The primary advantage of CFB gasification technology is that it enables the substitution of expensive fuels e.g. oil or gas with cheap solid fuels. These cheap fuels are typically different types of waste woods, bark or other biofuels. The process may also solve a waste disposal problem, providing a secondary economic and environmental benefit. The Lahden Lämpövoima Oy's Kymijärvi power plant gasification project will demonstrate the direct gasification of wet biofuel and the use of hot, raw and very low-calorific gas directly in the existing coal fired boiler.
The aim of the Lahden Lämpövoima Oy’s Kymijärvi power plant gasification project is to demonstrate the direct gasification of wet biofuel and the use of hot, raw and very low-calorific gas directly in the existing coal fired boiler. The gasification of biofuels and co-combustion of gases in the existing coal fired boiler offers many advantage 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. Furthermore, only small modifications are required to the boiler, and possible disturbances in the gasifier do not shut down the power plant.
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 Imatran Voima 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. The Kymijärvi power plant is a typical pulverized coal fired steam boiler producing high pressure steam for the steam turbine.
At the moment, about 300 GWh/a of different types of biofuels and refuse fuels are available in the Lahti area. On an annual basis, the available amount of biofuels and refuse fuels is enough to substitute for about 15 per cent of the fuels burned in the main boiler, equalling a maximum of 30 per cent coal.
Biofuels, as well as waste-derived fuels are local fuels. The energy density of fresh biofuel is only about 2.5 GJ/m3 (30 GJ/m3 in coal). Therefore, transporting biofuels or recycled refuse fuel (REF) from long distances is not, in an economical sense, an attractive solution. This is the main reason why biofuel-based power plants are typically quite small compared to coal fired power plants. The specific investment and operation costs are always much higher in small plants than in large plants. In addition, in small plants the power production efficiency is typically lower.
In Europe, it is typical that about 30 to 150 MW of biofuel energy is available within 50 km from the power plant. This amount can be gasified and utilized directly in mid- or large- size coal fired boilers. Thus, a power plant concept consisting of a gasifier connected to a large conventional boiler with a high efficiency steam cycle offers an attractive and efficient way to use local biomass sources in energy production.
CFB gasification
The atmospheric CFB gasification system is very simple. The system consists of a reactor where the gasification takes place of a uniflow cyclone to separate the circulating bed material from the gas and of a return pipe for returning the circulating material to the bottom part of the gasifier. All these 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 a lot of particles are conveyed out from the reactor into the uniflow cyclone. The fuel is fed into the lower part of the gasifier above a certain distance from the air distribution grid. The incoming biofuel is composed of 20 to 60 per cent water, 78 to 89 per cent combustibles and one to two per cent ash.
The operating temperature in the reactor is typically 800 to 1000°C, depending on the fuel and the application. When entering the reactor, the biofuel particles start to dry rapidly and a first primary state of reaction, namely pyrolysis, occurs. During this reaction, fuel converts to gases, char coal and tars. Part of the char coal goes to the bottom of the bed and it will be oxidized to CO and CO2, generating heat. These products then flow upwards in the reactor, and a secondary stage of reactions take place, which can be divided into heterogeneous reactions, where char is one ingredient in the reactions, and homogenous reactions where all the reacting components are in the gas phase. A combustible gas is produced which enters the uniflow cyclone and exhausts from the system together with some of the fine dust.
Most of the solids in the system are separated in the cyclone and returned to the lower part of the gasification reactor. These solids contain char, which is combusted with the air that is introduced through the grid nozzles to fluidize the bed. This combustion process generates the heat required for the pyrolysis process and subsequent mostly endothermic reactions. The circulating bed material serves as heat carrier and stabilizes the temperatures in the process.
The heat energy in the gas in three forms: as a chemical heat (combustion); as sensible heat (hot gas); and as carbon dust (combustion). In normal operation, the fuel feed rate will define the capacity of the gasifier and air feed rate will control the temperature in the gasifier. The coarse ash is accumulating in the gasifier and will be removed from the bottom of the gasifier with a water-cooled bottom ash screw.
The first commercial gasifier supplied by Foster Wheeler Energia Oy has replaced fuel oil in a lime kiln since 1983 at Wisaforest Oy, Jakobstad, Finland. Since then, similar gasification plants having the same basic technology have been installed at two pulp mills in Sweden and one mill in Portugal. These gasifiers produce lime kiln fuel from bark and waste wood, and they also utilize a part of the generated gas in drying plants.
Kymijärvi power plant
The Kymijärvi power plant was started in 1976. Originally, the plant was heavy oil fired 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°C/170 bar and 540°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 is operated in low capacity with natural gas the main fuel.
In 1986, a gas turbine generator set was installed at the plant from which exhaust heat was used for preheating the boiler feed water. The maximum electrical output of the gas turbine is 49 MWe when the outside temperature is -25°C.
The boiler uses 1200 GWh/a (180 000 t/a) of coal and about 800 GWh 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.
Gasification plant
It has been evaluated that approximately 300 GWh/a of different types of biofuels and refuse fuels are available in the Lahti area. On an annual basis, this amount is enough to substitute for about 15 per cent of the fuels burned in the main boiler.
The recycled fuel REF is produced from refuses of various origin, which come from households, offices, shops and construction sites in the area. The processing of REF will be started by the municipally owned waste management company Päijät-Hämeen Jätehuolto Oy in 1997. Besides these fuels listed, peat, demolition wood waste and shredded tyres are going to be used as fuels in the gasification plant.
Gasifier concept
The CFB gasifier consists of the inside refractory-lined steel vessel, where fuel is gasified in a hot fluidized gas-solid particle suspension. In the gasifier, biofuels and REF are converted to combustible gas at atmospheric pressure at a temperature of about 850°C. The hot gas flowing through the uniflow cyclone is cooled down in the air preheater before it is fed into the main boiler. Simultaneously, the gasification air is heated up in the air preheater before feeding it into the gasifier.
From the process point of view, the major difference compared to the gasifiers supplied in mid-80s is that fuel will not be dried in this application, and the moisture content of fuel can be up to 60 per cent. However, no considerable changes have been made to the design of the gasifier, air preheater and the gas pipeline, but the design is heavily based on the design of the those commercial scale atmospheric biomass gasifiers that Foster Wheeler Energia Oy supplied in the mid-1980s.
Some mechanical changes compared to the standard atmospheric biomass gasifiers have been made to accommodate the special nature of the fuel components to be used in the gasifier.
For fuels like REF, some wood wastes and shredded tyres which may contain different types of solid impurities (nails, screws, metal wires, concrete), the air distribution grid and the bottom ash extraction system have been modified.
The product gas for combustion is led directly from the gasifier through the air preheater to two burners which are located below the coal burners in the boiler. The gas is burned in the main boiler where it replaces part of the coal consumption. When the fuel is wet, the heating value of the gas is very low. Typically, when the fuel moisture is about 50 per cent the heat value of the gas is only about 2.2 MJ/m3. The design of the product gas burners is unique and is based on both the pilot scale combustion tests and the CFD modelling work.
Status
The hot combustion tests of the gasifier cycle were started at the beginning of January 1998 and the first switch-over to the gasification mode took place on January 14. During the first operation week, the start-up and shut-down cycles were tested and the stabilities of the main boiler and the gasifier were monitored in different cases.
After the successful tests, the gasifier has been in continuous operation since January 21 1998. Fuels like wood chips, bark, sawdust and wood waste from the wood working industry have been used. So far, the moisture content of the fuel mixture has been high at 50 to 55 per cent, but the operation of the gasifier as well as the operation of the gas burners has been stable in spite of the low heat value of the gas. The measured gas quality has matched well with the design calculations.
The load of the gasifier has been varied between 30 and 45 MW and most of the time the gasifier has been run at full load. The thermal input of the gasifier will increase when the fuel moisture content decreases. The next step will be taken on February 10 1998 when the test runs with REF start.
An economical transport distance for biofuels is typically only 30 to 80 km. Thus the amount of available biofuels within this range is limited, and the capacity of biofuel-based plants small. The construction of new biofuel plants is therefore an expensive option.
Instead, this concept, demonstrated at Lahden Lämpövoima Oy’s Kymijärvi power plant, offers an attractive solution for the use of different types of biomasses in heat and power production.
TablesTable 1. Reference list of Foster Wheeler Energia Oy ACFB gasifiers Table 2. The available local fuels on an annual basis in the Lahti area Table 3. Composition of recycled fuel (REF)