Heat value of biogas varies from 250 to 1000BTUs/cf, plus any variation in 
feedstock can effect other components (H2S, CO2, NH3), so passing the gas 
over iron fillings could bind the H2S, and bubbling it through lime-water 
might take out some CO2, but you'd either have to live with some changing 
heat outputs, or test frequently for CH4 conc. #]:-|)#
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