[Gasification] Drying

[Adam Carr, Renergy]

Taking into account that there are probably 3 sources of recoverable heat on most commercial gasifiers: i.e.1) Gas cooling circuit, 2) Engine exhaust, 3) Engine water jacket, there is more than enough heat to dry - say wood - down from ~50% to <20%. It is merely a question of the trade off-between time taken to dry (quicker = less expensive biomass storage space needed) and the capital cost of the dryer. Very generally speaking drying things faster involves greater technical sophistication/capital costs.

However, for me this is not the central issue. In the UK relatively small scale gasification(< 5MWe) is probably not commercially viable unless CHP, i.e. a market can be found for "waste heat", and so it follows that it is important to use as little energy for drying as possible so as to maximise revenue.

As a rule of thumb in most heat applications higher grade heat (i.e. from gas cooling and engine exhaust) has a greater commercial value than low-grade (water jacket) heat. This is certainly true of the project we are currently working on, so for me the key drying issue is can we dry ~one tonne an hour continuously using ~500KW of hot water at 90C thereby enabling us to derive revenue from the higher grade heat.

I have to say that opinions differ. The gasifier technology supplier we are currently contemplating has a commercially operated gasifier running (in Germany - I will shortly be visiting) for which it is claimed that a walking floor the last third of which is perforated through which warm air derived from water jacket is blown is capable of drying sufficient "chunks" ( ~50mmX50X50) to run the gasifier continuously. However specialist UK dryer manufacturers tell me this is simply not possible as at least twice as much heat would be required.

Persons interested in this topic might want to look at 2 research notes (NC-308 & NC-293) by Joseph B. Sturos (Google him) produced by Forest Services USDA. N.B. difference between drying "Chunks" as opposed to "Chips". I would be grateful if some of the more scientifically competent than me list members  (I am merely a bloated capitalist) could let me know whether they think some of the data presented in these papers might or might not have a bearing on this subject. There is also an ETSU paper produced by the DTI/Border Biofuels, but this relates to a monstrous machine capable of drying > 500K tonnes/yr.

Adam Carr