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Sunday, February 13, 2005

Biomass and codigestion

You can gasify material. You will find information on it if you check around under pyrolysis as it is generally called. The big disadvantage of pyrolysis is that you have to put a lot of energy into it to get a result, and the gas you get is mainly a mixture of carbon monoxide and hydrogen. It is very combustible but equally toxic. Pyrolysis has the advantage that it works on wood, which is indigestible for anaerobic bacteria. Lignin, the wood fibre material, just will not decompose without oxygen. Probably there was not much wood around when these bacteria evolved, many millions of years ago! If bagasse will decompose, and I have no idea, never having seen the stuff, then you will get more energy more safely this way.

I have been reading up on the codigestion projects in Switzerland, and they have all been chopping the vegetable matter and pasteurising it before use. Perhaps there are residual conservation chemicals (antibacterials?) that have to be removed before it will work properly.

The question of H2S is one that comes up more when you are storing the gas or feeding it into the mains gas supply. It is corrosive and there are strains of bacteria that thrive in it. We are permitted to keep it at work in concentrations less than 50 ppm. This is not considered dangerous, but you can definitely smell it!The traditional method of removing it is either washing or passing it over iron, usually both are used. The problem only really occurs with manure, vegetable matter has little sulphur in it, and it is very diluted. That is another advantage of codigestion, the manure is diluted, so the production of H2S and NH3 is also diluted. For small scale use I would simply ignore it.

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