The main types of biochar and their properties and expectative researches

Objectives Biochar production and application, as an effective low carbon use of by-products of industrial and agricultural production, have been widely recognized by researchers for their effects in soil improvement and crop production. However, the reports on the effects of biochar are discrepant greatly as the different feedstock and parameters in the production process of biochar. Summering previous researches will provide academic base for the effective use and production of biochar.

Major advances 1) The range of the carbon contents in biochar is 30%-90% with an average of 63.84%, and the carbon contents from different raw materials are in order: wood > straw > shell > manure > sludge. The carbon contents of straw biochar are mainly in 40%-80%, and those of wood biochar are mainly in 60%-85%. The range of biochar ash is 0-40% with an average of 15.52%, and the contents are in order: sludge > manure > straw > shell > wood. The ash contents of straw biochar are generally in 20%-35%, and those of wood biochar are mainly in 0-10%. The correlation coefficient between the carbon contents and the ash contents is -0.77, the correlation coefficients between the pyrolysis temperature and the carbon contents and the ash contents are 0.17 and 0.28 respectively. Adding biochar could improve soil properties. Biochar ash content usually plays a significant role on nutrient supplement of poor and sandy soil. 2) The range of specific surface area is 0-520 m²/g with an average of 124.83 m²/g, and the contents are in order: shell > straw > wood > manure > sludge. The specific surface area of straw biochar is generally in 0-200 m²/g and that of wood biochar is generally in 0-100 m²/g. The correlation coefficient between the pyrolysis temperature and the specific surface area is 0.48. The pore structure of biochar could reduce soil bulk density, soil density, and remove heavy metals in solution and soils. 3) The pH of biochar is in range of 5-12 with an average of 9.15, and in order: straw > sludge > manure > wood > shell. pH of straw biochar is generally in 8-11 and pH of wood biochar has an uniform distribution. The range of CEC is 0-500 cmol/kg with an average of 71.91 cmol/kg. CEC of straw biochar is generally in 0-100 cmol/kg and CEC of wood biochar is generally in 5-10 and 15-25 cmol/kg. The correlation coefficients between the pyrolysis temperature and pH and CEC are respectively 0.58 and 0.30. The biochar could reduce soil proton, improve pH and nutrient availability of acidic soil; biochar also gets the ability of ion exchange adsorption, improves the cation and anion exchange capacity, increase ability to protect fertilizer. 4) All the biochar were prepared at the paralysis temperature of 200℃-800℃, occasionally at 1000℃.

Suggestions and expectations The production and application of biochar are still in the initial stage in the world. Some researches should be considered in the future. Firstly, the differences in beneficial effects of biochar related manufacture parameters and raw materials should be systemically studied and a database should be setup accordingly; the suitable type of biochar should be made clear according to the expected target and the soil properties, to achieve the highest profit of biochar. More work is also needed for the selection and combination of biochar from different material sources, so to produce multiple functional products to meet the requirement of practical use.