Biological wood oxidation is a composting of wood substrate which aims at generating heat—for this reason, it is also known as "compost heating". During the wood oxidation process; microorganisms, mainly bacteria and fungi, degrade wood substrate into CO2, H2O, and subsequently release heat in the presence of oxygen. Mainly woody materials like branches, trunk, and leaves are used. These materials are rich in carbon and poor in nutrients like nitrogen. As a result, nutrient-rich additives such as manure, urine and food waste are used to enhance and accelerate the process. The temperature of wood composting usually varies from 20 to 70 °C, which fits well for building applications such as air heating, floor heating and domestic hot water service.
Biomeiler
One of the most common application of biological wood oxidation is in a compost heater or "Biomeiler". Biomeiler has been paid a lot of attention and the number of biomeiler has increased dramatically in the world. Biomeiler was firstly developed by a French inventor, Jean Pain, in 1970s. Now, it has been developed in Germany., Romania, Netherlands, Australia and some other countries like Italy, France, US, Belgium, Czech Republic.
A biomeiler is a system consisting of tons of shredded woodchips. It is usually a round construction with several meter width and several meter height. In the biomeiler construction, coils of hosing or piping are configured in the woodchips heap. These coils are completely filled with water, which runs through a heat-exchanger mechanism, like a radiator. The cool water is pumped in the pipes and heated by the biomeiler pile.
This system has been proposed to utilise agricultural waste, specifically shredded waste banana plants, to generate hot water but has not been implemented.
Turbomeiler
Another application of biological wood oxidation is turbomeiler. The turbomeilers were tested in Leeuwarden and Haarlem, the Netherlands. A turbomeiler is a large-high silo filled with woodchips. Compared with biomeiler, turbomeiler is a little bit more high-tech. The advantage of turbomeiler over biomeiler is that the woodchips can be filled and emptied continuously.
In the turbomeiler system, tons of woodchips are filled in a silo. Water is sprayed to the woodchips pile from the top, and the water percolates through the pile of woodchips via gravity. During percolation process, the percolated water is heated by the composting pile via conduction. The temperature of the water depends on the temperature of turbomeiler and the retention time. The percolated water is usually collected on the bottom of turbomeiler and flowed to a buffer tank for heat recovery. After heat recovery (usually by a heat exchanger), the water is then pumped from the buffer tank to the top of turbomeiler and sprayed again for next circulation. By doing the water spray, the wood chips are humidified and the heat is extracted.
Advantages of biological wood oxidation
The main advantages of biological wood oxidation are that it generates hot water with low cost and it can save money for heating water. It is estimated that the material cost (except for wood waste) of a 90 m3 woodchips pile is only 457 euro and the cost of tool / machine is 525 euro. Arie van Ziel and Paolo Zampieri reported that the total cost of building a biomeiler pile is 3000 euro and with reuse it can be possible to have a cost of 700 euro for next biomeiler process. Jorritsma also reported that the total cost (including woodchips, materials and labor) of a 135 m3 biomeiler pile is 6225 euro and it can save 6900 euro compared to using gas. Turbomeiler is more expensive due to the high cost of silo. However, it costs much less once the turbomeiler has been built. Even if the biological wood oxidation cannot heat the water to a high temperature when the heat generation rate decreases, it can save energy and money by preheating the water for further use.
Another advantage of biological wood oxidation is that the final product is wood compost, which is the final product of wood composting. The wood compost creates a new possibility generating soil conditioner. It was reported that biological wood oxidation could retain about 70% of the CO2 absorbed by wood (through photosynthesis process) in the form of compost.