District Heating Plants
The use of forest chips at district heating plants has increased significantly since the first systems came into operation at the beginning of the 1980s. While there were only three wood chip-fired district heating plants in 1984, the number has increased to approx. 50 plants today. The consumption of wood chips in the same period has increased to approx. 725,000 m3 l. vol per year which is equal to an amount of energy of approx. 1,800 TJ. At the end of the publication, there is a list of wood chip-fired district heating plants in Denmark.
Seen in an international perspective, the use of wood chips at district heating plants has increased tremendously during a relative short period of time.
Wood chip-fired district heating plants are established either in order to replace oil- or coal-fired district heating plants, connected to old district heating systems, or as new plants and systems (the so-called “urbanisation” projects). Wood chip-fired boilers at district heating plants are designed for the generation of heat in the range of 1 MW and 10 MW; the average being 3.5 MW.
Subsidies are granted under the State-Subsidised Promotion of Decentralised Combined Heat and Power and Utilisation of Biomass Fuels. It is obvious that this is financially beneficial to these projects, and it is assumed that the subsidy scheme is of great importance to the continuos enlargement of
the district heating supply based on biomass. “Urbanisation” projects are started from scratch. The heating plant, the district heating system and the consumer service installations thus all have to be established. These plants require a considerable total investment and have typically been implemented in small communities , wherefore wood chip-fired boilers used here are smaller than the average of 3.5 MW mentioned above.
About 10 to 20 manufacturers in Europe making turn-key wood chip-fired district heating systems. In addition a large number of manufacturers
are supplying small systems for farms and institutions or parts of systems.
The biomass technology has recently received increased interest by trade compagnies and industries. This is due to the fact that the companies no
longer can deduct energy and environmental taxes on indoor heating. Trade and industry are also offered the opportunity of being granted subsidies from the Energy Agencies for investments in installations which may reduce emissions of e.g. CO2 .
Hydraulic Feeding System
Many plants use this quite reliable feeding system. Wood chips fall from a hopper into a horizontal, square box, from where hydraulic feeding devices
force wood chips on to the grate. The construction of the system is of decisive importance to its reliability. If correctly designed as most often seen today, it is among the best feeding systems for wood chips.
Small systems (0.1-1 MW boiler nominal output) often have screw stokers feeding the boiler. At some plants, the screw stoker is positioned across the longitudinal direction of the grate. This gives a good distribution of the fuel over the width of the grate.
Grate with Feed Hopper
Some wood chip plants have a simple hopper that feeds the wood chips on to the grate. The system is known from coal-fired boilers with travelling grate and
requires that the height of the wood chips in the hopper will be high enough so as to function as an airtight plug between the feeding system and the boiler. The problem of the blocking of the hopper can be remedied by an appropriate design of the hopper, and as a last resort by mechanical stirring/scraping systems.
Wood chips are thrown into the combustion chamber by a rotating drum in a spreader stoker. Only a few plants use the system.
Wood chips are blown into the combustion chamber and fall on to the grate.
Spreaders and pneumatic stokers are often used in connection with combustion of wood chips with a high moisture content.