Literature analysis on properties and pyrolyzing conditions of biochars

ObjectivesApplication of biochar in agriculture has recently received great attention around the world for its potential to enhance carbon sequestration, crop productivity and environment quality of soils. This paper reviewed global literature on biochar study, analysed biochar property depended of the production conditions and explored future research needs.

MethodsData were retrieved from 402 publications available by the end of 2015, and were categorized in terms of study location regions, production/preparation conditions and properties of biochars.

Results1) The studied biochars were mainly prepared in lab (68.2%), while a minor portion (22.9%) commercially produced. 2) Both woods (44.3%) and crop residues (38.6%) were the main feedstocks, though biochars from China were more from crop residues. 3) Pyrolyzing temperature ranged mostly (71.3%) from 300℃ to 600℃, but more often between 400–600℃ with commercial production. 4) Among the properties tested for biochars, total organic carbon was the most frequently measured, and those of pH, EC, total nitrogen, ash content and specific surface area were often tested. While concentrations of potentially toxic contaminants of biochar were often ignored, the potential risk of PAHs in plant-derived biochar and heavy metals in sludge biochars is still to be studied. 5) In addition, most biochars were produced with feedstocks available in the study area, and measurements of biochar properties and the methods needed to be upgraded and standardized.

ConclusionsWhile global studies on biochar were progressively increasing, attentions were paid more on biochar from woods in European and America but from crop residues in the developing countries. Moreover, commercialized biochar production was less developed in the developing countries compared to the EU and US. There were vast differences in biochar properties and functions across feedstocks and production conditions. But biochar from crop straws or under medium pyrolyzing temperature was more or less balanced between the properties of various significance, which could be concerned as a commercial production scenario. In addition, there was limited knowledge of potentially toxic pollutants. Finally, a framework of a minimum set of biochar testing including properties and the applicable methods should be upgraded.