Your choices so far:
1 Fuel: solid; Biomass (solid)
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | 1 Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Solid biomass is the prime storage for solar energy chemically bound by the photosynthesis process. In most cases and properly handed, the solid biomass is in itself a fuel but sometimes there is a need for upgrading. Federal standard EN-14588 gives the following definitions:
Living, the solid, land-based, biomass is very wet with moisture contents anything from 30 to 65 or even more. Aquatic and marine biomass like algae will contain even more water. The cell walls in land-based biomass will consist mainly of cellulose, hemicellulose and lignin while most types of algae and kelp will not have cell walls. Since lignin is the constituent having the highest heating value, the result becomes that dry, land-based biomass generally will have a higher heat content than dry aquatic biomass. Aquatic biomass is mainly used in biochemical conversion processes such as anaerobic digestion, since the low content of lignin makes it readily digestible. Therefore, the resource for commercial solid fuel is land-based biomass, residues from forestry/forest operations, residues from agriculture and sorted waste fractions.
Lignin, cellulose and hemicellulose form a rigid structure and the higher the plant, the more rigidity is needed for it to support its own weight. Therefore, high plants like trees will generally contain more lignin than lower plants such as miscanthus or other types of grass. Hence again, dry woody biomass will usually have a higher heat content than dry agricultural biomass.
The production of solid biomass for energy purposes will contain a harvesting operation, transports and comminution or fragmentation. During this supply chain it is important that the biomass is given the chance to lose parts of its water content and with a properly designed supply chain the solid biomass delivered to the energy plant may well have moisture contents about 30% or less, only because of open-air drying.
At the same time it must be remembered that biomass is a living substance and if improperly stored may deteriorate rapidly. During deterioration, heat is evolved and a pile of wet biomass may well self-ignite if not properly planned.
The cell cavities will contain the nutrients and minerals necessary for the plant and these will constitute part of the ash content of the fuel. But during the operations in the supply chain, more minerals and contaminants such as road dust and soil will enter the fuel and the final ash content is only partly depending on the actual specie.
Solid biomass is the cheapest and most abundant fuel and should be selected for large-scale applications such as major CHP or tri-generation plants. With modern boiler technology, there are no real problems handling and combusting biomass fuels with up to and exceeding 50% moisture. However; It must be remembered that the boiler will be designed for best performance with a moisture content only within certain limits. Thus, a boiler aimed for wet fuels (40-60%) will not work well if fed with moist fuels (20-40%) and may even be destroyed if fed with dry fuels (0-20%). Similar limitations apply to ash content and -properties so that a boiler aimed for wood fuel may not work well with agricultural fuel.
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The very simple rule-of-thumb is that if an advanced (i.e. expensive) boiler has been bought, then the fuel may be simple (i.e. cheap). In case a cheap boiler has been bought (i.e. one without advanced combustion technology), then the fuel must be advanced (i.e. upgraded and more expensive). If this rule is violated, then technical as well as environmental performance will suffer! |
While the solid biomass is in itself an excellent fuel, depending on the water content at delivery, it may be changed into a number of other solid fuel by different processes.
The upgrading processes may be grouped into:
- Processes that do not affect the chemical composition
- Processes that change the chemical composition
Processes that do not affect the chemical composition
These processes involve grinding into a powder suited for pulverised fuel combustion and compaction into briquettes or pellets, mainly for the use in smaller applications. Mechanically durable pellets or briquettes, i.e. high-quality products, are well suited for use in small scales like single-family houses.
Commercial wood pellets will have a diameter of 6, 8, 10 or 12 mm, a particle density for the individual pellet exceeding 1100 kg/m3 and they will be mechanically robust. They will also have a low moisture content, typically about 12%, corresponding to an energy content (heating value) about 16-18 MJ/kg or almost 5 kWh of thermal energy per kg. Other materials, such as herbaceous biomass (e.g. straw) may also be pelletized but it must be remembered that not all biomasses lend themselves well for the production of durable pellets and that the content of contaminants and ash is not changed by the pelletizing process.
An important limitation is the fact that pellets must be protected from the weather. Pellets swell when they are exposed to humid conditions and finally degrade to their original form (initial particle size and density). The pellets quality can also be downgraded due to biological degradation by means of microorganisms, if not properly handled. Pellets are typically stored in containers or covered storages to diminish the effect of weathering.
Since pellets also have a smooth surface, a uniform shape and relatively constant properties as a whole, they lend themselves well to small-scale automatic feeding and firing systems.
Briquettes are blocks of compressed flammable matter used as a fuel. The most apparent difference between briquettes and pellets is the size; briquettes are generally bigger than pellets (from 20 mm to 100 mm in diameter).
Briquettes are more brittle compared to pellets and are less resistant for mechanical wear. The bulk density of briquettes is ∼500 kg/m3, which is significantly higher that the bulk density for saw dust and cutter shavings, the most common raw materials for the manufacturing of briquettes.
Pulverized fuel combustion is commonly used in large-scale coal fired plants and solid biofuel suitable for milling, such as non-durable pellets and briquettes, are well-suited to be used as a complementary fuel in such plants. Easiest, the biomass is milled together with coal and co-fired in pulverized coal power stations.
In many cases this may be the simplest and cheapest method to introduce biofuel into the national energy system. Experience from several types of plants and installations have for example been reported and documented by the International Energy Agency (IEA), clearly demonstrating the feasibility of this technology.
The types of mills most commonly installed in this kind of applications are ball mills or rod mills. To achieve the size required for pulverized fuel firing, i.e. < 0.8 mm approximately, in this type of mills, the material must be mechanically brittle. Biomass is usually not brittle – unless it is very dry. Hence, to make the material suitable for grinding, it should be pre-treated either via a pelletizing process or a pyrolysis process.
Due to the difference in ash- and combustion properties between coal and biomass, and also because of the lower heating value with biomass and hence the large amounts to be handled, it is usually not advisable to go beyond 10-15% of the total thermal input. However, most experience indicates that biomass input at these levels causes no major operational or environmental problems with the power stations.
Processes that change the chemical composition
These processes will be thermochemical processes, but unlike gasification the aim with these processes will be to maximise the yield of the solid material and to retain in the solid material the highest possible fraction of the energy in the original feedstock.
Low-temperature pyrolysis or torrefaction. When biomass is heated up – in absence of oxygen – to temperatures about 300 °C, a partial pyrolysis will occur and the material be dried. A pyrolysis at such low temperatures will not fully evaporate the heavier hydrocarbons produced but they will be retained in the dry residue rendering the product some hygroscopic properties. At the same time, the product becomes brittle, its heating value is increased and the density decreases. The main part of the ash will be retained in the solid product and hence the ash content will increase. Some fuel impurities, such as part of the sulphur and chlorine contents, will be released with the pyrolysis gas while others will retain in the solid product.
The heat required for the process represents a loss but can be supplied by combustion of the gaseous and liquid pyrolysis products and may be kept well below 10% of the total energy contained in the original fuel depending on the moisture content of the feedstock. The solid fuel thus produced can be used in a variety of processes and can also be delivered as pellets to increase density and improve transport economy.
High-temperature pyrolysis or charring. The higher the pyrolysis temperature becomes, the larger the fraction of the volatile components that are released during the process and the smaller the fraction of residual solid. Ultimately, about 70-80% of the dry substance may be released as pyrolysis products and only about 20-30% of the dry weight be retained as solid charcoal, so the density decrease is significant. Since practically all hydrocarbons are given off during this process, the product will not have any hygroscopic properties. The high process temperature will release the main part of volatile impurities such as sulphur and chlorine, which will then be present as hydrogen sulphide and hydrochloric acid in the pyrolysis gas.
The heating value of the charcoal, in this case almost pure carbon but with the main part of the mineral ash components still present may be as high as about 30-35 MJ/kg but the total energy used for the pyrolysis process will represent about 10-20% of the total energy contained in the feedstock – again depending on the original moisture content. The solid charcoal produced can be used in a variety of processes.