WO1999037738A1

WO1999037738A1 - A method for processing solid municipal waste

Info

Publication number: WO1999037738A1
Application number: PCT/FI1999/000045
Authority: WO
Inventors: Georgi B. Manelis; Victor P. Foursov; Evgueni V. Poliantchik
Original assignee: Fioter Oy
Current assignee: Fioter Oy
Priority date: 1998-01-22
Filing date: 1999-01-22
Publication date: 1999-07-29
Anticipated expiration: 2000-07-22
Also published as: AU2166199A; RU2150045C1

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 a, and the mass fraction of incombustibles in the processed SMW b, and the mass fraction of combustibles in processed SMW c, while the ratio A = ab/c is maintained within 0.022 to 0.1.

Description

A METHOD FOR PROCESSING SOLID MUNICIPAL WASTE

This invention pertains to methods for processing solid municipal wastes (SMW) , primarily highly humid ones, by means of pyrolysis and gasification of their organic part so as to produce hydrocarbon products of pyrolysis and fuel gas, which are used for energy generation. The method can be used for environmentally friendly and energy efficient processing/disposal of poorly combustible wastes. A number of methods for incineration of combustible wastes with energy generation is known. Among those, distinguished by their environmental friendliness 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 gasifying 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 gasifying agent correspondingly heats before it enters the combustion zone. In the combustion zone, the oxygen 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 reduction reactions. The temperature of the gas flow decreases 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 product gas (containing steam, hydrocarbon vapors, and tars) is withdrawn for further use.

A method for pyrolysis and combustion of combustible 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 proposes gasification of solid municipal waste in a coun- tercurrent of gasifying agent containing oxygen and also water and/or carbon dioxide. The maximum temperature 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 fraction of oxygen in the gasifying agent "a", mass fraction of incombustibles in the fuel charge "Jb", 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 drawback 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 efficiency 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 combustible gases) . This invention provides a method for processing 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 gasifying agent containing oxygen, steam, and carbon dioxide, 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 "jb" , mass fraction of combustibles 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 < ah/c < 0.1.

Thus it is possible to combine relatively high combustibility of the product gas with high energy 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 possibility to compensate dilution of the gasifying agent with the nitrogen of the smoke gas . The heat exchange with solid incombustible material provides a possibility to preheat the gasifying agent and thus to increase 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 gasifying 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 incorporating carbon monoxide and dioxide, steam, hydro- gen, methane, ethylene, propane, and other gases. Further 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 emitting 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. Finally, 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 conventional techniques. So the pyrolysis oils can be condensed and used as a hydrocarbons feedstock and uncon- densed gas used as fuel gas .

The smoke gases can be supplied as a component of gasifying agent either directly, or, alternatively, 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 product 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. Introduction 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 material "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 discharged 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 material mixed with waste and the rest of solid residue is directed for further processing or disposal. Air "A_λ" is supplied by fan 11 to the lower part of the reactor. To the same zone exhaust fan 12 supplies smoke gas "S". The product gas "G" is withdrawn from the upper part of the reactor and directed to gas cleansing unit 13. In the condenser liquid products "C" are isolated from the product gas. The product gas is directed 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 continuously and when the temperatures deviate from prescribed optimal values, the control parameters are adjusted. 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 content of steam and carbon dioxide in the gasifying agent. Upon this greater becomes contribution of endo- thermic reactions C + C0₂ → 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. Increase in the content of incombustibles (correspondingly higher b and lower c) with the above restrictions on A provides a possibility to increase combustion temperature since it enhances preheating of the gasifying agent due to heat exchange with solid residue of combustion. 8

The other features and advantages of this invention 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, textiles 4.9, leather and rubber 0.6, polymers 7.0, bones 1.0, metals 4.0, glass and stones 5.1, fines 9.1, having 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 CH₁₇₂O₀₇₆N_{0 Λ}S₀_₀₀₃. 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 consumption (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 gasification of 1 t SMW. The smoke gas produced comprises (vol. %) : N₂ - 53.9, C0₂ - 11.0, 0₂ -1.3, Ar - 0.6, H₂0 - 33.2%; yield of smoke gas is 3450 nm³ per ton of SMW. (A = 0.12)

IB. SMW is gasified as in 1A but with gasifying agent composed of smoke gas and air in 1:1 volume ratio. The smoke gas produced comprises (vol. %) : N₂ - 57.8, C0₂ - 11.8, 0₂ -1.3, Ar - 0.7, H₂0 - 21.3%; yield of smoke gas is 3220nm³ per ton of SMW. {A - 0.082) 1C. SMW is gasified as in IB but with gasifying 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 IB. (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 elemental composition of combustible part of SMW corresponds to formula CH_{X 8}O_0.75N₀ -S₀.₀₀₄.

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 consumption (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, C0₂ - 10.5, 0₂ -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. 10

% ) : N₂ - 53 . 8 , C0₂ - 11 . 4 , 0₂ - 1 . 2 , Ar - 0 . 6 , H₂0 - 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 without 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 is ~ 400 MJ higher than that in 2B,C,D. Note that processing of highly humid wastes according to the above described method requires generally lower A than described in RU-2079051, since ratio 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 process, as compared with the use of steam from external source, since heat loss with smoke gases at the stage 11

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 temperature of combustion of the product gas in the burner with the same gain in efficiency on aftercom- bustion stage.

Claims

12 CLAIMS

1. A method for processing combustible solid municipal wastes, which includes charging wastes into reactor, possibly in a mixture with pieces of solid incombustible nonmelting material; supplying to said reactor a gasifying agent containing oxygen at the part of the reactor where solid processing products accumulate; discharging from the reactor solid processing products, and withdrawal of the products of drying, pyrolysis and combustion products as the product gas, the gasification being achieved by successive stay of the wastes in the zones of heating, pyrolysis, coking, combustion (oxidation) , and cooling; control of the temperature in the combustion zone within 800 - 1300 °C through variation of at least one of the following parameters, mass fraction of oxygen in the gasifying agent "a", mass fraction of incombustibles "Jb" , and mass fraction of combustibles "c" in the mixture charged into reactor; burning of at least part of the combustible gas produced, c h a r a c t e r i s e d in that the smoke gas, preferably in mixture with air, is used as the gasifying agent, mass fraction of oxygen in the gasifying agent, mass fraction of incombustibles in SMW, and mass fraction of combustibles in SMW being chosen so as to meet the condition 0.022 < ah/c < 0.1.

2. A method according to Claim 1, char ac teri sed in that the wastes are dried with the heat of smoke gases used as the gasifying agent, the steam evaporated in drying being incorporated in the gasifying agent.