Burning of fresh woodchips – discussion in the Finnish forest energy sector

karelia-uas-wood-chip

In the autumn of 2015, a 10 MW grate boiler plant and a modern flue gas scrubber with a heat pump connection were commissioned at Kauhavan Kaukolämpö Oy’s Kauhava plant. The concept of the plant and operations is based on the burning of fresh woodchips – from harvesting timber through the logistical chain to burning and heat recovery. Based on the operational experience of the past winter, the concept of using fresh woodchips is working very efficiently, providing significant cost savings in the acquisition of fuel to the plant operator. Despite high moisture, the burning of woodchips can be controlled and specific emissions are low. 

As a concept, the burning of fresh woodchips is new and rather heretical. The traditional way to burn woodchips is to store the felled timber on roadsides and allow them to dry before chipping and burning. Practical issues of logistics have also contributed to this model. It can be assumed that the concept of fresh woodchips works efficiently in part because a significant portion of the volatile components contained in timber is included in combustion, rather than allowed to evaporate into air.

The use of fresh woodchips requires a completely new kind of thinking from the plant and logistics. The concept can also be applied to old plants if the structure of the boiler is suitable for burning moist fuel. Fresh fuel also sets some requirements for the fuel storage and fuel supply systems.

An essential part of the system is, however, a flue gas scrubber system that efficiently recovers heat from flue gases, condensing the moisture of the fuel vaporised in the boiler. The scrubber should work reliably and efficiently under all conditions, regardless of the variation in the return temperature of district heating. With a heat pump integrated in the scrubber, flue gases can be cooled efficiently to even below +30°C, making it possible to utilise a significant part of the condensation heat of the water contained in the flue gas. For the purpose of optimising and ensuring the profitability of heat production, a heat pump scrubber has, in practice, been established as the only potential recovery technology for lost heat.

Summarized from article by Mika Nummila: http://www.elomatic.com/en/elomatic/expert-articles/voc-emissions-of-timber-felled-for-fuel-%E2%80%93-a-significant-energy-loss.html

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