The yield of products from pyrolysis varies heavily with temperature. The lower the temperature, the more char is created per unit biomass. High temperature pyrolysis is also known as gasification, and produces primarily syngas from the biomass. The two main methods of pyrolysis are “fast” pyrolysis and “slow” pyrolysis. Fast pyrolysis yields 60% bio-oil, 20% biochar, and 20% syngas, and can be done in seconds, whereas slow pyrolysis can be optimized to produce substantially more char (~50%), but takes on the order of hours to complete. For typical inputs, the energy required to run a “fast” pyrolyzer is approximately 15% of the energy that it outputs.Modern pyrolysis plants can be run entirely off of the syngas created by the pyrolysis process and thus output 3-9 times the amount of energy required to run.Alternatively, microwave technology has recently been used to efficiently convert organic matter to biochar on an industrial scale, producing ~50% char.
The ancient method for producing biochar as a soil additive was the “pit” or “trench” method, which created terra preta, or dark soil. While this method is still a potential to produce biochar in rural areas, it does not allow the harvest of either the bio-oil or syngas, and releases a large amount of CO2, black carbon, and other GHGs (and potentially, toxins) into the air. Modern companies are producing commercial-scale systems to process agricultural waste, paper byproducts, and even municipal waste.
There are three primary methods for deploying a pyrolysis system. The first is a centralized system where all biomass in the region would be brought to a pyrolysis plant for processing. A second system would effectively mean a lower-tech pyrolysis kiln for each farmer or small group of farmers. A third system is a mobile system where a truck equipped with a pyrolyzer would be driven around to pyrolyze biomass. It would be powered using the syngas stream, return the biochar to the earth, and transport the bio-oil to a refinery or storage site. Whether a centralized system, a distributed system, or a mobile system is preferred is heavily dependent on the specific region. The cost of transportation of the liquid and solid byproducts, the amount of material to be processed in a region, and the ability to feed directly into the power grid are all factors to be considered when deciding on a specific implementation.
Unless crops are going to be dedicated to biochar production, the residue-to-product ratio (RPR) for the feedstock material is a useful gauge of the approximate amount of feedstock that can be obtained for pyrolysis after the primary product is harvested and the waste remains. The amount of crop residue available to be used for pyrolysis can be determined by using the RPR, and the collection factor (the percent of the residue not used for other things). For instance, Brazil harvests approximately 460Mt of sugar cane annually, with an RPR of 0.30, and a collection factor (CF) of 0.70 for the sugar cane tops, which are normally burned on the field. This translates into approximately 100Mt of residue which can be pyrolyzed to create energy and soil additives annually. Adding in the bagasse (sugar cane waste) (RPR=0.29 CF=1.0) which is currently burned inefficiently in boilers, raises the total to 230 Mt of pyrolysis feedstock just from sugar cane residues. Some plant residue, however, must remain on the soil to avoid heavily increased costs and emissions from nitrogen fertilizers.