CA2275646A1

CA2275646A1 - Method of gasifying solid fuels in a circulating fluidized bed

Info

Publication number: CA2275646A1
Application number: CA002275646A
Authority: CA
Inventors: Johannes Albrecht; Johannes Loeffler
Original assignee: Metallgesellschaft AG
Current assignee: Individual
Priority date: 1996-12-18
Filing date: 1997-12-01
Publication date: 1998-06-25
Also published as: AU5657598A; DE59702967D1; ES2155270T3; AU722068B2; EP0948583B1; WO1998027182A1; DE19652770A1; BR9714421A; EP0948583A1; JP2001506288A

Abstract

Combustible fuels, for example waste substances, biomass or coal, are gasified in a circulating fluidized bed with the addition of oxygenous gas at temperatures ranging from 700 to 1000 ~C. A fuel gas is obtained which contains dust and hydrocarbons, including higher hydrocarbons (C6+
hydrocarbons), and has a calorific value of between 2000 and 8000 kJ/m3. The dust-laden gas is passed through a separation chamber in which the higher hydrocarbons present in the gas are largely separated by the addition of gaseous oxygen at temperatures ranging from 800 to 1200 ~C and below the temperature of the ash melting point. The content of C6+ hydrocarbons present in the gas emerging from the separation chamber is at most 10 wt % of the initial content. The gas from the separation chamber is cooled and dedusted and passed through at least one bed or at least one reactor with granular solids which bind with pollutants. The gas-purification process is carried out without forming waste water.

Description

Method of Gasifyinq Solid Fuels in a Circulating~
Fluidized Bed Description This invention relates to a methad of gasifying solid fuels in a circulating fluidized bed, where the fuels are gasified in a gasification reactor by supplying oxygenous gas at tem-peratures in the range from 700 to 1000°C, a gas-solids mix-ture is supplied from the upper portion of the gasification reactor to a separator, from the separator there is withdrawn gas containing dust and hydrocarbons including higher hydro-carbons with more than 6 C atoms in the molecule (C6+-hydrocarbons) with a calorific value of 2000 to 8000 kJ/m3 and separate therefrom separated solids, and the solids are at least partly recirculated into the lower portion of the gasification reactor.
Such methods are known from DE-A-42 35 4I2 (to which corre-sponds US-A-5,425,317) and DE-A-44 12 004. In the known meth-ods, the gas coming from the separator and containing combus-tible components is gasified or burnt by forming liquid slag, and the liquid slag is removed from the process. The gas pro-duced during the combustion or gasification is cleaned in contact with washing liquid. It becomes necessary to process the used washing liquid or to dispose of the same.
It is the object underlying the invention to modify the known methods such that the entrained dust is removed dry, and wet washing with the formation of waste water in the gas cleaning is omitted. In accordance with the invention, this is achieved in the above method in that the dust-laden gas from the separator is passed through a separation chamber, where in the separation chamber the hydrocarbons contained in the gas are largely broken down by supplying gaseous oxygen at a temperature in the range from 800 to 1200°C and below the temperature of the ash melting point, thereby reducing the content of the higher hydrocarbons (C6+-hydrocarbons) in the gas to at most 10 wt-% of the content of these higher hydro-carbons in the gas coming from the separator, that the gas coming from the separation chamber is cooled, the cooled gas is passed through a deducting means and entrained dust is separated, that the cooled and dedusted gas is passed through at least one bed or a reactor with granular solids binding pollutants, and that the gas is subsequently dedusted.
Due to the conditions in the separation chamber and the con-ditions in the gas cleaning, there is no condensation and sublimation of the higher hydrocarbons (C6+) in the subse-quent gas cleaning means.
The solid fuels to be gasified can for instance be communal or industrial waste, biomasses or coals of various kinds. In the gasification of communal waste, the same is usually pre-sorted before the gasification, where in particular metal and glass parts are discarded. The remaining residual waste is then comminuted, e.g. to lump sizes of not more than 70 mm, before it is gasified. To increase the calorific value of the gas coming from the gasification reactor, the solid fuels may be dried before the gasification.

In the method in accordance with the invention no liquid re-sidual substances are produced. The ash withdrawn from the lower portion of the gasification reactor usually is so inert that it can still be utilized e.g. for road building, but at least the ash can easily be dumped. The entrained dust ob-tained in the dedusting means may contain heavy metals and is then disposed of in the usual way. Expediently, at least part of the entrained dust obtained is burnt or gasified in a com-bustion chamber at temperatures in the range from 1000 to 1500°C. It is recommended to supply the gaseous products formed in the combustion chamber into the gasification reac-tor.
Embodiments of the method will now be explained by means of the drawing, wherein:
Fig. 1 represents the flow diagram of a first method vari-ant, and Fig. 2 represents the flow diagram of a second method vari-ant.
In accordance with Fig. 1, the solid fuels to be gasified are supplied via line 1 to a gasification reactor 2, where they come in contact with hot gases and particles in the state of the circulating fluidized bed. Oxygenous fluidizing gas is supplied via line 3 and passed through a distribution chamber 4 with a grid 5 into the fluidized bed of the reactor 2. The oxygenous gas of line 3 may for instance be air or air en-riched with 02. The gasification in the reactor 2 is effected at temperatures of 700 to 1000°C and mostly at temperatures of at least 800°C. Ash is withdrawn through line 6 and, if necessary, dumped upon removal of metal components or sup-plied to a further usage, e.g. in road building.
At the upper end of the reactor 2 a gas=solids mixture leaves the reactor through the passage 8 and flows into a cyclone separator 9, from which dust-laden fuel gas is withdrawn through line 10. Solids obtained in the separator 9 are re-circulated into the lower portion of the reactor 2 through line I1.
The dust-laden gas of line 10 contains condensable hydrocar-bons and mostly carbonaceous entrained dusts. It is important to at least largely eliminate the higher hydrocarbons (C6+) and convert them to substances which do not condense at the given temperatures and partial pressures. For this purpose the gas is passed through a separation chamber 12, to which 02-containing gas, e.g..air, air enriched with oxygen, or technically pure oxygen is supplied through line 13. In the separation chamber 12 there are provided temperatures in the range from 800 to 1200°C and mostly 900 to 1100°C. It is im-portant that the temperature and the dwell time in the sepa-ration chamber 12 are chosen such that the formation of liq-uid slag is avoided and at the same time a sufficient break-down of the C6+-hydrocarbons is ensured.
The gas coming from the separation chamber 12 via line 15 contains various solids and ash particles, which here are re-ferred to as entrained dust. In a waste heat boiler 16 the gas is cooled to temperatures of about 150 to 300°C and is then supplied through line 17 to a dedusting means 18-. This may for instance be a cloth filter or an electrostatic fil-ter. The entrained dust obtained, which usually contains heavy metals, is withdrawn via line 19, and a part thereof may be supplied along the transpart line 20 to a combustion chamber 21. The residual entrained dust is removed from the process through line 22.
Oxygenous gas, e.g. air, air enriched with 02, or technically pure oxygen is supplied to the combustion chamber 21 through line 24, and the entrained dust supplied is burnt at tempera-tures in the range from 1000 to 1500°C. The solid or liquid or gaseous combustion products obtained in the process are altogether charged into the upper portion of the reactor 2, where they are absorbed by the fluidized bed. In contrast to Fig. 1, liquid slag from the combustion chamber 21 can also be withdrawn such that it does not get into the reactor 2, cf. Fig. 2.
From the dedusting means 18 a gas is withdrawn via line 25, which still has a disturbing content of pollutants. These pollutants are for instance mercury, chlorine and sulfur com-pounds. To largely eliminate these pollutants, the gas is first of all passed through an indirect cooler 26, and the temperature favorable for the subsequent treatment is ad-justed for instance in the range from 100 to 150°C. The cooled gas is supplied to a cleaning through line 27, where the formation of waste water is avoided. In one or several beds or reactors the gas to be cleaned is brought in contact with granular adsorbents. These adsorbents may for instance be arranged in the fixed bed, in the moving bed, or in the fluidized bed, or there may be used an entrained-bed reactor.
In the drawing a moving-bed reactor 30 is schematically rep-resented, to which through line 31 granular adsorbent is sup-plied, which in the reactor 30 farms a bed 33 slowly moving downwards. The gas to be cleaned flows through the bed in ap-proximately horizontal direction. The gas leaves the reactor 30 through line 35 and is passed through the filter 36 for dedusting, which filter may for instance be a cloth filter or an electrostatic filter. Cleaned gas leaves the filter 36 via line 37. The loaded adsorbent coming from the reactor 30 is discharged via line 38, mixed in line 39 with the solids separated in the filter 36, and withdrawn.
For the selection and application of suitable adsorbents known per se there are in particular the following possibili-ties: lime hydrate, activated carbon, hearth furnace coke or zeolites. The removal of mercury by means of a zeolite with a low aluminium content is described in the EP patent 638 351.
In the flow diagram of Fig. 2 the reference numerals already mentioned in conjunction with Fig. 1 have the meaning ex-plained there. In accordance with Fig. 2 the gas of line 27 containing pollutants is supplied to a spray-type absorber 40, to which lime milk and possibly other adsorbents are sup-plied through line 41. Gas and solids flow through line 42 to a filter 43, which may for instance be a cloth filter or an electrostatic filter. Via line 44, cleaned gas flows to an adsorber 46 for the separation of mercury, e.g. in a fixed zeolite bed, as is described in the EP patent 638 351. Chlo-ride-containing solids are withdrawn via line 45. In the com-bustion chamber 21, liquid slag is withdrawn through line 23, and the combustion gas is supplied through line 32 into the reactor 2.
Example:
To a procedure in accordance with Fig. 2 communal residual waste is supplied. The subsequent data have been calculated in part. The residual waste, which is delivered in an amount of 7500 kg/h, contains 24.5 wt-% moisture and 30 wt-% ash.
This waste is first of all dried to 5 wt-% residual moisture and is then gasified in the reactor 2 at 900°C and by supply-ing 6230 Nm3/h air. Per hour, 13000 Nm3 gas flow through line 10, the gas contains 48 g/Nm3 dust and 1 vol-% C6+-hydrocarbons. In the separation chamber 12 the dwell time of the gas is 1.5 seconds, air is supplied through line 13, and there is achieved an outlet temperature of 1000°C. The con-tent of C6+-hydrocarbons in line 15 is only 0.1 vol-%.
Through line 20, 400 kg/h dust are supplied to the combustion chamber 21, which is fed with 1860 Nm3/h air, and in which 1300°C are reached. Lime milk is supplied to the spray-type absorber 40, and the outlet temperature is maintained at _ 7 160°C. The adsorber 46 contains a fired zeolite bed for the removal of Hg.

Claims

1. A method of gasifying solid fuels in the circulating fluidized bed, where the fuels are gasified in a gasification reactor by supplying oxygenous gas at temperatures in the range from 700 to 1000°C, a gas-solids mixture is supplied from the upper portion of the gasification reactor to a separator, from the separator there is withdrawn gas containing dust and hydrocarbons including higher hydrocarbons with more than 6 C atoms in the molecule with a calorific value of 2000 to 8000 kJ/m3 and separate therefrom separated solids, and the solids are at least partly recirculated into the lower portion of the gasification reactor, characterized in that the dust-laden gas from the separator is passed through a separation chamber, where in the separation chamber the hydrocarbons contained in the gas are largely broken down by supplying gaseous oxygen at a temperature in the range from 800 to 1200°C and below the temprature of the ash melting point, thereby reducing the content of the higher hydrocarbons (C6+-hydrocarbons) in the gas to at most 10 wt-% of the content of these higher hydrocarbons in the gas coming from the separator, that the gas coming from the separation chamber is cooled, the cooled gas is passed through a dedusting means and entrained dust is separated, that the cooled and dedusted gas is passed through at least one bed or a reactor with granular solids binding pollutants, and that the gas is subsequently deducted.

2. The method as claimed in claim 1, characterized in that at least part of the entrained dust withdrawn from the dedusting means is reacted in a combustion chamber at temperatures in the range from 1000 to 1500°C by adding O2-containing gas.

3. The method as claimed in claim 2, characterized in that the gaseous products formed in the combustion chamber are admixed to the solids-containing gas formed in the gasification reactor.