There are five main byproducts of a biodigester; water, compost, methane, hydrogen sulfide (which stinks), and carbon dioxide. The compost is farm-ready that we use on our fields, it doesn't smell like manure, in fact, it has an "earthy smell", and is rather pleasant. The gases are processed and we extract CH4, S2O, and CO2. The process is complicated because we need to use the methane to make heat and pressure to remove the CO2 and S2O and dry and compress all the gases. About 40% of the methane is used for this process. We pump the CO2 into our greenhouses. The S2O is a waste gas that we "sell" to a local gas company that on-sells (after processing) to industrial users.
It is a complicated process that requires a great deal of surveillance and maintenance, but assuming no major breakdown, and that's a big assumption we should be able to recover the cost of our biodigesters within 5/7 years, which we consider a major achievement. For example, we have been "heating" the greenhouses for the past four days, as overnight temperatures have dropped rather dramatically (below freezing last night), that alone in LPG would have consumed 60 liters per day (we have 20 greenhouses, and they are large). The combination of boosted CO2 and heat has increased our greenhouse yields by 30%, and the coldest part of the year is yet to occur. Overall cost comparison is difficult because the price of LPG has dropped in the past few months, still, another cost we can control is a valuable addition.
In addition, we have been lucky several local farmers are using intensive farming technologies and produce excessive manure, that we have been able to acquire for almost nothing. We have made a deal with three farms in the area where we acquire all their manure to feed our biodigesters. Our calculation two years ago, when gas prices were about where they are today was a seven-year payback, had we purchased LPG in 2022 (we didn't because we were running down our supplies for the start-up of our biodigester), payback would have been only two years.
What is complicated in our system is that we produce manure all year round, and in winter it is a bit more since the cattle try to remain indoors. However, biodigesters work best when it is warm. So we need the methane in winter when it is cold but our biodigesters are less performant during the winter months. The methane is used for both heating and electricity, so during the high-producing summer months (we are talking about 30% more productive), we use the gas to power compressors so that the gas can be stored in tanks, that works fine and by the end of the hot summer, our three tanks are full (methane energy density is 10% higher than LPG), so in winter we produce less from our biodigesters but consume more energy (the heating), because of a financial windfall 18 months ago, we were able to build two separate units rather than one so that over the past eight months energy production has met more than 100% of our energy requirements (excluding diesel). Twice this last summer we sold power to the grid, to get rid of excess methane, we cannot vent in the atmosphere (it is a condition of the approval to build). Next year we may add an additional storage tank if the price is right.
We introduced biodigesters for two reasons; first, over the medium term, it reduces our farm's energy costs and reduces our carbon footprint dramatically. Second, because of increasingly stringent water quality standards for farm wastewater, it solves the future cost problem of disposing of cow slurry (which is a big thing).
Note: the increased concentration of CO2 is very small, it constitutes about a 10% increase in the CO2 and it's done mostly at night -- when the plants absorb the gas. The exception is the area where we grow leafy greens, but again concentrations are low and no worse the being on city streets.
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