Biomeiler are extracting heat from a special compost heap and are therefore also known as "Compost Heating". Their function are based on the principal of composting. During this process aerobic bacteria are converting organic matter (in this case wood chips) into heat, carbon dioxide and ammonium.
Jean Pain created his works as a way to care for forests and also inadvertently came to create systems for creating liquid combustable fuel and heat for buildings. He states that a 50 ton heap of cleared brushwood cut to pieces was able to create 60 degrees Celsius water from an inflow of 10 degrees Celsius at a rate of 4L per minute for a period of 6 months. I've also heard the statement that composting woody biomass could result in more energy release than combustion? Could we calculate these rates and see if it is a viable option for hydronic heating systems. While a large amount of woody materials are needed for the process the end product is high-grade compost which indicates a very regenerative process.
Richard Perkins interviewing JPs nephew and discussing the applications of JP compost: