WO1981002303A1

WO1981002303A1 - Combined production of electrical energy and fuel from peat

Info

Publication number: WO1981002303A1
Application number: PCT/SE1981/000041
Authority: WO
Inventors: P Collin
Original assignee: Stora Kopparbergs Bergslags Ab; P Collin
Priority date: 1980-02-18
Filing date: 1981-02-16
Publication date: 1981-08-20
Also published as: IE810308L; SE8001272L

Abstract

Dried peat fuel with or without addition of solid or liquid fuels is gasified (10) with oxygen or air enriched in oxygen at a pressure above atmospheric The produced gas with high calorific value is cooled (16) and purified (8) from dust and condensed water. The purified gas is burnt (18) with air and drives a gas turbine (21-23) and is then cooled in a waste heat boiler (4). The gas is then used for drying (2) of peat to a dried peat fuel. The gas turbine as well a steam turbine (27) coupled to the waste heat boiler are driving electric generators (24, 27) producing electric energy.

Description

TITLE OF INVENTION:

COMBINED PRODUCTION OF ELECTRICAL ENERGY AND FUEL

FROM PEAT.

TECHNICAL FIELD: The invention relates to a method for production of electrical energy from peat combined with the drying of additional peat to a peat fuel substantially consisting of dried peat.

BACKGROUND ART: According to a known method energy is produced in a gas turbine by combustion of a gas produced by gasifi-. cation of peat with air or oxygen. The sensible heat of the combustion gases leaving the gas turbine is used for drying of wet peat, which in a simple way is done by directly contacting gas and finely divided peat in drying apparatus of different kinds.

Depending on the construction of the dryers the drying may be performed cocurrently or counter-currently. Finely divided peat may be produced by disintegration e.g. in a disc shredder or by the Freztorf or milled-peat method, which directly delivers a peat-powder with a water content of 50-60 % by weight.

In said method exhaust gases leaving the turbine at a temperature of 250-300°C have been used for the drying. When the gases enter the dryer at this temperature, a com¬ paratively high temperature can be allowed in the moist gases leaving the dryer and nevertheless a substantial amount of the sensible heat of the gases is used for the drying. At an exit temperature of the moist gases of about 100 C there is no risk of saturating the gases with steam and reaching the dew-point, where the ability of the gases to absorb water expires. However, there may be problems when drying peat with exhaust gases at as high a tempera¬ ture as about 300 C because of the fire hazard of the material. This fire hazard is augmented in this case by the high content of free oxygen, introduced when burning the gases and adapting the temperature of the combustion gases for the turbine operation. The British patent 670 884 describes a method for producing a carbonized fuel from peat. The heat for dryi and carbonization is supplied by reheating and recircula ting the exhaust gases from the drying and carbonization, which first are purified from oil and tar. The gases are reheated in heat exchangers after first having been com¬ pressed and are expanded in turbines between the heat exchangers. The heat is supplied to the heat exchangers combustion gases obtained from a gas producer and burnt admixture of air. The producer is supplied with fuel as oil, tar or solid fuel which may be the peat coke produc in the carbonization plant.

When burning peat much water is formed, because o the chemical composition of the peat, but to this must b added, that peat fuel usually has a moisture content of 25-50 %. From this follows that the gases from the total combustion of peat or from gasification and burning have such a high moisture content that the dew-point of the gases becomes too high for using them for drying of peat at low temperature. This means that only part of the sensible heat content of the gases can be used. In moder gas turbine plants with efficient utilizatio of the ex¬ haust heat from the gas turbine e.g. for production of steam for use in steam turbines or other steam engines the total degree of efficiency for the machinery may pas 45 %. In such cases the temperature of the exhaust gases is below 200°C and often between 150-100°. It is impossi to utilize the heat of these gases for drying, because t dew-point is too high to allow a temperature reduction i the gases, leaving the dryer, down to a temperature belo 100°C and preferably down to 40-50°C. DISCLOSURE OF INVENTION:

According to the invention a gas of high calorific value is produced by gasification, i.e. partial combustion, of dried peat and/or coal or hydrocarbons with oxygen or air enriched in oxygen,to at least 50 % oxygen, at a pressure above atmospheric preferably 10-20 bar (1-2 MPa) . The gas is purified of dust and dehydrated by cooling, whereupon it is combusted with air and expanded in a gas- turbine. The exhaust gas from the gas turbine passes a waste heat boiler and is then used for drying moist peat. The gasification is performed under such conditions that a substantial amount of coolant must be added to keep the temperature within appropriate limits. The coolant may be carbon dioxide, steam or water e.g. water contained in peat. This means that, if the peat shall be gasified by oxygen gas, the drying of the peat need not achieve complete dryness but the dried peat may contain 5-20 % moisture de¬ pending on the conditions at the gasification. It may be mentioned that theoretically it is possible to gasify peat containing 40 % by weight water if the dry substance has a heat value of at least 4500 KCal/kg (18,9 MJ/kg) and simul¬ taneously maintain a temperature of the gasification of 1000-1100°C. The preferred limitation of the moisture content to 5-20 % by weight depends principally on the con- venience of gasifying the peat as a fine grained free flowing powder, which can be blown into a gas producer and which, under the temperature conditions prevailing in the producer, is gasified in a very short time. Starting with 50 % moisture the peat is preferably ground to a particle size of 0-3 mm. This powder then is dried according to the invention to about 15 % moisture content. The powder may be further pulverized in a high speed hammer mill to Q-AOO ili , the moisture on the same time sinking to about 10 % by weight. The powder processed in this way has proved to be suitable for gasification. The gas produced at the gasification of this powder has a high calorific value and a small volume because of the high pressure at the gasi fication. This small volume advantageously influences the size of the apparatus and facilitates the realizing of an efficient gas purification, which is needed to se a continuous operation of the turbine. This is of speci importance if the peat is gasified together with sulphu containing fuels as coal or oil. When using peat alone gas purification is performed in a simple way substanti by scrubbing with water and water solutions with an alk line reaction. Carbon dioxide may be recovered from the scrubbing liquids and returned to the reactor as coolan possibly used for injecting powdered fuels. The gas fro the gasifier may be cooled before the purification or c when purified. When cooling the gas to 20-30 % the gas separation from condensed water contains only 15-25 gra of water per kilogram of gas, which is only 25-30 % of saturation amount at 50 C. This leads to a dew-point we below 50 C also after combustion with air in the burner the gas turbine*. As mentioned before the fuels must be injected i the producer in a finely divided state, i.e. solid fuel as powder, and liquids as spray. Finely divided material peat and other solid fuels is blown into the producer p ferably using the cooled gas from the producer, which m be tapped off after the gas purification and recirculate through the fuel injection, or by carbon dioxide, which may be recovered from the gas purification. At a suitab performed gasification the heat content of the fuel is converted with small losses to the gas from the produce if the sensible heat is included in the calculation. As sensible heat may amount to 10-15 % of the heat produce it suitably ought to be utilized e.g. by heat exchange different appropriate agents such as water, steam or un condensable gases. The oxygen or the oxygen enriched air preferably produced by fractional distillation of air and in order

i c: achieve the pressure needed in the most economical way, it has appeared suitable to perform the compression of the air for the burner of the gas turbine in two steps and between the two steps withdraw the needed amount of precompressed air for the oxygen producer. The oxygen received then can, with a small addition of energy, be supplied at a pressure convenient for the gasification.

BRIEF DESCRIPTION OF DRAWINGS AND MODE FOR CARRYING OUT THE INVENTION: ' Milled-peat .1 is fed to a grinding .station 2, and is ground to a particle size of 0-3 mm. From the grinder the peat powder is passed into a dryer 3, which stands under some overpressure. The peat is dryed from 50 % to 15 % by weight of moisture by exhaust gases from the waste heat boiler 4. The gases are cooled from about 115 to 60 C in the dryer and then released 5 to the atmosphere. After grinding the peat to a particle size below 400 m in a fine grinding station 6, the peat is passed into a feeder 7, standing under overpressure. The peat-flour is blown by purified gas from a gas purification station 8 compressed by a compressor 9 to about 20 bar (2MPa) into a gas pro¬ ducer 10. The overpressure in this is suitably 18 bar. Oxygen is introduced for partial combustion of the peat. To advantage even other fuels 11 as pulverized coal, oil etc are introduced into the gas producer. Pulverized coal suspended in water or in oil is pumped as well as oil into a mixer 12, from which the fuels are injected by steam 13 or carbon dioxide 39 as a fine spray into the producer 10. Oxygen received from an oxygen plant 14 under a pressure of 6 bar is compressed 15 to 15-18 bar before introduction into the producer 10.

From the gas producer 10 a hot, about 1500°C, gas with high calorific value and a overpressure of 18 bar is withdrawn. The gas passes a heat exchanger 16, in which the sensible heat is transferred to steam 17 , thereby cooling the gas to about 100°C. The cooled gas is passed still under pressure, through the purifying station 8, where it is purified in one or more steps from dust and possibly other ingredients as carbon-dioxide by scrubbin with water or water solution containing suitable absorbi agents. Hereby the gas is cooled to near room temperatur and freed from entrained water drops in a demister. The gas then will contain only 2 % of moisture.

The gas with high calorific value produced in thi way is, provided that its sulphur content is low enough, admitted to the combustion chamber 18 of the gas turbine where it is burnt with air under an overpressure of 16 bar. The air is introduced into the combustion chamber 1 after compression in the two steps 19, 20 of a compresso driven by the high - and medium - pressure steps 21, 22 of the gas turbine. In the low pressure step 23 of the gas turbine driving an electric power generator 24, a large part of the energy content of the gas is transform to electric energy 25. The exhaust gases leaving the turbine are passed a waste heat boiler 4 to generate steam, fed to a steam turbine generator 26, 27, which produces electric energy 28. The low pressure steam from the steam turbine is con densed to water in a condensor 29 and returned to the wa heat boiler 4.. From the steam turbine 26 also steam 13 m be withdrawn for injection of additional fuel into the g producer.

The exhaust gases leaving the waste heat boiler 4 at a temperature of 115°C has a somewhat higher pressure than the atmosphere thereby surmounting the gas resistan in the dryer 3.

If the sulphur content of the additional fuels, coal and/or oil, is too high, the gas from the purificat station 6 must be deprived of its content of hydrogen sulphide formed from the sulphur in the gasification process. This can be done by scrubbing with a suitable absorption liquid e.g. water solutions of mono- or diethanolamin.in a hydrogen sulphur absorbing station 30. The hydrogen sulphide absorbed is desorbed by steam 17 in a stripper 31 and the absorption liquid reused. The stripped hydrogen sulphide then is converted to elementary sulphur 32 by cathalytic oxidation with air 33 in a Claus furnace 34.

In the purification station 8, washed out dust, is dewatered and returned 35 to the coarse ground peat before the drying station 3. In^* this way its content of coal is utilized in the gas producer 10 and fly ash is returned to^' the producer and together with other mineralic ingredients is discharged as granulated slag 36. From the absorbing solutions in the purifications station 8 possibly carbon dioxide may be recovered 38 and after com¬ pression 9 be used for injecting peat or other fuels 39 and simultaneously act as coolant in the producer.

The dried peat fuel not needed in the gas producer is withdrawn 37 and used for other purposes. Additional fuel as coal and/or oil is needed if the peat supplied to the first grinding station 2, has a water content above 30-35 % by weight. At a water content of 50 % in milled-peat about 40 % of totally supplied fuel must be coal and/or oil.

INDUSTRIAL APPLICABILITY:

The following data are valid for a combined power plant comprising a gas turbine unit supplying 75 MW elec¬ tric power at 375 kg/s gases from the producer combined with a steam turbine generator supplying 60 MW, when the exhaust gases from the waste heat boiler has a temperature of 115 C. They show the advantages of the invention.

If such an combined power plant at 4000 h/year operating time is fired with only heavy fuel oil it con- sumes 138 000 metric tons of oil at the mean electric power of 135 MW. If the combined power plant is integrated in a peat treating system according to the invention it may the same working time and mean power be fed with 297 00 metric tons milled peat (50 % H_0) and 56 000 tons heav fuel oil. I.e. the milled peat in this case replaces 82 000 ton oil.

It is possible to fire the milled peat without drying in a conventional steam power plant. In this cas the said quantity of milled peat will replace 67 000 to oil. The efficiency of a peat fired steam power plant i however, only 29 %, compared with 38 % for a combined g turbine-steam power plant. This means that power genera tion from milled peat according to invention supplement with oil when needed produces 38 • 82.000 , _r __. , . . - ^ — ₆₇ nnn ⁼ *^'⁶ times more electric energy from the same quantity of milled peat than is possible in the co • ventional steam power- plant.

The proposed method for converting peat with or without addition of other fuels, to a gas with high calorific value for production of electric energy in co bined power plants, comprising gas turbines combined wi steam turbines, has the following advantages.

1. Raw peat with moisture content of 30-60 % by weight can be dried with heat unusable for power production in such an amount, that the dried peat totally or to a substantial part can cover the need of fuel for a combined power plant.

2. About 60 % more electric energy can be recovered fro the peat than is possible in a conventional peat fir steam power plant.

3. High pressure and consequently small gas volume faci tates the gas purification. Sulphur in the fuel is c verted to hydrogen sulphide, which is easily removed as elementary sulphur. This makes it possible to use sulphur containing additional fuels without environ¬ ment risks through emission of exhaust gases containing sulphur dioxide.

4. Surplus water is removed at the gas purification step making it possible to dry water containing solid fuels with the exhaust gases of low temperature always re¬ ceived at a combined power plant.

5. The gas with high calorific value is combusted in the burning chamber of the gas turbine at comparatively low temperature of 1000 -1200 C, which gives a low formation of nitrogen ox:ides. This is highly desirably in all combustion from the_^view point of clean air preservation.

Claims

CLAIMS :

1. Method for producing electric energy combined with production of dried peat fuel, characterized in that a gas with high calorific value is produced from a solid or liquid fuel by gasification (10) with oxygen or air enriched in oxygen at a pressure above the atmospheric pressure, that the generated pressurized hot gas is cooled (16) and purified (8) from dust, condensed water and other contaminations, that the purified gas is burnt (18) under pressure with air and then drives a gas turbine (21, 22, 23) coupled to an electric generator (2 that the hot exhaust gases from the gas turbine are cooled in a waste heat boiler, coupled to an electric generator, to a temperature below 200 C and that they thereafter are used for drying of peat to a dried peat fuel.

2. Method according to claim 1, characterized in that th gas of high calorific value is produced from dried peat alone or with addition of coal or solid or liquid hydro- carbons of varying amount and composition by gasificatio (10) with pressurized oxygen or air enriched in oxygen, controlling the reaction temperature by addition of wate steam or carbon dioxide, that generated pressurized hot gas entirely or partially is cooled (16) by heat exchang ing with a suitable liquid or gaseous medium for utilizi • the sensible heat of the .gas, that the gas under main¬ tained pressure is purified (8) from dust and other con¬ taminations in a purification system, in which it also i freed from an substantial amount of water by cooling, th the purified gas under pressure is burnt (18) by air in order to work a gas turbine (21, 22, 23) coupled to an electric generator (24) , that from the gas turbine comin hot exhaust gases at a pressure above atmospheric are ad mitted to the waste heat boiler (4) producing steam used for driving a steam turbine (26) coupled to an electric

O

^■ . generator (27) , that from the waste heat boiler (4) under overpressure discharged to 200 C or below cooled, exhaust gases are used for drying (3) of moist peat, and that hereby produced peat fuel entirely or partially are 5 used in the gasification (10) .

3. Method according to claim 1 or 2, characterized by gasification under an overpressure of 5-20 bar.

4. Method according to claim 2, characterized by control of the reaction temperature at the gasification introduced

10 by moist peat fuel.

5. Method according to claim 2, characterized by control of the reaction temperature at the gasification by carbon- dioxide recovered at the purification of the gases pro¬ duced by the gasification.

-j_5 6. Method according to claim 2, characterized in that the gas with high calorific value produced in the gasi¬ fication after the gas purification but before the gas turbine burner has a temperature not above 50 C.

7. Method according to claim 2, characterized in that the 20 pressure in the exhaust gases from the waste heat boiler is high enough for transporting the peat through the dryer.