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    Magazine MNM Service Magazine Journal Academy
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News Service - Technical supplement March 1999 Biomass Supplement ISSN 1562-479X In this edition ... Foreword
IJmuiden,
Potential and Utilisation of Biomass In estimating the energy consumption potential of biomass in the Federal Republic of Germany, we should differentiate between:
The residual material from farming and forestry totals so far, an unused potential of about 10 million tce p.a. (tons coal equivalent per annum), which corresponds to 2% of the primary energy consumption. Other sources mention values between 1.5 and 3.8%. Through the cultivation of energy crops, another potential of between 6 and 14 million tce p.a. could be tapped, corresponding to 1.2 to 3% of the primary energy consumption. So, together, these two give a total potential of 3.2 to 5%. Other authors establish a potential for solid biomass ranging from 2 up to a maximum of 15% of the primary energy consumption.
In Germany, bio-genetic solids currently contribute 0.8 % to of the primary energy consumption. Half of the amount consists of firewood, the other half, basically, industrial residual wood, with small amounts contributed by waste wood and a minor amount of straw. Combustion TechnologiesIn principle, for biomass utilisation, those combustion technologies are adequate which today are used for solid fuels. One of the essential criteria for the choice of the combustion system is the size of the plant to be erected. In this respect, differentiation is made between small furnaces up to a thermal output of 15 kW, middle-sized firing systems up to 1 MW, and industrial plant. Small furnaces are used in the household sector for water and room heating purposes, with outputs of 15 kW. They will not be regarded in this report. Utilities up to a thermal output of 1 MWth are used in commercial businesses. The widely used firing types are shaft furnace and underfeed firing systems. Studies of the emission behaviour of existing plants in industry and trade reveal, during operation with transient processes, higher emissions of dust, carbon monoxide and hydrocarbons due to incomplete combustion. These emissions are often caused through discontinuous operation of the fuel supply during start-up and shutdown, but also in the case of partial load when the output is controlled by connection and disconnection of the fuel supply and the fuel-air ratio is not set at its optimum. Newly developed underfeed firing systems, such as are used in Austria for wood chips, show, however, that combustion and operation are possible with low emissions also in the output range up to 1 MW. Firing systems with an output higher than 1 MWth are operated for heat, process heat and steam production mostly as combined heat and power generation plants. Today the upper limit of plants that are fuelled exclusively with biomass is in the range of 50 to 100 MWth since fuel acquisition, transport and logistics of the supply for higher outputs become too costly. In this output range, the prevailing system used is grate firing which is adequate for fuel types that are moist, problematic and/or in lumps and require little in terms of fuel preparation. What distinguishes fluidised-bed combustion systems most are lower emissions, but they require more in terms of installation and thus are economical only from output capacities above 10 MW. Pulverised-fuel firing systems for biomass are practical in particular if the fuel is delivered already reduced in size. For coal as fuel in industrial plants, pulverised-fuel firing is the predominant technology because of its remarkable features such as a high power density, good controllability and complete burnout. In the case of fuels with shares of fines and coarse matter, a combination of pulverised-fuel and grate firing may be practical, too. For the combustion of straw bales, in Denmark a one-off design, the so-called cigar burner, has proved to be a reliable technology.
In the thermal power range around and higher than 1 MWth, grate firing, which in terms of process engineering belong to the group of fixed-bed firing systems, is the predominant technology for biomass combustion in Germany. At present, pusher-type grates, travelling-grate stoker and reciprocating grates are used for the combustion of woody biomass. Problematic fuels such as humid wood residuals or high-ash bark residuals are also suitable. The grate construction type mostly found for wood combustion is the pusher-type grate. The grate is fed from the fuel storage via screw conveyor or hydraulic stoker. Through the movement of the grate, the fuel gets carried from the feeding until the end of the grate. It dries, pyrolyses and combusts completely during advance on the grate. Beneath the grate, primary air is blown in, and secondary air, above the grate and in front of the partly refractory-lined secondary combustion zones. For the combustion of residual wood from industry, grate firing is often employed in combination with a pulverised-fuel burner for powdery residues.
In Germany, there are an increasing number of grate firing systems for residual wood exploitation, but also for forestry wood chips. One typical example is a pusher-type grate firing, comprising a multicyclone for particulate removal, which was put in service in 1998 in the town of Baden-Baden (Baden-Wuerttemberg). This firing has a thermal power of 3.2 MW. The plant is connected to a district heating system, which supplies several community facilities with heat. Common use is also to install smaller plants, which, via residential heating system, supply only single facilities with heat. A representative example on industrial scale may be the bark-fuelled CHP station Klenk in the town of Oberrot. It consists of two boilers with a thermal power of 36 MW in total. There are two purposes served: the production of process steam and heat, and also, through partial expansion of the steam gases in steam turbines, generation of electricity, possible up to 5 MW. Multicyclones and an electrostatic precipitator are used for flue gas cleaning. Grate Firing for Gramineous BiomassIn grate firing systems, it is also possible to combust straw as the sole fuel. In Denmark, the burner types widespread are cigar burners, combined with a following inclined grate. In Germany, operational experience in this respect is confined to a plant with a cigar burner installed in the town of Schkoelen, and a pusher-type grate firing with preceding bale cutter in the town of Jena. Thermal exploitation of gramineous biomass has not satisfactorily matured in Germany, and is clearly less common than the practice of wood combustion. The so-called cigar burner is, in principle, a kind of grate firing system. The bales though are not fed to the grate but first set on fire at the front before they get slowly pushed into the combustion chamber. Unburnt layers of straw, breaking away, fall as lumps onto the inclined grate where they combust completely. They can, however, cause increased CO emissions. The ash is carried out on the grate. The advantages of this technology are:
Disadvantageous features are the narrow range of fuels and the restriction to a bale type, the dimensions of which have to be well maintained.
In 1993, at thAt time, the only plant of this type in Germany, went into service in the town of Schkoelen, Thuringia. It is designed for a rated load of 3.15 MW and produces heating with a maximum temperature of 120 °C. A cyclone dust collector and a flue gas filter are installed for flue gas cleaning. At this plant, measurements were taken of the emissions and the composition of residual matter as functions of the fuel. For smaller output levels, also grate-firing systems related to the pusher-type are used. In these types, the large bale is first cut into several slices which then are put onto the grate. A plant of this kind has been in service in the town of Jena since 1995.
The straw-burning boiler is designed for 1.7 MW thermal heat output. Besides straw, this boiler is also suited for combustion of whole plants and hay from landscape conservation. The size of the cuboid bales is variable within a certain margin so that different baling presses already existing in farming can be utilised. The fuel is transported to the slice cutter by automatic loading. This machine puts the bale upright and cuts pieces from it of ca. 30 cm in length. These pieces are fed to the boiler at a freely selectable rate, according to load demand. The combustion principle is based on a water-cooled grate firing with pre-gasification. A cyclone and a bag filter are used for flue gas cleaning. In straw combustion, some peculiarities have to be taken into account in comparison with thermal exploitation of wood. In contrast to wood, the lower ash softening and initial deformation temperatures of gramineous biomass can lead to agglomeration of the fuel and thus diminishes the perviousness to air of the grate and hence hinders the combustion process. Temperatures below the ash softening and initial deformation temperatures can be achieved in these cases by means of low piled fuel and less heat load of the grate. The maximum furnace temperature should not exceed 800 to 900 °C. Another possible means to prevent ash fusion and slagging on the grate is given by additional water cooling of the grate bars. Fuel-bed agitation does not reliably take remedial action because it may result in an incomplete burnout. Bubbling Fluidised-Bed CombustionIn fluidised-bed combustion, the prepared fuel is combusted at 800 to 900°C in a fluidised bed, which consists in inert matter (sand, for instance) by 95 to 98 % and contains combustible matter only by 2 to 5 %. Blowing in fluidising air via the nozzle tray creates the fluidised bed. The fuel can be fed onto the top of the bed by spreader stoker, or directly into the bed via screw conveyors. Devolatilisation of the fuel and burning of the fixed carbon evolve inside the bed. In biomass combustion, in particular, a large part of the volatile components are burned in the secondary combustion chamber. The intensity of mixing and combustion, caused by the process, the good heat transfer inside the fluidised bed as well as the non-interacting of the residence times of the particles and the flue gases allow a wide range of fuels with regard to moisture, composition and preparation. Fluidised-bed firing is suited in particular for the combustion of several, even widely divergent, kinds of fuel. It has advantages over grate firing especially when fuels with high moisture content, such as sludges, shall be fired. The low combustion temperature in fluidised-bed firing is the reason why problems such as slagging and fouling are avoided. The only hazard of sintering of the fluidised bed occurs in the case of high-alkali fuels, such as straw, which possibly excludes the application of the fluidised bed technique in the case of firing this type of fuel alone. Yet for combustion of wood as single fuel or a blend of wood and gramineous biomass, the fluidised bed firing is an adequate technique. In Germany, a plant of this type with a thermal heat output of 35 MW will go into service in March 1999 in Schongau-Altenstadt, Upper Bavaria. The maximum electrical power production is intended to reach 11.5 MW, and the heat output, up to 25 MW, involving, however, a reduction of the electrical power. The fuel to be used shall be wood above all and hay from landscape conservation. Dust in the flue gases will first be precipitated in a cyclone from where the gases enter the bag filter downstream. The design plan includes in addition a dry sorbent injection process with calcium hydroxide for capturing acid flue gas components. The sorbent is injected directly into the fluidised bed at a controlled rate.
Co-combustion in coal fired furnaces is a cost-effective, reasonable and technically feasible solution for biomass utilisation. Where woody biomass involves only minor effects on the firing and the downstream cleaning units, gramineous biomass may entail limitation of the biomass share. In this latter case arise problems in particular due to the contents of chlorine and alkalis. The effects of these substances are slagging in the furnace, corrosion in the area of the final superheaters, deactivation in the case of DENOX catalysts in high-dust arrangement, and quality deterioration of the residuals. In Germany, co-combustion has been tested in a few industrial plants. In the USA, ample experience has been gained with wood co-combustion. Plants for co-combustion of biomass and lignite or bituminous coal are not in service in Germany. There have been intensive investigations though both on experimental and on industrial plants. Since 1993, the Institute of Process Engineering and Power Plant Technology (IVD) has investigated the combustion behaviour of biomass both in mono-fuel combustion and in co-combustion in pulverised-fuel and in fluidised-bed furnaces. The investigations comprise, besides combustion experiments regarding emissions, slagging and corrosion, the preparation of the biomass prior to combustion in the pulverised-fuel furnace. One of the important basic requirements for these investigations is the analysis on the composition of the fuel and of the residual matter, carried out in the IVD laboratory for fuel analyses. In the power station of the Bayernwerk AG in Schwandorf, Bavaria, investigations were carried out into co-combustion of pelletised straw and whole plants in a 350 MW lignite-fuelled boiler. The biomass component in these experiments was between 5 and 15% by weight. A higher component could not be investigated due to problems with the preparation. The pellets were milled in the beater-wheel mills together with the coal; only 2, however, of the available 4 mills were employed to mill the biomass. So, during the milling, up to 40 % of biomass was in the mills, which filled up due to the higher volume and the very diverse milling behaviour of the biomass in comparison to coal. With shares up to 15% in the firing, the experiments were successful and any serious alterations regarding plant operation could not be observed. In the VEAG company‘s power station at Luebbenau, co-combustion investigations with wood chips were carried out in a lignite boiler. Here up to 7% by wt of biomass was milled together with the lignite. After the first mill run, however, the chips were not ground to a sufficiently small size so that part of them got unburnt into the wet ash extractor. This matter is usually recirculated to the mill though. After this second mill run, the chips had the required small size to complete combustion thoroughly. In these experiments, a decrease of the sulphur-dioxide emissions was established while the other emission values remained unchanged.
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