Kymijärvi passes the gasification test
A power plant consisting of a gasifier connected to a large conventional boiler offers an attractive and efficient way of using local biomass sources for energy production. Such a plant has been operating in Lahti, Finland, for a year. PEi looks at how it has performed so far.
Juha Palonen, Jorma Nieminen,
Foster Wheeler Energia Oy,
Lahden Lämpövoima Oy is a Finnish power company producing power and district heat for the City of Lahti. The company is 50 per cent owned by the city of Lahti and 50 per cent by Fortum Oy, which is the largest utility power company in Finland. Lahden Lämpövoima Oy operates the Kymijärvi power plant located near the city of Lahti in Southern Finland.
To keep the energy prices as low as possible, Lahden Lämpövoima is continually looking for the most economical fuel sources while trying to improve the environmental performance of its energy production.
Accordingly, Lahden Lämpövoima Oy has set up the Kymijärvi power plant gasification project to demonstrate, on a commercial scale, the direct gasification of wet biofuel and the use of hot, raw and very low calorific gas directly in a coal fired boiler.
The gasification of biofuels and co-combustion of gases in the existing coal-fired boiler offers many advantages such as: recycling of CO2; decreased SO2 and NOx emissions; an efficient way to utilize biofuels and recycled refuse fuels; low investment and operation costs; and utilization of the existing power plant capacity.
This project is supported by the EU Thermie programme. The partners in this EU Thermie demonstration project are: Lahden Lämpövoima Oy from Finland, Foster Wheeler Energia Oy from Finland, VTT (Technical Research Centre of Finland) from Finland, Elkraft Power Company Ltd. from Denmark and Plibrico Ab from Sweden.
The experiences of the first operating year 1998 have been excellent. The availability of the plant has been good and the results, with regards to product gas quality, gasifier bottom ash, main boiler filter ash and main boiler flue gas, have also fulfilled expectations.
Kymijärvi power station
The Kymijärvi power plant started in 1976 as a heavy-oil-fired plant but in 1982 was modified for coal firing. The boiler is a Benson-type once-through boiler. The steam data is 125 kg/s 540 degreesC/170 bar/540 degrees C/40 bar and the plant produces electric power for the owners, and district heat for Lahti city. The maximum power capacity is 167 MWe and the maximum district heat production is 240 MW. The operating hours of the boiler is about 7000 h/a. In the summer, when the heat demand is low, the boiler is shut down. In the spring and autumn, the boiler operates in low capacity, with natural gas the main fuel.
In 1986, the plant was furnished with a gas turbine connected to the heat exchanger preheating the boiler feed water. The maximum electrical output of the gas turbine is 49 MWe, when the outside temperature is -25 degrees C.
The boiler uses 1200 GWh/a (180 000 ton/a) of coal and about 800 GWh/a of natural gas. The boiler is not equipped with a sulphur removal system. However, the coal utilized contains only 0.3 to 0.5 per cent sulphur. The burners are provided with flue gas circulation and staged combustion to reduce NOx emissions.
Atmospheric CFB gasification
The plant can gasify saw dust; wood residues (bark, wood chips, wet and fresh wood residues, etc.); and dry wood residues from the wood working industry such as plywood, particle board, cuttings, and recycled fuel (REF). In addition to these fuels, peat, demolition wood waste, and shredded tires are also used.
The atmospheric CFB gasification system is simple. The system consists of a reactor where the gasification takes place, a uniflow cyclone to separate the circulating bed material from the gas, and a return pipe for returning the circulating material to the bottom part of the gasifier. All these components are entirely refractory lined.
Typically, after the uniflow cyclone hot product gas flows into the air preheater, which is located below the cyclone. The gasification air, blown with the high pressure air fan, is fed to the bottom of the reactor via an air distribution grid. When the gasification air enters the gasifier below the solid bed, the gas velocity is high enough to fluidize the particles in the bed. At this stage, the bed expands and all particles are in rapid movement.
The gas velocity is so high that many particles are conveyed out of the reactor and into the uniflow cyclone. In the uniflow cyclone, the gas and circulating solid material flow in the same direction – downwards – both the gas and solids are extracted from the bottom of the cyclone, a difference compared to a conventional cyclone.
The fuel is fed into the lower part of the gasifier above the air distribution grid. The incoming biofuel typically contains 20-60 per cent of water, 40-80 per cent of combustibles and 1-2 per cent of ash.
The operating temperature in the reactor is typically 800-1000 degrees C depending on the fuel and the application.
The gasifier was connected to the main boiler on December 7, 1997 and after the refractory lining warm up, the first combustion tests with solid biomass fuel were performed on January 9, 1998. The very first gasification tests were carried out on January 14, 1998 and the unit has been in continuous operation since Week 4, 1998.
The gasifier was shut down for the summer maintenance on June 2 and because of the extremely low electricity price in Finland in summer/autumn 1998, the main boiler was put in to operation in the beginning of September and the gasification plant two weeks later, i.e. September 21, 1998. During the first operating year approximately 4730 hours of operation in the gasification mode was achieved and the availability of the gasification plant was 81.8 per cent (highest monthly availabilities up to 93 per cent).
Operating experience during this first operating period has been excellent. Few problems have occurred in the gasification plant and the availability of the plant has been high since the beginning. Most of the problems, especially in the beginning, were related to the fuel processing plant. Lack of fuel and operational problems at the fuel processing plant decreased the availability of the whole plant during the first half of 1998.
With regard to the gasification plant itself, the problems were related mostly to the wire content of the shredded tires. Since there is no additional separation of metal wires after shredding, wires blocked the ash extraction system and the gasifier had to be shut down on several occasions. With all other fuel fractions, the operation of the gasifier was good.
The operating conditions in the gasifier i.e. temperatures, pressures and flow rates have been as designed and the process measurements with regards to the product gas, bottom ash and fly ash composition have been very close to the calculated values. Due to the high moisture content (up to 58 per cent) of the gasifier fuels, the heating value of the product gas has been low, typically only 1.6-2.4 MJ/nm3.
Table 1 summarizes the main fuels that were used during the first operating year. Table 2 presents the produced energy divided between different fuel fractions.
The stability of the main boiler steam cycle has been excellent. The large openings that were made for the low Btu gas burners have not caused any disturbances in the water/steam circulation. The product gas combustion has been stable even though the moisture content of the solid fuel has generally been high and the heating value of the gas very low. The stability of the main boiler coal burners has been normal even though the product gas burners were arranged very close to the lowest level coal burners.
During the start-up and the commissioning phase of the gasification plant, a one-year monitoring programme was started. In this period, the operation of the fuel preparation plant, the gasification plant and the main boiler were monitored. In general, three types of measurements were carried out during the monitoring phase: corrosion/deposit formation monitoring in the main boiler with probe testing; standard measurements including product gas analyses with continuous gas analyzers; and complete measurements including detailed measurements and analyses of gasifier product gas, main boiler flue gas as well as fuel and ash samples from both the gasifier and the main boiler.
At the end of December, 1997 before the gasifier was connected to the main boiler, a reference test run was carried out in which the main boiler was fired with 100 per cent coal. In this reference test, the main boiler flue gas emissions were measured and samples of filter ash and coal were taken and analyzed in detail. During the first gasifier operating season (January- June) six test runs were carried out. During the second operating season (September to December) no special test measurements were performed beyond monitoring the process with the analyzers on site.
With regard to the corrosion/deposit formation monitoring, the tests were started at the beginning of November, 1997. Including the reference tests, there were a total of 14 probe tests during the first operating year.
In the test runs with normal measurement routines, the product gas was analyzed with continuous IR analyzers on site and in more detail with a gas chromatography analyzer. Solid material samples of fuels, bed materials and ashes were also analysed. The main boiler flue gas composition was analyzed with the continuous analyzers on site.
In the test runs with complete measurements, more detailed analyses have been performed for the product gas and the flue gas. With regard to the product gas these measurements have included tars, particulates, ammonia, hydrogen cyanide, alkalies and sulphur components. Furthermore, as regards the flue gas, components like dioxins, furans, PAHs, benzenes, phenols and heavy metals have been measured and analyzed.
The results have been generally as expected. The product gas quality has been close to the calculated values and the effect of the gasifier to the main boiler emissions has been marginal. Perhaps the most positive phenomenon has been the decrease in the main boiler NOx emission when product gas is combusted in the main boiler.
Gasifier product gas: For the whole of 1998 the moisture content in the fuel mixture was quite high varying – typically 45 to 58 per cent. This resulted in a relatively low product gas heating value, typically only 1.6 – 2.4 MJ/nm3.
The measurements have shown that the use of contaminated materials (e.g. gluelam containing nitrogen and sodium in glue) increases the concentration of ammonia, hydrogen cyanide and alkalies in gasifier product gas.
Gasifier bottom ash: The main components of the gasifier bottom ash were bed materials, i.e. sand and limestone. Small amounts of solid impurities such as metal pieces, pieces of concrete, glass, etc. were also found in the bottom ash.
Typically, the carbon content in gasifier bottom ash was less than 0.5 per cent. No signs of chlorine were found in the analyses. The analyses and gasifier balance calculations, showed that much of the alkalies fed into the gasifier were extracted in bottom ash through the bottom ash extraction system.
Main boiler flue gas: The main boiler emissions were perhaps of greatest interest in the measurement programme of the monitoring phase. The changes in the emissions were low.
The dust content in the flue gas after the ESP decreased by 10-20 mg/nm3. The most probable reason was because of the increase in the flue gas moisture content, which in turn enhanced the operation of the ESP.
Perhaps the most positive phenomenon has been the decrease in the NOx emission. According to the measurements the NOx content of the main boiler typically decreased approximately 10 mg/MJ, equalling a decrease of 5 to 10 per cent from the base level. The two most obvious reasons for the decrease are: the reburning effect of ammonia and, far more importantly, the cooling effect of the low Btu, high moisture product gas in the bottom part of the boiler. The cooling effect meant the formation of thermal NOx was lower in the coal burners located in the lower part of the boiler. The low sulphur content of biofuels, also caused the main boiler SOx emission to decrease approximately 20-25 mg/MJ. The use of REF fuel and shredded tires in the gasifier, however caused the HCl content of flue gas to increase by about 5 mg/MJ when the gasifier was in operation.
Deposit formation monitoring: Corrosion probe monitoring tests showed no indication of abnormal deposit formation/fouling or corrosion could be seen in the test coupons. Further, no signs of abnormal deposit formation or high temperature corrosion could be found during the inspection of the boiler heat transfer surfaces (furnace walls, superheater section, economizer and air pre-heater) during the summer maintenance.
Today technologies using biofuels in heat and power production are of great interest. Biofuels have many environmental benefits compared to fossil fuels, and the utilization of biofuels is one way of reducing CO2 emissions.
The results from the first operating year have been encouraging. About 230 GWh of energy from biomass was produced with the gasifier. Several different types of fuel fractions were used: biomass (wood chips, bark, saw dust, etc.), REF, railway sleepers (chipped on site), shredded tires, plastics, etc.
Because of the excellent process behaviour of the gasifier and low impact on emissions, no limitations as regards gasifier fuels or utilization of ashes have been set by the authorities. All fuel fractions that were tested are permitted to be used as gasifier fuels today.