Your choices so far:
1 Biomass (solid); 2 Fuel: 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 |
| 1 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 | 2 Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
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. 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).
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 or torrefied material, 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.
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.
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.