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Biochar and Bio-oil

Left - a nutrient-poor oxisol; right - an oxis...Image via Wikipedia
The growing concerns about climate change have brought biochar, a charcoal produced from biomass combustion, into limelight. Biochar is a carbon-rich, fine-grained residue which can be produced either by ancient techniques (such as covering burning biomass with soil and allowing it to smoulder) or state-of-the-art modern pyrolysis processes. Combustion and decomposition of woody biomass and agricultural residues results in the emission of a large amount of carbon dioxide. Biochar can store this CO2 in the soil leading to reduction in GHGs emission and enhancement of soil fertility. Biochar holds the promise to tackle chronic human development issues like hunger and food insecurity, low agricultural productivity and soil depletion, deforestation and biodiversity loss, energy poverty, air pollution and climate change. Thus, biochar could make a difference in the energy-starved countries of Asia, Africa and Latin America as well as the industrialized world with its vast array of benefits. 

The two main methods for biochar production are fast pyrolysis and slow pyrolysis. The biochar yield is more than 50% in slow pyrolysis but it takes hours to complete. On the other hand, fast pyrolysis yields 20% biochar and takes seconds for complete pyrolysis. In addition, fast pyrolysis gives 60% bio-oil and 20% syngas.

Bio-oil is a dark brown liquid and has a similar composition to biomass. It is composed of a complex mixture of oxygenated hydrocarbons with an Bio-oil has a much higher density than woody materials (three to six times, depending on form), which reduces storage and transport costs. Bio-oil is not suitable for direct use in standard internal combustion engines. Alternatively, the oil can be upgraded to either a special engine fuel or through gasification processes to a syngas and then bio-diesel. 

Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store. Since the oil has a density of about 1200 kg m-3, it can be conveniently transported over long distances. Current end-use possibilities are as a boiler fuel for stand-alone heat or in combined heat and power (CHP) using the steam cycle after either diesel or gas turbine electricity generation. The majority of these options have been found to be technically feasible. In addition, bio-oil is also a vital source for a wide range of organic compounds and speciality chemicals. 


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