Solid Fuels
2.2 Renewable Solid Fuels
2.2.3 Fuel Characteristics of Biomass
2.2.3.1 Biomass from Farming and Forestry Molecular Structure
Biomass essentially consists of macromolecular organic polymers – lignin, cellu- lose and hemicellulose. Cellulose is by far the most common organic substance. It is a polysaccharide consisting solely of glucose chains which are held together by
2.2 Renewable Solid Fuels 43 Table 2.9 Components of biomass (% by wt) (Kicherer 1996)
Lignin Cellulose Hemicellulose Ash Other
Hardwood 26–31 40–48 19–25 1 3
Softwood (coniferous wood)
22–25 35–43 21–30 1 3
Wheat straw 18 32 37 8 5
Miscanthus 18 40 34 3 7
hydrogen bonds in crystalline clusters, forming the framework of the cell walls. Cel- lulose is an important raw material for the chemical industry (cellulose production).
Hemicellulose or polyoses are structurally similarly to cellulose, but also contain other sugar types as basic building blocks, not only glucose chains. Lignin, one of the lignocellulose substances, is a three-dimensional aromatic branched-chain macromolecule; it acts as a binder for the cellulosic tissue. Lignin is responsible for the lignification of the cell walls. Table 2.9 shows the molecular composition of the various biomass types. It is observable that woods have higher lignin contents than herbaceous plants (Kleemann and Meliß 1993; Kicherer 1996; CMA 1995).
Moisture Content
The moisture content of fuel derived from biomass is generally higher than the respective moisture content of hard coal. Straw and whole cereal plants immediately after the harvest may have moisture contents up to 40%, but they can be reduced to below 20% within 2–3 days by field drying, provided the weather is favourable (Hartmann and Strehler 1995; Clausen and Schmidt 1996). With energy-grass crops like Miscanthus, moisture contents below 20% can also be achieved by choosing to harvest in spring, after the leaves and petioles have dried (Lewandowski 1996).
Values below 20% are required for herbaceous biomass so that it can be stored while avoiding the formation of moulds and spores (Wieck-Hansen 1996; Clausen and Schmidt 1996).
Wood in a fresh state contains between 40 and 60% moisture. This content can be reduced by partially drying the unchopped, uncut wood in the forest or, in the case of woodchips, by a subsequent drying process in a storage area. With coarse woodchips, the dry state is achieved by natural air circulation, while for fine wood- chips, forced ventilation is necessary. Given sufficient drying time (several months) and ventilation, the moisture content can also be reduced to less than 20% (Hart- mann and Strehler 1995; Kaltschmitt 2001).
Calorific Value
The lower heating value (LHV) of dry ash-free ligneous and herbaceous biomass ranges between 17 and 21 MJ/kg; the calorific value is between 16 and 20 MJ/kg.
Ligneous biomass has a somewhat higher calorific value than herbaceous biomass.
Basically, however, the calorific value of biomass is determined by its moisture
44 2 Solid Fuels
Fig. 2.15 Calorific value as a function of the moisture content
content; starting out from the dry matter, it diminishes with an increasing mois- ture content (see Fig. 2.15). Up to 60% moisture, the calorific value of wood may be between 6 and 18 MJ/kg. Air-dried wood with 15–20% moisture has a calorific value between 14 and 15.2 MJ/kg.
Volatile Matter, Residual Char, Ash
Figure 2.16 compares the contents of volatile matter, fixed carbon and ash of straw, wood, hard coal and brown coal. Biomass has a markedly higher volatile matter con- tent than hard coal. As the fuel is heated in the furnace, the volatile matter is released
Fig. 2.16 Volatile matter, residual char and ash contents of various biomasses and coals
2.2 Renewable Solid Fuels 45 and homogeneously burned. This way, a small residual char fraction remains, which has a high porosity and hence is very reactive. Ligneous biomass, as a rule, has a low ash content. Herbaceous biomass types have ash contents similar to hard coal if the ash content is referred to the calorific value.
Elemental Composition
Table 2.10 shows the composition of different biomass types, including typical val- ues for the constituents as well as their ranges. Biomasses have significantly lower fractions of carbon, while their oxygen contents exceed that of coal many times over. The hydrogen fractions are somewhat higher than that of coal. The high oxy- gen fractions and the associated partial oxidation of fuel molecules mean a lower calorific value of dry ash-free matter in comparison to coal.
Relevant to pollutant formation are the trace elements nitrogen, sulphur and chlo- rine. Figure 2.17 displays the contents of these compounds in various solid fuels (with respect to their calorific values).
Compared to hard coal, all biomass types are distinguishable by significantly lower sulphur contents (again, with respect to the calorific value). On top of this, SO2 that is formed during the combustion of biomasses may be bound by the ash, so that the SO2 emission limits can be met without sophisticated desulphurisation engineering.
The content of nitrogen in the fuel depends on the biomass type and the way it is cultivated. While wood contains very little nitrogen, straw as fuel can mean nitrogen inputs to firing in the same order of magnitude as, or higher than, hard coal. Nitrogen contained in the grain of whole cereal plants is significantly higher in concentration. For perennial grass plants like Miscanthus, a transfer of the nutrients (nitrogen, potassium, phosphorus) from the sprouts to the rhizome occurs in late summer, so that the nitrogen content in the plant matter above ground decreases (Lewandowski 1996). Biomass in general is an excellent fuel in regard to apply- ing primary combustion-engineering measures, given that most of the nitrogen is released into the gas phase during the combustion of volatile matter.
A much more problematic constituent than nitrogen and sulphur in the fuel is chlorine, which is the cause of operational problems as well as pollutant emissions problems. Chlorine contents in herbaceous plants are in some cases far higher than that of coal. Cereal straw, in this respect, has the highest values. Wood, in contrast, has low chlorine contents. Chloride is taken up from the soil by the roots of energy crops. Chloride is found naturally in soils but is also part of fertilisers, in the form of potassium chloride (KCl). In coastal areas, the chlorine content of plants is higher, due to the higher salt concentration in the air. Tests are being carried out to reduce the chlorine content of biomass by replacing the potassium component of the fer- tiliser. Results of such tests are that the chlorine content could be reduced to a third.
In the case of open-air storage of straw, most of the chloride is leached by rain (Wieck-Hansen 1996).
46 2 Solid Fuels
Table2.10Fuelcompositionofbiomasstypes(Kaltschmitt2001;Lewandowski1996;HartmannandStrehler1995;ClausenandSchmidt1996;Obernberger 1997;Spliethoffetal.1996) HardcoalBrowncoal StrawWoodMiscanthusWholecerealplants(comparison)(comparison) TypicalTypicalTypicalTypical valueRangevalueRangevalueRangevalueRangeG¨ottelbornFortuna Moisturecontent [%]1510–204520–602010–301510–20755 LHV,raw[MJ/kg]14.812.5–16.49.65.7–15.514.011.2–16.614.912.5–16.627.98.7 LHV,dryash-free [MJ/kg]18.717.5–19.019.518.5–20.018.518–1918.717.5–1930.222.2 Ash%dry4.53–70.50.3–42.51.5–5.04.03–789 Volatilematter% dry7875–818070–858078–8478.075–8135.153 C47.046–485049–524847–5047.046–4874.362.8 H6.05.4–6.45.85.2–6.16.05.2–6.56.05.3–6.854 N0.50.3–1.50.20.1–0.70.30.1–0.41.40.4–1.71.50.5 S0.150.10–0.20.05<0.10.10.02–0.130.10.07–0.1110.5 Cl0.40.1–1.10.02<0.10.30.1–0.40.30.25–0.50.2 O(difference)41.543.442.841.29.523.2
2.2 Renewable Solid Fuels 47 Fig. 2.17 Ranges of nitrogen,
sulphur and chlorine contents in biomass compared to hard coal
Ash Fusion Characteristics
Wood has ash fusion temperatures like hard coal, in the range of 1,200–1,400◦C.
Straw has significantly lower initial ash deformation temperatures (ca. 900◦C), so more severe fouling and slagging problems have to be expected. Figure 2.18 draws a comparison between the ash fusion characteristics of various types of biomass and fossil fuels. The comparison also reveals the great scattering of values within the same biomass type.
700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550
Pine Oak Beech Oats Wheat Wheat
Temperature [°C]
Melting range Softening range Melting range
Softening range
European hard coals
Different woods
Different straw
samples Miscanthus Total plants European
hard coals Miscanthus Total plants
Fig. 2.18 Ash fusion temperatures of various biomass types
48 2 Solid Fuels Table 2.11 Ash composition (%) of a wood (spruce) and a straw compared with one hard and one brown coal type
Straw Spruce Hard coal Brown coal
SiO2 65.43 29.61 43.46 11.07
Al2O3 0.59 2.59 27.83 8.05
Fe2O3 1.17 6.73 9.93 5.03
CaO 9.47 37.06 5.21 31.19
MgO 1.76 5.38 2.75 4.02
K2O 18.07 9.52 3.54 0.10
Na2O 0.20 1.97 1.18 0.10
SO3 0.98 3.21 4.42 40.24
TiO2 0.10 0.31 1.08 0.20
ZnO 0.00 0.21 0.10 0.00
P2O5 2.25 3.42 0.49 0.00
The low fusion temperatures of herbaceous biomass can be put down to the com- position of the inorganic ash components. Comparing the components, it can be seen that Si, Al and Fe dominate in the ash of hard coal, while Si, K and Ca dominate in biomass ash. For ash of herbaceous biomass in particular, the melting point is lowered by its high potassium content, which, with respect to the calorific value, is about 4–20 times as much as the content in hard coal. Table 2.11 shows the ash compositions for a wood type (spruce) and a straw type compared to one hard and one brown coal.
Densities of Biomass Types
The density of a fuel type has an influence on the transport method and the associ- ated costs, the necessary storage space and the required fuel preparation and feeding.
For biomass, this density is significantly lower than for fossil fuels and depends not only on the fuel type (straw, wood, cereals, C4grass plants), but also on what form the fuel is in (i.e. bales, chaff, chips, pellets, powder, shavings).
Table 2.12 shows the density of various types of biomass, including variations for different forms of particular biomasses.
Table 2.12 Densities (at a moisture content of 15%) of various biomasses (kg/m3) (Kicherer 1996;
Hartmann and Strehler 1995)
Biomass Density Bulk density
Herbaceous biomass:
Large-size cubic bales Round bales Chaff Pellets
Straw 150 120 70 520
Miscanthus 130 120
Whole cereal plants 220 190 130 560
Grain Grain 700
Wood Cordwood Chips Pellets
300–500 200–300 650
2.2 Renewable Solid Fuels 49 Table 2.13 Energy densities of various biomasses
Fuel
Density ρ[kg/m3]
Lower heating value (LHV) [MJ/kg]
Energy density [GJ/m3] Straw, large-size
cubic bales
150 14.4 2.2
Straw, chaff 70 14.4 1.0
Straw, pellets 520 14.4 7.5
Whole plant, large-size cubic bales
220 14.4 3.2
Miscanthus, large-size cubic bales
130 14.4 1.9
Wood chips 250 15.3 3.8
Hard coal 870 28 24.4
Brown coal 740 10 7.4
The form of preparation that has become generally accepted for ligneous biomass is that of woodchips; for herbaceous biomass, according to experience in Denmark, big bale systems seem to be most suitable for straw. Further compaction in the field or in the forest is not beneficial for transport, but means additional costs and energy expenditures.
Due to the low densities of biomasses and their low calorific value, the resulting energy densities lie about one order of magnitude below the density of hard coal and significantly below the density of brown coal (see Table 2.13).