byfuel on natural gas or oil fed boiler. The smoke gases produced in latter case can also be used in this process since they contain carbon dioxide and steam.

Depending on the combustion regime in the boiler, the oxygen content in smoke gas can vary and at high oxy-

gen excess the smoke gas can be directly used as the gasifying agent.

The mixture charged enters the preheating zone wherein it heats to 300°C owing to heat exchange with the combustible product gas. The product gas is withdrawn from the preheating zone. Here the name product gas refers to aerosol comprising pyrolysis tars as vapors and fine droplets and generator gas in- corporating carbon monoxide and dioxide, steam, hydro- gen, methane, ethylene, propane, and other gases. Fur- ther the charge enters the pyrolysis zone, where it heats to 300-500°C due to heat exchange with gas flow and combustible materials undergo pyrolysis emit- ting volatiles to the gas and forming carbonaceous residue. Further the mixture containing pyrolyzed waste enters the coking zone where coke is formed from the organic matter of waste at 500-800°C. Further the mixture containing coked combustibles enters the gasification (combustion) zone where preheated gasify- ing agent reacts with coke at 800-1300°C to yield combustible gas and solid residue of combustion. Fi- nally, the solid residue enters the cooling zone where owing to heat exchange of the solid residue with coun- tercurrently supplied gasifying agent the latter is preheated.

The above classification of the zones is in part arbitrary, they might be defined alternatively, say according to gas temperature or composition and state of the reactants. However, for any notation cho- sen the distinctive feature is that owing to counter- flow of the gas and the charge, the gasifying agent (oxidant gas) preheats due to heat exchange with the solid residue and further hot gaseous products lend their heat to fresh mixture charged into reactor.

Upon completion of the process, the solid residue of processing is discharged from the reactor.

The product gas withdrawn from the reactor can be di-

rectly burnt in gas burner of a boiler; alternatively it can be cleansed or processed according to conven- tional techniques. So the pyrolysis oils can be con- densed and used as a hydrocarbons feedstock and uncon- densed gas used as fuel gas.

The smoke gases can be supplied as a compo- nent of gasifying agent either directly, or, alterna- tively, after being used for preliminary drying of the waste. In the latter case one can achieve both lower humidity of the waste charged into reactor and reduced overall volume of recycled smoke gas; correspondingly less becomes dilution of the product gas with nitrogen and higher becomes combustion temperature of the prod- uct gas.

Thus, unlikely to the methods known from the previous art, this invention makes possible pyrolysis and gasification of SMW without additional heat supply and with high energy efficiency. The energy necessary to support the process is supplied by combustion of a fraction of combustible part of the waste. Introduc- tion of steam and carbon dioxide to the gasifying agent provides a possibility to enhance content of combustible components (hydrogen and carbon monoxide) in the product gas, while use of the smoke gas allows one to avoid additional energy expenditure to produce steam, only water contained in the waste is used in the process.

The figure schematically presents a possible materialization of the process.

Waste"W"is prepared in crasher 1, further in mixer 2 it is mixed with solid incombustible mate- rial"I"and then charged into shaft kiln reactor 4 through lock 3 at its upper part. In reactor 4 the mixture successively passes through heating zone 5, pyrolysis zone 6, combustion zone 7, and cooling zone 8. Solid processing residue"R"is continuously dis- charged via lock 9 with the rate controlled so as to

maintain combustion zone at certain elevation from the reactor bottom. The solid residue is fractionated on sieve 10 and a part of it is recycled as solid mate- rial mixed with waste and the rest of solid residue is directed for further processing or disposal. Air"A1" is supplied by fan 11 to the lower part of the reac- tor. To the same zone exhaust fan 12 supplies smoke gas"S". The product gas"G"is withdrawn from the up- per part of the reactor and directed to gas cleansing unit 13. In the condenser liquid products"C"are iso- lated from the product gas. The product gas is di- rected for combustion with air"A2"in steam boiler 14.

A fraction of smoke gas"S"is directed to drier 15, where waste"W"is dried with the heat of smoke gas.

The temperatures in respective zones are measured con- tinuously and when the temperatures deviate from pre- scribed optimal values, the control parameters are ad- justed. In case the temperature in the combustion zone exceeds the prescribed limits, the fraction of oxygen in the gasifying agent is decreased through higher content of smoke gases and correspondingly higher con- tent of steam and carbon dioxide in the gasifying agent. Upon this greater becomes contribution of endo- thermic reactions C + CO2 X 2 CO C + H20 CO + H2 and temperature in the combustion zone drops.

Alternatively, when the temperature in the combustion zone falls below prescribed limits the fraction of smoke gases in the gasifying agent is reduced. In- crease in the content of incombustibles (correspond- ingly higher b and lower c) with the above restric- tions on A provides a possibility to increase combus- tion temperature since it enhances preheating of the gasifying agent due to heat exchange with solid resi- due of combustion.

The other features and advantages of this in- vention are disclosed in the following nonrestrictive examples.

Example (1).

Processed is solid municipal waste of the following composition (wt. %): paper and cardboard 38.2, food residues 28.6, wood and leaves 1.8, tex- tiles 4.9, leather and rubber 0.6, polymers 7.0, bones 1.0, metals 4.0, glass and stones 5.1, fines 9.1, hav- ing humidity 47% and calorific value of 5.87 GJ/t. Ash content of dry mass is 27%. The elemental composition of combustible part of SMW corresponds to formula CHl72Oo76NolS0003 This composition is typical of Moscow SMW.

1A. [Processing according to RU-2079051] SMW is gasified with addition of 10 wt. % of solid inert material in the processing mixture and supply of the gasifying agent comprising 200 g steam per 1 kg of air. The product gas is burnt with supply of secondary air so as to maintain volume concentration of oxygen in the smoke gas at 2% (on dry gas basis; i. e. overall stoichiometric ratio of oxygen is 1.1). Total air con- sumption (sum of that fed as gasifying agent constitu- ent and secondary air fed to the gas burner) is about 3 t per ton of SMW. For the specified parameters of gasification 200 kg of steam is consumed for gasifica- tion of 1 t SMW. The smoke gas produced comprises (vol. %): N2-53.9, C02-11.0, O2-1.3, Ar-0.6, H20 -33.2%; yield of smoke gas is 3450 nm3 per ton of SMW.

(A = 0.12) 1B. SMW is gasified as in 1A but with gasify- ing agent composed of smoke gas and air in 1: 1 volume ratio. The smoke gas produced comprises (vol. %): N2- 57.8, CO2-11.8, O2-1.3, Ar-0.7, H2O-21.3%; yield of smoke gas is 3220nm3 per ton of SMW. (A = 0.082)

1C. SMW is gasified as in 1B but with gasify- ing agent composed of smoke gas and air in 7: 10 volume ratio, the smoke gas withdrawn from boiler at 250°C, being directed for drying of SMW. Smoke gas dries ca.

50 kg of water from a ton of SMW. This water as steam enters the gasifying agent The composition of the smoke gas and yield of the smoke gas are the same as in 1B. (A = 0.098) The heat loss with smoke gas (primarily as the condensation heat of the steam in the smoke gas) in 1A is 500 MJ/ton of SMW higher than that in 1B, C.

Example (2).

Processed is SMW preliminary treated with re- covery of a part of metal, glass, textiles, plastics, and cardboard suited for reprocessing. The material gasified has humidity of 50%, calorific value of 4.3 GJ/t, and ash content of 15% per dry mass. The elemen- tal composition of combustible part of SMW corresponds to formula CHl8Oo75Nolsooo4- 2A. [Processing according to RU-2079051] SMW is gasified with addition of 15 wt. % of solid inert material in the processing mixture and supply of the gasifying agent comprising 200 g steam per 1 kg of air. The product gas is burnt with supply of secondary air so as to maintain volume concentration of oxygen in the smoke gas at 2% (on dry gas basis; i. e. overall stoichiometric ratio of oxygen is 1.1). Total air con- sumption (sum of that fed as gasifying agent constitu- ent and secondary air fed to the gas burner) is about 2.5 t per ton of SMW. The smoke gas produced comprises (vol. %): N2-49.3, CO2-10.5, O2-1.1, Ar-0.6, H20 -38.6; yield of smoke gas is 2950 nm3 per ton of SMW.

(A = 0.109) 2B. The waste is gasified as in 2A but with gasifying agent composed of smoke gas and air in 1: 1 volume ratio. The smoke gas produced comprises (vol.

%): N2-53.8, CO2-11.4, O2-1.2, Ar-0.6, H2O- 33.0; yield of smoke gas is 2720 nm3 per ton of SMW (A = 0.072).

2C. The waste is gasified as in 2B but with gasifying agent composed of smoke gas and air in 5: 10 volume ratio, the smoke gas withdrawn from boiler at 250°C, being directed for drying of SMW. Smoke gas dries ca. 30 kg of water from a ton of SMW. This water as steam enters the gasifying agent The composition of the smoke gas and yield of the smoke gas are the same as in 2B. (A = 0.087).

2D. The waste is gasified as in 2B but with- out addition of solid inert to the processing mixture (this is possible since preprocessed SMW is regular in composition and dimensions of pieces. The composition of the smoke gas and yield of the smoke gas are the same as in 2B. (A = 0.022).

The heat loss with smoke gas per ton of waste in 2A ils"400 MJ higher than that in 2B, C, D.

Note that processing of highly humid wastes according to the above described method requires gen- erally lower A than described in RU-2079051, since ra- tio A characterizes heat exchange in the zone of solid residue cooling, whereas the necessity to evaporate substantial quantity of water with its high latent heat of evaporation makes necessary to shift the heat exchange balance so as let more heat enter the drying zone. Reduction of A below the abovementioned limit is disadvantageous, because it lowers preheating of the gasifying agent prior to it entering the combustion zone.

A comparison of the above examples shows that use of smoke gases as a component of the gasifying agent in gasification of combustible wastes provides a possibility to enhance energy efficiency of the proc- ess, as compared with the use of steam from external source, since heat loss with smoke gases at the stage

of aftercombustion becomes lower. Additionally to that this allows one to exclude special apparatus for steam production. Use of the smoke for preliminary partial drying of the fuel provides possibility to reduce vol- ume of the smoke gas recycled and enhances the tem- perature of combustion of the product gas in the burner with the same gain in efficiency on aftercom- bustion stage.