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methane or biogas - best options? (how to cook with methane or store power from a gasifier)

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We are installing a 3kw solar power array with 6kw inverter and already have 24v x 800ah batteries. This would give us our power requirements unless we have a few cloudy days together, and we are looking for options to cover us for this, so that we can consider disconnecting from the grid and saving $120 a year on connection fees.

At present we have a "chippie" wood heater with wetback that gives us heating and hot water. It will also just fit one medium pot but is not very effective with this, as it made to heat hot water and store heat (long, tall and narrow with fire bricks inside). Making an indoor rocket-stove fire to cook on is not an option in New Zealand, and installing a second fire place with oven/stove would be several thousand.
We do also a petrol generator, and could use that to back up electricity if required (I think it does not have a DC output, and we would need to buy a battery charger to have it charge the deep-cycle batteries - which is a few hundred I think).

Since installing the wood heater, our power bill has been $80-120 (200-400kw) per month. We actually use more in summer, as a major user of our electricity is our sand-trap pump to irrigate and feed stock (i.e. when we need more pump there will be plenty of solar power to support it), and we have a few fridge/freezers that would obviously use more in summer.

The other major user of electricity would be our electric stove-oven, and if we could avoid this on cloudy days, we could probably extend our batteries another day. Mum is discarding a gas stove-oven for free, and we are finding out if we could run this from a portable gas bottle as a back-up, and we already have a portable gas camping stove. We could use this when a few cloudy days in a row are predicted.

We are also looking at making our own methane, and at wood gasification. Methane production seems the easiest/safest option to make and store ourselves, but I have been told that we may need special stoves and ovens to burn it on? Any advise on this? We also don't want a high maintenance system, but we do have pig manure from two pigs that we would like to collect on a regular basis and heat process for hygiene sake, and we are building larger wood-chip composters to heat a greenhouse that we could build a methane midden into, so if we could just dump the pig manure in the composter every few days then it could be part of the farm routine.

Wood chips and twigs also abound, and so a gasification unit also would be a good option if we could get it to make electricity to recharge our batteries and be a back-up power source. Making one, however, seems a lot more complex. I have found the link to these models ready made: http://vulcangasifier.wix.com/vulcangasifier#!evolution-series/c9l8
The turn-key system seems too expensive for what we want, but the smaller micro-series gasifiers seems a good option if they would recharge our deep-cycle DC batteries. What would be required to have these recharge a deep-cycle battery array?
Things I like about this option is that it could also be placed in the greenhouse as supplement heat this on the cloudiest days, and it can happily sit unused in the garage for months through summer. We are actually in the process of arranging a shipment from the USA, and could put one of these in if it would suit our requirements.

Any other suggestions? Would it be most time and cost effective to just use the back-up bottled gas and generator when required (we are in NZ where petrol is over $2 per litre).
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Location: Pittsburgh PA
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Annie, with biogas, you do need a different burner. There are small efficient, and cheap Japanese and Indian counter top burners. You can convert a stove, but thats all it will be good for, unless you did some major overhauls, and made one half for bottle gas, and the other biogas? The reason why, is this: biogas has a much higher percentage of carbon dioxide, compared to commercialy provided methane fuels. This means that biogas has a different fuel to air ratio, which creates a low flame speed and a loss of flame stability. There are options however not cheap. Carbon scrubbers, i have never seen a homemade or cheap scrubber personally, and as far as i know, require a fair amount of gas pressure. The other problem with biodigesters in a mild or cold climate is, they need to maintain a constant and fairly high temperature to preform optimally, or at all. I have seen burried digesters in the floors of a green house, but it only worked so so. If you need to heat your greenhouse anyways...then it might be an option, and an added benifit from heating and heat storage.
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