EP0750731B1 - Destructive, especially complete combustion, process - Google Patents

Abstract

For the purposes of the complete destruction, especially combustion, of carbon compounds like plastics, hydrocarbons, uncoated or coated paper, slurries and timber waste mixed with halogen-containing material like PVC, i.e. practically sorted domestic rubbish, the hydrocarbons are first mixed in a preliminary stage (7) with hot sand in a water vapour atmosphere, during which water gas is separated off. The degassed residue is taken with the now cooled sand to a fluidised bed combustion process and the gaseous fraction formed in the preliminary stage (7) is taken to a post-combustion process using the fluidised bed exhaust gas as the oxidizer. The hot exhaust gas produced can now be cooled in the ordinary way.

Description

The invention relates to a method for the complete combustion of carbon compounds, such as. B. plastics, hydrocarbons, uncoated or coated paper, sewage sludge and with halogen-containing material such as PVC, mixed wood waste, in a fluidized bed combustion chamber (1) according to the preamble of claims 1 or 2.

It is known that plastic waste burns quickly with the formation of soot, resulting in a slag-like flame-retardant melt which contains relatively large hydrocarbon molecules, soot but also dioxin components through condensation and causes a loss in calorific value due to the unburned. It is also known that the sand used for fluidization melts due to excessively high bed temperatures and gives rise to difficulties due to grain size changes in the fluidized bed. The formation of soot and high-molecular hydrocarbons leads to increased CO values in the flue gas and increases the organic carbon content in the exhaust gas from the incineration plant. High combustion chamber temperatures improve burnout. In a fluidized bed, however, the temperature of the combustion chamber is limited by the ash softening point.

From DE-OS 25 32 994 a process for drying and optionally pyrolysing products is known, water and / or usable products being removed from the material to be burned by a heat treatment process. However, no two-stage combustion is disclosed. PCT 93/11388 shows a similar process for drying substances to be burned, which, in contrast to our invention, intensifies the combustion and thus promotes clumping and only partial combustion in organic substances. From DE-OS 20 38 545 a system for the destruction of solid waste in a combustion device under pressure and with the inclusion of a gas turbine is known, the exhaust gases of which are used to treat the solid waste to be burned, which after its coking and before its combustion is still used for waste water purification , whereby water gas is generated.

DE-A1 4026272 discloses the staged incineration of waste by degassing and incineration in two separate fluidized beds. The degassing takes place in a flue gas atmosphere by converting CO ₂ into CO, which means that only a small amount of heat can be removed from the plastic gasification process by chemical setting, so that the plastic ultimately melts and encrusts. The resulting flammable exhaust gases are burned in a downstream combustion chamber without additional fuels at temperatures above 1200 ° C. In the gasification zone, when there is a lack of oxygen, the combustion is throttled, and when plastic waste is burned, the same melts, so that lumps form which give rise to disturbances in the fluidized bed. In order to counter this disadvantage, in our invention the heat is supplied not by combustion but by by the hot sand from the combustion, whereby at the same time gas gas is generated in a heat-consuming process by the gasification in a water vapor atmosphere, so that the sand material is cooled and high-energy (hydrogen-containing) exhaust gases are produced, the combustion of which enables a higher temperature to be reached in the downstream combustion chamber. In our invention, heat is chemically bound and later released again in the actual combustion.

DE-A-3726643 discloses the drying of moist brown coal before it is burned, the vapors being recirculated, that is to say no conversion of the hydrocarbons takes place. JP-A-56042008 discloses a three-stage combustion in a divided combustion chamber, the fuel to be burned being first pyrolyzed in the absence of air in the presence of a hot heat transfer medium and then being burned in a fluidized bed after overflowing a weir. The heat transfer medium is then returned hot to the pyrolysis chamber after the burnout. The combustible gases generated during pyrolysis and residual combustion are then burned in a common room.

The invention has set itself the task of eliminating the disadvantages mentioned and simplifying the plant required for this, the combustion of the carbon compounds being controlled so that the formation of slag is avoided and the combustible part is completely converted into heat, with the volatile substances are first split off in a water vapor atmosphere at temperatures between 300 ° C and 850 ° C, whereby the calorific value of the substance is reduced before the actual combustion, so that the combustion temperature certainly remains below the softening of the sand and the fluidization property of the sand is not influenced by grain size.

A major advantage of the invention is seen in the possibility of removing sewage sludge, which is first dried under vacuum conditions and steam formation and then degassed and burned.

This object is achieved by the characterizing features of claims 1 or 2.

Essential embodiments of the invention are specified in subclaims 3-10.

The invention is shown, for example and schematically, in the connected FIGS. 1 and 2.

1 shows a combustion based on the principle of a circulating fluidized bed. In the case of a circulating fluidized bed, the combustion chamber (1) is essentially filled with a turbulent gas / solid mixture which is separated into a gaseous and a solid fraction in a downstream cyclone (2). The solid is returned to the combustion chamber (1) via line (3, 4), and is enriched with fuel via line (11). In most cases, sand is used as a solid, which in its fine-grained form is a fluidizing agent for the fuel. The fuel sand mixture is fluidized in the combustion chamber (1) by air injection (12), the combustion being controlled by supplying secondary air (5) and the tertiary air (6) to achieve complete combustion. Ashes are removed in line (3 or 4). In the case of combustion of carbon compounds, such as. B. Plastics or hydrocarbons, uncoated or coated paper, sewage sludge and also wood waste with or without PVC coatings, abbreviated to rubbish, arise locally due to the rapid combustion of the plastics, particularly high temperatures that melt the fuel and form slag-like residues and the make it more difficult to burn off the substances to be burned and form lumps which cause difficulties in fluidizing in the fluidized bed and include unburned substances. For the complete breakdown of these carbon compounds, a preliminary stage (7) is provided in the form of a fluidized-bed fluidized fluid chamber in which the volatile substances are split off from the carbon compounds in a water vapor atmosphere, with an endothermic conversion of the water vapor into hydrogen and the volatile carbon into carbon monoxide . This measure reduces the calorific value of the carbon compounds, so that a lower combustion chamber temperature is reached during the actual combustion in the combustion chamber (1), at which slag formation is no longer possible. Of the The conversion of plastics in a steam atmosphere does not take place quantitatively to CO, CO ₂ and H ₂ . Some of the hydrocarbons remain in the form of aliphatic C _x H _y compounds. The formation of aromatics is suppressed because they only form at very low oxygen partial pressures. At higher temperatures, aromatics already form at higher oxygen partial pressures and are therefore more difficult to suppress. The formation of soot and aromatics is undesirable and leads to the precursors of dioxins. So indirectly for the formation of dioxins.

The precursor (7) passes hotter sand as heat transfer medium, which is cooled by the melting and gasification process and the endothermic water gas formation. In this sense, the fluidized bed combustion is expediently designed as a circulating fluidized bed, the circulating solid being sand and / or ash, in which carbon compounds to be split are introduced via a steam supply (13), the carbon compounds melting as a result of the high temperature of the sand but as a result of Lack of oxygen can not burn and spread in the sand. The volatile gaseous components react with the water vapor for the heat-consuming water gas reaction and hydrogen and carbon monoxide are formed. It is essential that the gas-forming reaction takes place in a highly turbulent zone, that is to say in a fluidized bed, so that the melted, partly decomposing substances are evenly distributed in the sand and no drop formation occurs. The amount of sand is expediently more than 30 times greater in weight than that of the carbon compounds to be mined.

This fuel-sand mixture is now burned out with air supply in the actual fluidized bed combustion, so that practically burned-out sand and combustion exhaust gas are separated in the cyclone (2). This exhaust gas can now be mixed in a further combustion chamber (8) with the water gas formed in the preliminary stage (7), whereby the temperature of the exhaust gas rises further as a result of the combustion that occurs, which then is beneficial in the connected heat exchangers (9 and 10) Formation of water vapor, in particular for the generation of electrical energy, can be cooled.

So that the gas does not contain undesirable components such. B. hydrochloric acid or hydrogen sulfide enters the combustion chamber (8), this can be cleaned in a dry absorption chamber (14) from the undesirable gas components and the heat exchanger (10) leaving the exhaust gas is low in pollutant. Part of this gas can be reintroduced into the fluidized bed of the combustion chamber (1) to improve the fluidization or temperature control of the fluidized bed.

In the case of sewage sludge incineration in particular, it makes sense to operate the preliminary stage (7) with negative pressure, the steam required being generated by vacuum evaporation in a degasser (15) and the sewage sludge thereby being dewatered or the liquid separated from the sewage sludge being evaporated.

It is essential that the distribution, melting and decomposition of the fuel takes place in a water vapor atmosphere and the resulting gaseous products are then burned with excess air, the temperature rise during the combustion of the now simplified hydrocarbon compounds resulting in no increased aromatics formation and thus also no dioxin formation or formation Dioxin precursors leads. The CO and H ₂ formation (water gas reaction) thus lowers the temperature during the decomposition of the fuel without aromatic compounds and / or soot being formed.

Within the scope of the invention, as shown in FIG. 2, a stationary fluidized bed can also be used, or the gas generated can also be blown into the fluidized bed, thus simplifying the circuit. If necessary, one of the two heat exchangers (9 and 10) can also be arranged between the fluidized bed and the injection of the degassing gas. The latter measure is particularly effective when the gas is blown in above the stationary fluidized bed.

In a simplified embodiment, the fuel is introduced into the fluidized bed (plenum 20) via line (11) with steam supply (13) into the combustion chamber 1 ', the edge zone (18) of the fluidized bed in the combustion chamber 1' being fluidized with steam, during the oxygen input for combustion via the air injection (12) or recirculation gas injection he follows. As a result, the actual combustion in the center (19) of the fluidized bed is achieved and can be regulated simply by supplying air, while the fuel gasifies by supplying heat to the sinking sand (arrow 17 ') in the peripheral zone (18) and in some cases splits the fluidizing water vapor into hydrogen, the released oxygen is set off by the hot carbon with the formation of CO. These gaseous gasification points rise in the fluidized bed (arrow 17) and burn through the primary and secondary air supply (16) above the fuel inlet.

This results in a circulation of the sand (arrow 17 and 17 ') in the fluidized bed, which flows upwards in the center and downwards in the peripheral zone. Hot gas is thus continuously fed to the gasification in the edge zone (18), which is cooled in it and heated again at the combustion zone in the center (19) during its ascent.

Claims (10)

1. Process for complete combustion of carbon composites, for example plastic materials, hydrocarbons, uncoated or coated paper, sewage sludge and mixed wood wastes with halogen containing material, such as PVC, in a fluidised bed combustion chamber (1) in the presence of fluidising material in a solid a gaseous form, for example sand and water vapour, and the carbon composites are initially partially decomposed in a pre-stage (7), whereafter still combustible remainders are combusted and gaseous reaction products produced in the pre-stage (7) are combusted with the waste gas of the fluidised bed combustion chamber (1), characterised in that, for the purpose of preventing a formation of high molecular condensation products, the conversion of carbon is carried out in two stages, and carbon composites to be combusted are entered with at least 30 times the weight volume of hot sand from the recirculation as a heat carrier in low oxygen atmosphere into a turbulent fluidised bed, which is fluidised by delivery of water vapour (13), of the pre-stage (7) of the fluidised bed combustion chamber (1), and the carbon composites are split in a water vapour atmosphere whilst forming water gas and light volatile low molecular hydrocarbons, and discharging decomposing reactions of light volatile low molecular hydrocarbons, which formed during gasification by melting or splitting the hydrocarbon composites, in a water vapour containing atmosphere at temperatures above 400°C, and gaseous reaction products, such as water gas and light volatile low molecular hydrocarbons, are separated from sand and remaining solid and low volatile portions of the carbon composites, and in the fluidised bed combustion chamber (1) the remaining solid or low volatile portion of the carbon composites, such as hydrocarbons and produced carbon, are combusted with further combustible materials, such as coal and/or gaseous reaction products from gasification reaction, with a surplus of air whilst heating the sand.
2. Process for complete combustion of carbon composites, for example plastic materials, hydrocarbons, uncoated or coated paper, sewage sludge and wood wastes mixed with halogen containing material, such as PVC, in a fluidised bed combustion chamber (1) in the presence of fluidising means in a solid and gaseous state, for example sand and water vapour, and the carbon composites are initially partially decomposed, and still combustible remainders are subsequently combusted, and produced gaseous reaction products are combusted with the waste gas of the fluidised bed combustion chamber (1), characterised in that, for the purpose of preventing a formation of high molecular condensation products, the conversion of carbon is carried out in two stages, and carbon composites to be combusted are entered with at least 30 times the weight volume of hot sand from the internal recirculation as a heat carrier in low oxygen atmosphere into a turbulent edge zone of the fluidised bed combustion chamber (1), which is fluidised by delivery of water vapour (13), and the carbon composites are split in a water vapour atmosphere whilst forming water gas and light volatile low molecular hydrocarbons, and discharging decomposing reactions of light volatile low molecular hydrocarbons, which formed during gasification by melting or splitting the hydrocarbon composites, takes place in a water vapour containing atmosphere at temperatures above 400°C, and gaseous reaction products, such as water gas and light volatile low molecular hydrocarbons, are separated from sand and remaining solid and low volatile portions of carbon composites, and in the central zone of the fluidised bed combustion chamber (1) the remaining solid or low volatile portion of the carbon composites, such as hydrocarbons and developed carbon, are combusted in an oxidising atmosphere with other combustible materials, such as coal and/or gaseous reaction products from gasification reaction, with a surplus of air whilst heating the sand.
3. Process according to Claim 1, characterised in that degasification is performed with a drop in sand temperature, and light volatile components react in the water vapour atmosphere whilst using up energy preferably to hydrogen and carbonmonoxide, and remaining still combustible solids finely distributed in the sand are combusted together with the sand in a downstream fluidised bed combustion chamber (1) with oxygen delivery (air blown in 5, 6, 12).
4. Process according to Claim 1, characterised in that in the pre-stage (7) a controlled supply of oxygen, in particular in the form of air or recirculation gas, is delivered in addition to the delivery of steam (13), so that the temperature of exiting, partially degassed substance/sand mixture is maintained whilst dropping only insignificantly below the temperature of entering hot sand, in particular between 450°C and 700°C.
5. Process according to Claim 1, characterised in that water, water vapour and air or oxygen are fed into the pre-stage (7) for the purpose of temperature regulation, and during gasification in the pre-stage (7), the pressure is maintained at below 1 Bar absolute.
6. Process according to Claim 1, characterised in that gaseous reaction products produced during gasification are, after having left the pre-stage (7), delivered to a gas purification (14) where formed hydrochloric acid and other halogen composites, and if appropriate hydrogen sulfide, are removed (Fig. 1).
7. Process according to Claim 6, characterised in that the distribution of gaseous reaction products formed in the pre-stage (7) in the fluidised bed combustion chamber (1) is carried out via injectors which use recirculation gas with higher pressure or water vapour as propellant medium, or preferably the required water vapour for the pre-stage is obtained from cooled down hot waste water by generating a negative pressure in a vacuum degasifier with vapour jet apparatuses.
8. Process according to Claim 2, characterised in that gasification and combustion are carried out in a fluidised bed (20), and the fuel (11) and the vapour (13) are premixed under low pressure, preferably negative pressure, and then blown into an injector by water vapour at higher pressure into the edge zones (18) of the fluidised bed (20), mixed with hot sand and evaporated in a low oxygen atmosphere, and the gaseous reaction products are discharged upwards out of the gasification zone and combusted with recirculation gas and/or primary air locally above the gasification zone, and the low volatile fuel portions and solid residues are ducted with the sand into a downstream combustion zone (19) at the side or below the degasification zone and thereafter combusted with air (12) and/or recirculation gas (Fig. 2).
9. Process according to Claim 2, characterised in that substances to be combusted are mixed with sand of more than 600°C, which is extracted from the fluidised bed of the combustion chamber, in an edge zone (18) with water vapour and at least partially gasified, and the hot sand which is entered into the edge zone (18) as a heat carrier delivers energy required for producing hydrogen (Fig. 2).
10. Process according to Claim 2, characterised in that solids discharged from the edge zone (8) are heated by way of supplying oxygen (12) to above 600°C and thereafter returned into a water vapour containing atmosphere for further degasification by way of separation (Fig. 2).

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