WO1985002454A1

WO1985002454A1 - Method of decreasing particulate and gaseous emissions and recovering heat when burning or smelting substances containing ash and sulphur

Info

Publication number: WO1985002454A1
Application number: PCT/FI1984/000085
Authority: WO
Inventors: Pertti Sarkomaa
Original assignee: Pertti Sarkomaa
Priority date: 1983-11-23
Filing date: 1984-11-22
Publication date: 1985-06-06
Also published as: FI84561B; FI834283A0; FI834283A; EP0162094A1

Abstract

A method for the combustion of fuels containing ash and sulphur or for the smelting of substances and the recovery of heat so that simultaneously inorganic compounds (ash) and, when required, sulphur oxides, are separated from the gases. In the method according to the invention the combustion or smelting is performed in a manner known per se by introducing oxygen in one or more stages. Gases containing melts and vapours can be cleaned by means of a smelt cyclone before the cleaning and heat recovery part. In the method according to the invention, the gases containing melts and vapours are conveyed to a particle cooler in which the temperature of the particles has been chosen so, typically T <= 900oC, that the smelts and vapours stick onto the surface of the particles in the form of a solid layer. If the new particles being introduced to the particle cooler consist partly or entirely of a material forming sulphate, sulphur oxides can be simultaneously separated from the gas.

Description

METHOD OF DECREASING PARTICULATE AND GASEOUS EMISSIONS AND RECOVERING HEAT WHEN BURNING OR SMELTING SUBSTANCES CON¬ TAINING ASH AND SULPHUR

Technical Field:

The present invention relates to a method for producing energy from fuels containing ash (e.g. coal , peat, wood, bark) or for smelting concentrates or other substances in such a way that the inorganic compounds in the flue gases (the ash of fuels among others) are separated partly within the combustion chamber or smelting chamber and eventual ly in a smelt cyclone disposed after said chamber, but mainly within a so cal led particle cooler disposed after the com¬ bustion or smelting chamber,, into which cooler the flue gas enters at such a high temperature that the ash is either molten or vapourized. The conditions within the particle cooler can be chosen regardless of the combustion or smelting method so that the separation of the inorganic compounds and the removal of the environmental ly or othei— wise harmful gaseous oxides (SO and NO among others) can be carried out in the most effective way.

Because of the increased use of coal since 1973, the environmental impacts of energy production have attracted a lot of attention. When looking at the development in the entire world, it can be estimated that the increased energy demand wi l l be met mainly by means of coal and fission-based nuclear energy.

The compe itiveness of conventional energy (coal , biofuels) is weakened essential ly by the increasing invest¬ ments required by environmental protection (NO , SO dust). This is particularly emphasized with regard to the most important source of conventional eπery, i.e. coal .

Background Art:

Known combustion methods can be divided into the following main groups:

1. Suspension or pulzerized fuel combustion: for pulverized fuels, l iquid fuels, gaseous fuels

2. Fluidized bed combustion: suitable for al lmost al l fuels

3. Stoker combustion: for lump, sol id fuels.

Suspension combustion is today the only method used in high capacity units. Coal is mil led into fine powder with an average particle size of 50 to 150 μ m and is burned by means of several diffusion burners in a combustion chamber or furnace of a boi ler.

Suspension combustion of sol id fuels containing ash, such as coal , is carried out nowadays so that the ash particles are cooled by means of radiation into a sol id state before the cool ing surfaces disposed in the end of the radiation portion in order to prevent slagging of the surfaces (e.g. superheaters). Especial ly in big units, (Φ -

100 MW ) this leads to big radiation chambers with a power/- volume ratio as low as 0.1 to 0.2 MW/M 3. Since the combus¬ tion temperature cannot be control led accurately, it is not possible to achieve efficient SO capture in this kind of a boi ler without a separate wet scrubber using an alkal ine solution. The big coal-fired boi lers based on suspension combustion are thus considerably bigger and more expensive than gas and oi l boi lers of equal capacity. As environmental protection wi l l make more stringent demands, a further burden wi l l be the separation system required in restricting the SO- emissions. Nor is it possible to recover the ash of the fuel without separate preci pi tators .

Fluidized bed combustion has rapidly become more general in units burning sol id fuels and having a thermal capacity below 100 MW as it is suitable for very different kinds of fules, primari ly due to its stabi l ity which is based on its wide temperature capacity. In fluidized bed combustion, the temperature can be maintained almost stable in the entire combustion chamber, which makes a simultaneous

OMPΓ combustion and absorption of sulphur oxides into Ca-based bed material possible. Under oxidizing conditions, the SO gases react with ca-lcium or other suitable metals and form sulphate (MeSO.) . Today there is a wide variety of commer¬ cial ly avai lable fluidized bed reactors for sulphurous fuels which perform a s imu I ataneous combustion and SO absorption in one and the same perfectly stirred chamber. The essential drawback of this method is that sulphates are decomposed at temperaratures exceeding 900 C and form SO- and metal oxides and an efficient SO absorption cannot be reached at x _rt a temperature exceeding 900 C.

Under reducing conditions it is principal ly possible to bind sulphur into a metal sulphide, but in practise an efficient sulphide absorption requires a very high tempera¬ ture, T > 1300 C. At this high a temperature, the ash of the fuel usual ly smelts and fluidized bed combustion cannot be used due to sintering of the bed material . The lower l imit of the temperature range is l imited on the other hand by the request for sufficient reaction speed. The practical l imit for proper coal combustion (soot, PAH, CO emissions) is T - 850 °C.

The decomposing of sulphate, smelting of ash and the requirement for efficient combustion l imit the temperature range of combustion and SO absorption in the same reactor to the narrow range of 850 C - T - 900 . The ash bound to the so cal led organic matter of the fuel cannot be recovered notably by any method based on fluidized bed combustion, but the boi lers always have to be provided with dust separation equipment. Due to the narrow temperature range, the turndown ratio of the boi ler is l imited and a compromise between the combustion efficiency and a good SO absorption has to be made .

Attempts have been made to solve these problems by dividing the bed material of a fluidized bed reactor into two flows, and by cool ing one of these flows, a constant temperature (typical ly 850 °C) can be maintained in the combustion chamber within the entire capacity range. These solutions lead to a-compiex construction which-has not been proven feasible in commercial units. Another essential drawback of this method is that the ash flow passing through the cyclone (vapourized, smal l particles with a diameter of approximately < 30 μm) cannot be recovered, but a separate dust separation equipment is required.

European patent appl ication 27 280 describes a method of improving dust separation by agglomerating the ash particles of a secondary cyclone in a local hot spot of the fluid bed. Ash agglomeration in a local hot spot of the bed is to be considered as a known method with several sl ightly varying appl ications. It is characteristic of al l these methods that most of the fluid bed operates at temperatures under the ash smelting or sintering temperature and the agglomeration of the ash is carried out by increasing the temperature local ly.

US Patent 4,198,212 describes a method of treating the product gas of coal gasification, wherein the organic im¬ purities (tars, acids, coal) contained by the fluid bed gasifier are separated by leading the gases through a cooled fluid bed, whereupon the impurities condensate on the sur¬ face of the bed material . In a gasification reactor, the bed material used is inert coal . A corresponding separation capacity may be achieved in a conventional counter-flow gasifier, in which the gases are discharged through a bed formed by new gasification material .

Stoker combustion is suitable only for sol id, lumped fuels. By a proper preparation of the fuel (e.g. pel letizing of peat), it is possible to get a better dust separation than in the known combustion methods described above. Stoker combustion cannot be appl ied in high-capacity units, because the ash easi ly smelts on the stoker and thus causes opera¬ tional disturbances. With regard to gaseous emissions.

O PI stoker combustion is not a good choise, since the binding of the SO compounds into metal sulphates is in practise not possible. Also with- regard to nitric oxi des , • convent i ona I stoker combustion cannot be recommended, since the general control labi l ity of a stoker combustion process is poor. There are always local areas which have s imu I ataneous ly a high temperature and excess oxygen. In practise the flue gases of stoker boi lers also require dust separation equip¬ ment .

Disclosure of the Invention:

By means of the the method according to the present invention the aforedescr ibed drawbacks can be el iminated to a great extent.

The method in accordance with the patent appl ication is characterized in that combustion or smelting is carried out in a manner known per se so that the inorganic compounds are either smelted or vapourized. The combustion can be performed as conventional combustion, suspension combustion or multi-stage combustion. In multi-stage combustion, the sol id fuel is first gasified and the combustion is then continued by adding oxygen into the gasification products so that the ash of the gas entering the particle cooler is either molten or vapourized. Conventional fluidized bed combustion cannot be used instead of the combustion of the method according to the invention, since the smelting of the ash would cause sintering of the bed. In staged combustion, however, a fluidized bed gasifier can be used as the first stage. Staged combustion can be recommended, since it does not require pulverizing of the fuel (the average particle size in dust combustion is approx. < 150 μm) and in the gasification stage the nitrogen of the fuel can be trans¬ formed into molecular nitrogen (N.,) . It is known that molecular nitrogen (N-) is transformed into nitrogen oxide slower than nitrogen bound in the fuel . Furthermore, the excess oxygen in the combustion fol lowing the gasification can be maintained a^*t a low level , since the compounds pre¬ sent in the last combustion stage are mainly gaseous.

After the combustion or smelting, the gases containing vapourized and smolten inorganic compounds are conveyed to a particle cooler in which the inorganic melts and vapours of the gases are layered on the surface of the particles in a sol id, state. The sol id material forms spherical pel lets which are suitable for further processing. In case the gases contain noxious oxides (e.g. SO ) , the particle material can be chosen so (e.g. CaCO-) that the oxides bind themselves to the material (e.g. CaSO„) . The temperature of the particles is set by means of cool ing to such a range - in the absorp¬ tion of sulphur typical ly 500 to 900 °C - that the condi¬ tions are favourable for the noxious substance to react with the particle material .

The particle cooler mentioned in this invention is an apparatus in which particles colder than the gas are mixed with the gas, whereby the molten and vapourized ash con¬ tained by the gases is agglomerated on the surface of the particles in a sol id state. In order to maintain the tempe¬ rature difference between the particles and the gas flow, the particles are cooled inside and/or outside the particle cooler. The wal ls of the chamber of the particle cooler can be uncoo I ed or partly or entirely cooled. A conventional fluidized bed cooler or a circulating fluidized bed cooler are examples of a particle cooler, but the operation of a particle cooler can be achieved by means of various arrange¬ ments of the gas and particle flows and cool ing etc. Based on the reciprocal movement of the gas and the particles the particle cooler can be a para I 1 e I -f1 ow cooler, a counter- flow cooler or a para 11 e I -fi ow/countei—flow cooler and the direction of the particle flow can be either perpendicular to or obl ique with regard to the direction of the gas flow. The longitudinal axis of the chamber of the particle cooler is preferably in a vertical direction, but it can also be in horizontal or incl irred position.

The present invention offers five essential advantages First, the separation of the inorganic compounds of the gas, e.g. the ash of flue gases, is so efficient that consider¬ able investment savings are created in the gas cleaning equipment and the heat of the gases is more easi ly recovered in smelting. Second, a certain level is achieved in the SO separation with a lower Ca/S ratio than e.g. in fluidized bed combustion, wherefore the separation of sulphur is less expensive.. Third, the method solves the problem of the vapourized ash compounds present in pressurized combustion of sol id fuels in gas turbine appl ications. Fourth, when using the method of the invention, the boi ler and the boi ler plant are smal ler and there is no need for a big combustion chamber for the cool ing of the gases by means of radiation to a temperature below the smelting point of the ash, which is an essential advantage, especial ly with regard to a large boi ler. Fifth, the combustion can be carried out under optimum conditions for each fuel separately from sulfur absorption and, as far as is required, from the recovery of heat.

Brief Descr i Pt_jon_oj_Mthe Drawings :

The invention wi l l be further described in the fol lo¬ wing with reference to the accompanying drawings in which; -

Fig. 1 i l lustrates a combustion method for the combus¬ tion of fuels having a low ash content;

Fig. 2 i l lustrates a combustion method for two-stage combustion of fuels having a high ash content;

Fig. 3 i l lustrates a method in accordance with the invention for the smelting of oxide concentrates; Fig. 4 illustrates a method in accordance with the invention for the combustion of sulphur containing fuel having a low ash conrtent; and

Fig. 5 illustrates a method in accordance with the invention applied in an open gasturbine process using a soI id fueI .

Referring now to the drawings. Fig. 1 illustrates a combustion method for fuels having a low ash content. The burning of the fuel takes place in a combustion chamber 1 into which fuel and oxygen are brought. The combustion is carried out at such a temperature that the ash contained by the gas which is conducted to the lower chamber 2 is vapourized or molten. A heat recovery part is disposed above a discharge opening 3 of the chamber 2. Said heat recovery part comprises a particle cooler 4 and eventually heat surfaces 5. The temperature of the particle cooler 4 is chosen so that the inorganic vapours and melts contained by the gases are agglomerated on the surface of the particles in a solid state. If required, an efficient separation of sulphur oxides from the gases can be achieved in the particle cooler 4. The clean flue gases are discharged through an opening 9. The inorganic matter which has been separated in the particle cooler can be removed through a separate outlet 10 of the cooler or together with the melt separated into the chamber 2. The molten ash flows through an opening 6 on the bottom of the chamber 2 to a granulating chamber 7 and is discharged together with the cooling water by means of a pump 8.

Fig. 2 illustrates a method according to the invention for two-stage combustion of a fuel having a high ash content. The gasifying (under-stoichiometric) combustion is carried out in a chamber 11 in such a way that ash may remain in a solid form in this stage. Excess air required

O PI for further combustion is brought to the process via a connecting means 12 disposed between the gasifying chamber 11 and a smelt cyclone 13. Thereby the temperature rises above the smelting temperature of the ash components. The molten ash separated in the smelt cyclone 13 flows through an opening 16 to a granulation chamber 17. The ash separated in the particle cooler is removed either through a separate outlet means 20 or from the bottom of the granulation chamber 17 by means of a pump 18. The heat recovery part comprises a particle cooler 14 and eventual ly a secondary cooler 15. Clean gases are discharged through an outlet opening 19. .

The problems of suspension combustion (expensive boi ler, expensive flue gas cleaning, pulverization of the fuel) can be avoided by the above described method, since the combustion chamber can be dimensioned entirely in accordance with the requirements of combustion and heat recovery is carried out in a smal l-sized cooler which can be bui lt ful l of heat transfer surface. The smal l size of the boi ler creates savings also in the boi ler construction i nvestments .

In comparison with the most popular method, fluidized bed combustion, investment savings are obtained (gas cleaning, smal l-sized and simple boi ler) and a good sulphur separation is achieved within a wide capacity range.

Fig. 3 i l lustrates a method in accordance with the invention for the smelting of a concentrate or other soiid substance. Fuel , the matter to be smelted and the oxidizer are brought into a combustion chamber 21 and the combustion is carried out so that the sol id material smelts. In case the sol id material is to be reduced, the combustion in the chamber 21 can be of an under-sto i chi ometr i c nature. Oxygen containing gas is brought into the reaction through a connecting duct 22 disposed between the chamber 21 and a smelt cyclone 23. This is carried out so that the viscosity of the molten inorganic compounds contained by the gases is suitable for the smelt cyclone 23. In the smelt cyclone, the molten compounds are separated mainly on the wal ls and flow further down to a lower furnace 27 where the phase separa¬ tion of the smelt takes place. Slag is discharged through an opening 26 and the heavier phase through an opening 28.

Gases containing vapours and melts are conveyed to a particle separator 24 where the vapours and the melts stick onto the surface of the particles in a sol id, state. Pel le- t i zed inorganic matter is discharged through an outlet opening 30.

Clean gases are cooled further in a cooler 25, if required, and removed through an outlet opening 29.

The smelting method can also be used in the smelting of stone and glass, wherefore it can be appl ied e.g. in the insulation industry in the manufacture of glass wool and mineral wool . The combustible gases formed in the smelting of the wool process can be uti l ized in the drying stages of the process, since when using the smelting in accordance with the invention they are clean enough.

Fig. 4 i l lustrates a method in accordance with the invention for the combustion of high-sulphur fuels having a low ash content. Combustion is carried out in a chamber 31 as conventional flame combustion. Flue gases containing sulphur oxides are conveyed to a particle cooler 34 where the temperature of the particles has been chosen so that SO sets into calcium sulphate and simultaneousl the vapourized ash compounds (e.g. vanadium compounds) are condensated on the surface of the particles.

The Ca-bearing material required for the absorption of sulphur is introduced through an inlet opening 32. The material containing sulphate is discharged through an outlet opening 33. The cleaned gases can be cooled further in a cooler 35, if required.

It is, of course, clear that the thermal energy recovered in the coolers of Figs. 1 to 4 can be used e.g. in processes, power plants and for heating.

Fig. 5 i l lustrates the method in accordance with the invention appl ied in an open gasturbine process which uses sol id fuels. A compressor 41 compresses air, part of which is used in a gasifier 42 as an oxidizer. Most of the air is conducted through cool ing pipes 43 of the gasifier and cool ing pipes 44 of the particle cooler into a combustion chamber 45 of the open gasturbine to act there as combustion air or, a minor part of the air, as secondary air of the staged combustion so that the ash contained by the gas is in the form of a vapour or a melt having such a viscosity that the separation of molten ash in a smelt cyclone 47 is efficient. Most of the ash is removed through an opening 48 on the bottom of the cyclone. Gases containing molten ash and vapours are conveyed through an opening 49 to a particle cooler 50 where the molten and vapourized ash sti l l con-, tained by the gases (containing e.g. alkal i and vanadium compounds) sticks on the surfaces of the particles mainly in the lower part of the particle cooler. Under reducing conditions, sulphur does not set into sulphate, wherefore the particle cooler 50 separates primari ly melt and vapours.

Material is introduced into the particle cooler through an opening 51 and discharged from the particle cooler through an opening 52.

The cleaned gases are conducted through an opening 53 to the -combustion chamber 45 of the open gasturbine. If required, also air 54 is conducted to said chamber from the compressor 41.

Gases 55 of the combustion chamber are conveyed to a gasturbine 56 which rotates both the compressor 41 and a generator 57.

In order to absorb the sulphur oxides gases 58 dis¬ charged from the turbine are conveyed to another particle cooler 59 which contains Ca-compounds and the temperature of which has been chosen so that an efficient SO absorption is reached.

Clean flue gases having a low sulphur content are pre¬ ferably conveyed to a flue gas boi ler or they can be used as such for drying or heating.

O PI

Claims

j__c a im:

1. A method for the combustion of fuels containing ash and sulphur or for the smelting of substances and the reco¬ very of heat in such a manner that at the same time in¬ organic compounds (ash) and, when required, sulphur oxides, are separated from the gases, characterized in that the combustion or smelting is carried out in a manner known per se in such a way that the inorganic compounds melt or vapourize and the gases containing molten or vapourized com¬ ponents are conveyed to a particle cooler the particles of which have such a temperature that the inorganic compounds which stick onto the surface of the particles remain in a so I id state.

2. A method according to claim 1 characterized in that particles introduced into the particle cooler form sulphate in order to remove sulphur oxides from the gases.

3. A method according to claim 1 or 2 characterized in that molten components are removed from the gases in a smelt cyclone before the gases are conducted to the particle coo I er.

4. A method according to any of claim 3 characterized in that recovered smelt is divided into separate phases.

5. A method according to any of claims 1 to 4 charac¬ terized in that the separation of the melts and vapours and the separation of the sulphur oxides is performed in sepa¬ rate particle coolers.

6. A method according to any of claims 1 to 5 charac¬ terized in that the thermal energy recovered in the particle coolers is used in an industrial process or in a power plant.

7. A method according to claim 6 characterized in that part of the clean gases from the particle cooler are re¬ cycled to the smelt chamber.

8. A method according to any of the preceding claims, characterized in that the combustion is under-sto ichi ometri c and the cleaned gases are used as raw material for an i n-

^■^fREX£F'

O PI WIPO dustrial process or a power plant.

9. A method according to claims 1 to 6 characterized in that sol id fuel rs gasified in an a i r-coo I ed- gas i fi er and the temperature of the gases produced is raised by combustion so that the inorganic substances smelt and/or vaporize, molten substances are removed in a smelt cyclone before the gases are conducted to an air-cooled particle cooler and the cleaned gases are conducted after combustion to a gas turbine driving a compressor which produces compressed coo I ing a i r.