Summary

The processing of wood – mainly residues but partially also full stems and short rotation coppice – to wood chips for energetic use is gaining considerable momentum. Wood chips are an economic fuel if the appropriate measures are taken to utilize it in a combustion system. Poor qualities – especially fuels with large needle-like chunks – will result in the so-called bridging, which means the build-up of obstructions leading to blocked material flow. The water content of commercially available wood chips is a common source of annoyance. Often the water content is decisively higher than the optimal 20-25%. High water content will result in rotting processes during storage, poor combustion properties with drastically increased emissions and difficulties in ignition. To cope with the variability of the fuel, the boiler technology has to be advanced. Construction and manufacture of the boiler has to be ingenious and robust, leading to considerable investment costs in quality boilers. Roughly the same amount as spent for the boiler has to be calculated for the peripheral devices. The economics of wood chip boilers improves with increasing power demand, since the comparably low price of wood chips may compensate for the high investment cost only this case. Operation of a wood chip boiler greatly depends on the quality of the peripheral devices (fuel hopper and fuel conveyor, buffer tank and hydraulic design), and even more on the quality of the wood chip purchased. The technological steps are described as the travel of a wood chip from the bunker through the conveyor, the backfire prevention device and the stoker into the furnace. Here different combustion technologies are discussed, and the flue gases are accompanied through possible flue gas treatment devices into the chimney. The emissions from wood chip boilers are in most cases decisively higher than those from a comparable pellet boiler. In areas with ambitious environmental regulations this may lead to the necessity of retrofitting a flue gas treatment system.

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