RESULTS FROM A NOVEL STAGED COMBUSTION TECHNOLOGY FOR THE CONVERSION OF VARIOUS BIOMASS FUELS WITH LOW ASH MELTING POINTS M. Bolhàr-Nordenkampf, F. Gartnar, I. Tschanun, S. Kaiser Austrian Energy & Environment; Siemensstrasse 89, Vienna, Austria Phone: +43-1-250 45 46 32; Fax: +43-1-250 45 128; Mail:
[email protected] ABSTRACT: Biomass fuel exists in various forms, traditionally as wood, bark, harvesting residues sewage sludge and organic waste resulting from agricultural industry. With the actual on or near site availability of biomass fuels and the trend for independent power production the demand for reliable biomass fired power plants is increasing. The favourable technology for combusting these biomass fuels is the bubbling fluidized bed combustion. The applied combustion technology is Austrian Energy’s “ECOFLUID” bubbling fluidized bed. Advantageous is the principles of a substochiometric bed operation which allows bed temperature control in the range between 650°C -850°C. Therefore, also fuel with low ash melting temperature can be burned. The applied staged combustion concept results in a homogenous temperature profile in the furnace and first pass of the boiler and thus low NOx emission. By using refractory lined superheaters corrosion problems can be minimized although high steam parameters can be obtained. These properties enable the ECOFLUID bubbling fluidized bed to handle a broad fuel range with different heating values as well as corrosive fuels. An overview of operating results and experience made during initial operation, and plants under construction as well as in start up procedure will be given. Keywords: gasification, combustion, atmospheric bubbling fluidized bed (ABFB)
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INTRODUCTION
Austrian Energy & Environment is a world wide supplier of fluidized bed technology. The reference list comprises plants with steam generating capacities from 9 t/h to 290 t/h and a fuel range from coal, biomass, sludge, RDF to various industrial wastes. The use of renewable fuels in industrial power plants is rising continuously. The driving forces are the Kyoto protocol for CO2 reduction resulting in state support for green power electricity, substitution of imported primary energy and multi-fuel concepts together with RDF. Biomass fuel exists in various forms, traditionally as wood, bark, harvesting residues sewage sludge and organic waste resulting from agricultural industry. With the actual on or near site availability of biomass fuels and the trend for independent power production the demand for reliable biomass fired power plants is increasing. The favourable technology for combusting these biomass fuels is the bubbling fluidized bed combustion. An overview of operating results and experience made during initial operation, and plants under construction as well as in start up procedure will be given. 2
level. This substoichiometric bed operation allows the control of the bed temperature in the range between 650°C-820°C. Therefore, also fuel with low ash melting temperature can be burned without any sintering problems in the bed. The standard operation temperature of the fluidised bed is approximately 760°C. The gasification gases rising from the bed are fully combusted by adding secondary air to the boiler. This causes a rise in temperature and oxygen content, as seen in Fig. 1. The turbulence in this area of the first pass results in very low CO-values in the fuel gas. Temperature Profile [°C]
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COMBUSTION TECHNOLOGY
The applied combustion technology in all evaluated biomass combustion plants is the “ECOFLUID” bubbling fluidised bed. The main feature of this technology is the principle of staged combustion of the fuel. The oxygen level in the fluidised bed is limited and hence only a part of the fuel is combusted, whereas the rest of the fuel is gasified. This can be achieved by adding a substoichiometric amount of oxygen (lambda approx. 0.35) to the fuel. However, in order to keep a constant lambda and temperature in the bed, this would result in a fluctuation of the fluidisation air volume flow and hence in fluidisation of the bed in accordance to the heating value of the fuel. Since this effect is not desired, the primary air is mixed with recirculated flue gas. This allows the control of lambda and the bed temperature as well as keeping the fluidisation of the bed at a constant
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Figure 1: Temperature and oxygen profile in the first pass of the fluidised bed boiler For high calorific fuels recirculation gas is injected above the secondary air to “cool” the flue gases in the post combustion chamber. This staged combustion concept results in a homogenous and moderate temperature profile in the furnace and first pass of the boiler and thus low NOx emission. If needed, NOx emissions can be easily controlled by installing a SNCR at the appropriate temperature level in the boiler. By using refractory lined superheaters in the first pass and the second pass corrosion problems can be minimized although high steam parameters can be obtained. These properties
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OPERATING EXPERIENCES
In the following the performance range of 14 biomass fired FBC plants using the staged combustion technology with different biomass fuels is evaluated as well as detail operation results of one plant are presented. 3.1 Operating experiences from various biomass fired FBC-Plants 14 biomass fired FBC-plants were investigated concerning correlations between the lower heating value (LHV) of the fuel and the boiler efficiency as well as the necessary lambda in the bed to obtain the desired combustion temperature of 760°C. Fig. 2 shows the correlation of lower heating value and boiler efficiency. The boiler efficiency was calculated according to DIN 1942 for all the plants. Therefore the obtained results are comparable. The fuel range covers paper sludge originating from paper industry at an approximate heating value of 2.5 MJ/kg up to waste wood with a heating value up to 18 MJ/kg, depending on the water content of the fuel. Fuels within this range of heating values can be combusted without additional firing using the ECOFLUID fluidised bed combustion technology. As it can be seen in Fig. 2 there is quite a good correlation between the boiler efficiency and the lower heating value of the biomass. The boiler efficiency depends strongly on the LHV of the fuel in the range of 2 to 8 MJ/kg (approximately 10 percent points for a change of 6 MJ/kg in the heating value). Whereas from 8 to 18 MJ/kg lower heating value the boiler efficiency increases only by 3 percent points.
oxygen level and hence lambda in the bed is controlled by mixing flue gas with an oxygen content of about 4,5 Vol%wet with combustion air. From Fig 3. it can be seen that fuels with lower heating values between 2.5 - 6 MJ/kg require high lambda values to operated the bed in the desired temperature range. With lower heating values of 2.5 MJ/kg the lambda value is approaching the value one, which means, that stoichiometric combustion conditions are necessary to maintain the desired temperature. The higher lambda values indicate also higher water contents of the fuel, since this water has to be evaporated in the bed by the heat yielded from combustion. Clearly it can be seen that the correlation between the necessary lambda and the lower heating value flattens at 8 MJ/kg up to 18 MJ/kg. For standard biomass fuels like wood chips lambda ranges between 0.35 and 0.45. 1.0
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enable the ECOFLUID bubbling fluidised bed to handle a broad fuel range with different heating values as well as corrosive fuels (e.g. net calorific values in a range of 3 to 20 MJ/kg) [1-4].
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Figure 3: Correlation between the lower heating value and the lambda in the bed
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Figure 2: Correlation between the lower heating value and the boiler efficiency Fig 3. shows the correlation between the lower heating value of the fuel and lambda, necessary to kept the desired 760°C bed temperature. As explained in the introduction, it is desired to operate the bed at substoichiometric conditions to obtain a low bed temperature and avoid ash sintering in the bed. The
3.2 Biomass fired power plant TIMELKAM / AUSTRIA The technical concept comprises a biomass combined heat and power plant (CHP) with a new bubbling fluidised bed boiler. For electricity production an existing steam turbine is used. With an estimated operation of 8000 h/year 95 GWh of electricity and 88 GWh of district heat can be produced, which corresponds to a supply of 26.000 and 5.800 private households with electricity and heat, respectively. As fuel wood chips and wood residues forest industry as well as from paper industry (bark, saw dust, grinding dust) and waste wood is used. The permit was granted in 2002, the erection was started at the end of 2004 and the boiler reached the PAC in February 2006. Green power feed in tariffs were granted for ten years according to the Federal Green Power Regulation. The plant consists of the following subsystems: • Fuel feeding • Fluidized bed steam generator • Combustion air system and burners • Bed material and ash handling • SNCR Denox plant • Flue gas cleaning
Basis for the design of the fluidized bed combustion Considering the fuel properties and the range of the heating values, the decision was made to use an ECOFLUID bubbling bed with substoichiometric combustion in the bed and a secondary air supply for the post combustion chamber. Table I: Design data Design Values Max. fuel heat rate Lower heating Value Moisture Ash content Fuel flow Control range Combustion temperatures
50.0 MW 6.000 ÷ 16.000 kJ/kg 12 ÷ 60 % max. 6 % max. 26 t/h 60 ÷ 100 % 700 ÷ 900 °C
the evaporator waterwalls. The bubbling fluidized bed is situated in the lower part of the boiler. The waterwalls in the fluidized bed and a part of the post-combustion chamber are covered with refractory material due to erosion protection and thermal reasons. The primary air enters the bubbling fluidized bed via the open type air distributor. In the lower third of the combustion chamber, the cross section is reduced to enhance the mixing of the gasification gases and the secondary combustion air. The secondary air is distributed over this reduced cross section together with the recirculated flue gas for temperature control in the post combustion chamber. This results in a staged combustion and a temperature increase to 870°C up to 950°C at the maximum in the post combustion chamber.
The plant is approved according to the § 29 Abs. 1 Z3 Abfallwirtschaftsgesetz (AWG), which sets emission limits for renewable fuels. The emissions are related to 12 % O2 content in dry flue gas. Table II: Emissions relate to 12 % O2 content in dry flue (half hour mean values). Pollutants according decree Carbon monoxide CO Sulphur dioxide SO2 Nitrogen oxides NOx as NO2 Unburnt organic carbon Corg Dust Hydro chloride HCl Hydro fluoride HF Mercury Hg Dioxins and Furans PCDD/F
Emission limit 90 mg/Nm3 45 mg/Nm3 210 mg/Nm3 25 mg/Nm3 25 mg/Nm3 9 mg/Nm3 0.63 mg/Nm3 0.04 mg/Nm3 0.1 ng/Nm3
The emission values for CO, NOX and organic matter have to be obtained with the fluidized bed combustion respectively with the SNCR. Together with the wood wastes incombustible parts like stones, nails, screws and metals are usually fed to the fluidized bed, where they collect and hinder the fluidization. To prevent this, an open type air distributor in AE&E –design was built. This air distributor enables the drain of bed material over the whole bed cross section during operation. To maintain a constant fluidization of the bed at a constant bed temperature flue gas is added to the primary air to the bed. To control the temperature in the post combustion chamber recirculated flue gas is used, too. Plant description Fluidized Bed Combustion and Steam Generator The steam generator is designed for the following data: Table III: Design figures Live steam flow Live steam pressure Live steam temperature Guaranteed boiler efficiency Feedwater temperature
56 t/h 41 barg 440°C 89% 110 °C
The steam generator is a bottom supported 3-pass type with natural circulation, where 3 passes are integrated in
Figure 4: Cross-section of the boiler Three superheaters are situated in the third pass. To avoid high temperature corrosion, the final superheater is arranged between superheater one and two. The fourth pass, built as carbon steel casing, contains the economizer, which cools the flue gases down to 160°C. Solid Fuel System The fuel is transported by a belt conveyor to the boiler house, were it is dropped into a fuel distribution bin. From this bin the fuel is distributed to the two silos in the boiler house by means of two speed controlled screws. From this point two independent fuel feeding lines exist, each designed for approx. 90% of the total fuel flow. The fuel is discharged from these silos by push-rod systems to the metering bins. The metering to the boiler of the fuel mixture is carried out by speed controlled double-screws, which drop the fuel into the feeding chutes. The fuel is distributed into the bed by means of air spouts. Burner System Start up of the fluidized bed boiler is carried out with natural gas burners, heating the bed material to the ignition temperature of the wood. Two burners are installed in the freeboard of the bubbling-fluidized bed, another two in the post combustion chamber to ensure the desired emission limits by maintaining the 850°C for two
seconds during start-up and also for fuels with very low heating value. Bed Material and Ash Handling The bed material together with the impurities is drained continuously from the furnace hoppers with vibrating conveyors. The coarse parts are separated from the fine particles and sand in an air classifier. The fine material is separated from the classifier air by a separator and can be either returned into the bed reducing the sand consumption of the boiler or can be sent to the ash silo for disposal. The ash from the second pass is collected by a water cooled screw and conveyed pneumatically to the ash silo. The ash from the economiser and the bag house filter is collected in one ash blow pot. However, the filter ash can be recirculated to the bag house filter to save adsorbent for the dry flue gas cleaning. Denoxing of the Flue Gases To keep the NOx Emissions for all possible fuel mixtures below the limit, a selective non-catalytic reduction system (SNCR) is installed. As SNCR-media urea is used and injected into the post-combustion chamber at a suitable temperature level. Flue Gas Cleaning A bag filter removes the dust from the flue gas. To remove any chlorine, sulphur and dioxin emissions, hydrated lime and activated carbon can be injected into the flue gas duct in front of the filter as adsorbent. The ID fan ensures the balanced draft in the combustion chamber. Operation Results The commissioning started in August 2005 and the plant was handed over to the client after successful trial run in February 2006. The guaranteed efficiency was measured by a third party and showed a value more than one percent higher than the guaranteed value. During start-up problems occurred in the fuel preparation and supply from the external fuel system, which was not scope of supply of AE&E. Some of the problems were caused by the strong winter and hence the shortage of biomass fuel. The content of fine particles in the fuel was too high and caused at the beginning problems in the conveying lines and especially in the level control of the metering bins. The installed capacitive level measurements were changed to microwave based ones which are insensitive to the dust transported with the biomass fuel. The design of the ECOFLUID fluidized bed combustion was confirmed during start up and the following commercial operation by the achieved operation data. All emission values are far below the guaranteed limits. At normal operation with biomass close to the design fuel even an operation of the SNCR is not necessary. The furnace could be operated easily even with very fine fuel, which was far below the fuel specifications and an operation with 100% waste wood keeping the emission limits was possible, too. Finally, it can be stated that this power plant is a big step forward in the green energy production from Biomass.
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SUMMARY AND CONCLUSION
From the correlations of the lower heating value the strong dependency of the efficiency in the lower ranges (2 – 8 MJ/kg) of the LHV can be seen. An increase of the heating value by drying the fuel with very low calorific value can increase the boiler efficiency significantly. To achieve high boiler efficiency fuels with a high LHV are necessary. Looking at the correlation of the lambda value and the lower heating value to obtain the desired bed operation temperature of 760°C it can be seen that for very low heating values nearly stoichiometric operation is needed. This represents also the lower limit of fuels which can be combusted with this technology at the desired conditions. At these conditions the whole heat is released in the bed and hence nearly no post combustion occurs in the top of the first pass. Therefore, only low superheating temperatures for the steam are possible. A possibility to raise the steam parameters is co-firing of other fuels in the post combustion chamber. For fuels with higher heating values this problem does not occur since only part of the fuel is combusted in the bed. The yielded gasification gases can than be combusted in the post combustion chamber to achieve high steam parameters. In summary it can be concluded that the staged combustion technology together with a substoichiometric fluidised bed operation has well proofed its suitability for even difficult biomass fuels even with a low ash melting points. 5
REFERENCES
[1] Tschanun, I., Holarek, M., Gartnar, F., Glatzer, A. (2001): Westfield, Fife/Scotland becomes World’s first to burn Poultry Litter in FBC, Proc. of the Power Gen Europe 2001, 29-31.Maqy 2001, Brussels, Belgium. [2] Bolhàr-Nordenkampf, M., Gartnar, F., Tschanun, I., Kaiser, S. (2005): Operating Experiences from FBCPlants using Various Biomass Fuels, Proc. of the 14th Bioenergy Conference, 17-21 Oktober 2005, Paris, France. [3] Tschanun, I., Mineur, M., (2003): Biomass Combustion with State of the Art Bubbling Bed Steam Generators, Proc. of the Power Gen Europe 2003, 6-8 May 2003, Düsseldorf, Germany. [4] Tschanun, I., Franz, P., Gartnar, F. (1998): Klärschlammverbrennung bei der Vera GmbH – Hamburg mit dem Wirbelschichtverfahren der Austrian Energy, Proc. of the 1st Berliner Klärschlammtagung “Integrierte Klärschlammentsorgung”, 25 – 27 May 1998, Berlin, Germany.
