Experiences in Europe and List of Biomass Co-firing Plants
In the Netherlands, the capacity of coal-fired installations amounts to 4,000 MWe, divided over 8 units (generating capacity under the power sector is 17,700 MWe). The first co-firing trials have been executed in 1993 with 5 to 10 wt % demolition wood, sewage sludge and pet cokes. In the 1 MWth coal-fired KEMA test boiler positive results from these tests in terms of combustion performance, ash quality and emissions behaviour have led to successful introduction of co-firing in all Dutch coal-fired power plants. Four electricity generation companies (EPON, EPZ, EZH and UNA) have all developed plans to modify their conventional coal fired installations to accommodate woody biomass as a co-fuel. Fly ash implications for co-combustion: A Key issue is the validation of the quality of the ashes produced during these trials.
From the results of these unique co-firing trials it was concluded that co-firing of waste wood up to about 25%, chicken manure up to a few percent and RDF (refused derived fuel) up to about 15% (on an energy basis) is possible without for instance significant influence on the properties of the fly ash. Therefore the ash can be sold to cement industries.
Co-firing units in the Netherlands range from 420 to 650 MWe capacity. Co-firing fuel experience 1996-2000 include: Pet cokes (dried),sewage sludge, paper sludge, (waste) wood, hydrocarbon gas, biomass pellets, citrus pellets, municipal waste, coffee grounds, cacao shells, animal fat, meat and bone meal.
Gelderland power station (see below)
Each mill operates at about 1.8 tonnes per hour, with a final product density of 200 to 240 kg/m3. A metering system fed the powder into four separate burner injection lines, each capable of conveying 1.1 to 3.5 tonnes per hour. Four special wood burners with a capacity of 20MWth each were mounted in the side of the boiler (two on each side) below the lowest rows of the existing 36 coal burners. At present Electrabel is feeding the pulverized wood directly into the pulversied coal transport lines. There is a reduction of fly ash as wood produces a lot less than coal.
There is some mixed biomass pellets used in co-firing for the Maasvlakte plant (see below). The pellets are made from sewage sludge, waste wood and paper sludge available in the vicinity of the plant. The pellets are blended with raw coal on the conveyor belts. BioMass Nederland started in the beginning of 1998 with the production of pellets consisting of biomass (pruning), sewage sludge and paper sludge. The yearly production is estimated to be 150,000 tonnes of material with a higher heating value of about 16 MJ/kg. This amount of fuel will replace 30,000 tonnes of coal and thereby leading to an equivalent reduction in CO2 emissions.
Denmark is also progressive in co-combustion experiences and operation. Straw is a popular feedstock due to the high yields and high cereal. Feeding of animals was optimised through nutrition and researched a decrease in cattle has meant a reduction in the demand for straw. Co-combustion represents a means to use this plentiful by-product. In contemporary times the burning of straw in hot water boilers for farm heating became very popular and it is still done today. The problems for implementing straw for electricity production included the relatively low density of straw, the requirement for handling and processing and the high salaries in Denmark. The typical farmer has 50ha of straw producing crops, where each hectare yields 4 tonnes of straw.
Straw moisture must not exceed 30% and should be relatively evenly distributed as loads of very wet straw cause problems in handling equipement. Suppliers of straw are rewarded for dryer straw, therefore it is in their interest to store the straw for longer periods before the plant can receive it. Spring is the time for purchasing and contracting straw for the coming season and the average price is 3.4 €/GJ. In seasons with rainfall in the harvesting season, the rain leaches potassium and chlorine from the straw which is advantageous for burner operation.
Since there is no pulp production industry in Denmark, there is substantial potential for forestry residues. Some higher quality products are already being supplied to power stations. There are some lower quality residues being used to make pallets and chipboards. After a slow start, the collection of residues from Danish forests is now taking off at a considerable rate and in 2004, 50% of extracted forest products were used as biofuels. The Baltic countries were the first to export products such as wood chips, logwood and whole stems, at a reasonable price favourable enough to cover transportation costs. Imports of chips from hardwood species from America is also see in Denmark.
The Avedore CHP co-firing plant (see below) in Copenhagen, Denmark, supplies a great deal of heat for the district heating network. The plant is based on the Parellel co-firing technique whereby the biomass, in this case straw and wood is combusted in a separate combustion chamber but the steam and heat produced is linked to the coal fired system.
Planned outages have been mainly due to maintenance of hammer mills and debalers. Both components are subject to severe wear and make change of wear parts or repair necessary. The wear parts in the debalers, i.e. fixed and rotating cutters are expensive and time-consuming to re-place. Therefore repair is performed on site by adding wear-resistant weld material to critical parts for every 2000 hours of operation. Down time is typically one working day. The frequency of change of hammers in the hammer mills is based on the amount of straw processed, typically once a week. The replacement is performed within 30 minutes. Co-firing of straw does not have any impact on the maintenance of the boiler and auxiliary systems.
Experience in Denmark: Studstrup Power Station (see below), unit 1 was converted to use straw in 1995 as part of a 2 year demonstration programme (now demolished). The facility saw a fully commercial straw pre-processing plant which handled 20tonnes/hr and corresponded to 20% total energy input. The burner system was modified. Performance of the boiler was very positive for further co-firing, however, in 1998 fly ash from co-firing was not allowed to be used in the cement or concrete production so co-firing was stopped from this point. Some years later the requirements for fly ash use for cement was revised and the 350 MWe Studstrup Power Station Unit 4 was converted to co-firing of straw on a 10% energy basis in the first quarter of 2002. The coal is mainly imported from South Africa and Colombia. Most of the straw comes from wheat but also barley, oats, hay and rape straw.
Changes to the burner system were slight, they included: oil lance and flame scanner were relocated in order to clear the core of the burner for pneumatic straw feeding. Straw-to-air ratio and straw-size distribution have been optimised to the opposed wall-firing giving a loss of ignition (LOI) in the bottom ash at almost the same level as when firing coal alone. There are no problems regarding bottom ash, which is sold for brick production. Measurements of LOI in fly ash show that co-firing of straw generally improves the carbon burn out. In some cases char particulates from straw can be observed visually in the fly ahs. But this does not contribute significantly to the LOI. The char particulates are broken down in the ash handling equipment and are not observed in the delivered fly ash. NOx emissions at 10% co-firing was seen as the same or slightly reduced as coal alone firing. The nitrogen content in straw varies depending on the fertiliser used and the climatic conditions. Considering the introduction of potassium and chlorine into the combustion system for, the possibilities of corrosion, slagging or fouling are increased. However, potassium chloride reacts with coal ash and sulphur to create potassium alumina-silcate, potassium sulphide and chlorine (released as a gas, therefore almost no chlorine is found in deposits). And in general straw at 10% has a negligible impact on corrosion rates. However, between 1996 and 1998 deposits were investigated on Strudstup 1 but this was handled by increased soot blowing. At a straw share of 20%, some slagging problems occurred. In Studstup Unit 4, deposit formation has not been studied yet but during the first 3 years of operation no fouling problems have been observed and increased soot blowing has not been necessary.
A lower content of sulphur than coal and a higher level of chlorine in straw means that flue gas chemistry is also changed. These changes can be handled with problem in the flue gas desulphurisation plant. High dust SCR catalyst deactivation is another possible issue for biomass co-firing. However tests revealed there is no distinguishable difference for deactivation of HD catalyst exposure to flue gas from coal or from straw co-firing. Observed deactivation is caused by ash poisoning and formation of ash surface layers. Increased deactivation is related to the formation of sub-micron particles from potassium, therefore it is largest for coal with low ash or high sulphur content and high for straw with high potassium content.
The sale and use of fly ash is required for the successful implementation of co-firing. Limits on the alkali content in fly ash had before restricted co-firing. However, the revised European standard EN450-1 and a compliance programme in connection with the Danish concrete industry has meant straw/coal fly ash is now allowed.
In Studstrup Unit 1 tests were performed co-combusting miscanthus and triticale. Tests showed that the straw processing plant and the boiler was capable of handling the fuels. Some losses were observed, Loss of ignition in bottom ash and a higher energy consumption for pre-treatment of miscanthus.
More country based reports to follow.