Pyrolysis biochar systems, balance between bioenergy and carbon sequestration

Abstract

This study aimed to investigate the extent to which it is possible to marry the two seemingly opposing concepts of heat and/or power production from biomass with carbon sequestration in the form of biochar. To do this, we investigated the effects of feedstock, highest heating temperature (HTT), residence time at HTT and carrier gas flow rate on the distribution of pyrolysis co-products and their energy content, as well as the carbon sequestration potential of biochar. Biochar was produced from wood pellets (WP) and straw pellets (SP) at two temperatures (350 and 650°C), with three residence times (10, 20 and 40 min) and three carrier gas flow rates (0, 0.33 and 0.66 l min À1). The energy balance of the system was determined experimentally by quantifying the energy contained within pyrolysis co-products. Biochar was also analysed for physicochemical and soil functional properties, namely environmentally stable-C and labile-C content. Residence time showed no considerable effect on any of the measured properties. Increased HTT resulted in higher concentrations of fixed C, total C and stable-C in biochar, as well as higher heating value (HHV) due to the increased release of volatile compounds. Increased carrier gas flow rate resulted in decreased biochar yields and reduced biochar stable-C and labile-C content. Pyrolysis at 650°C showed an increased stable-C yield as well as a decreased proportion of energy stored in the biochar fraction but increased stored energy in the liquid and gas co-products. Carrier gas flow rate was also seen to be influential in determining the proportion of energy stored in the gas phase. Understanding the influence of production conditions on long term biochar stability in addition to the energy content of the co-products obtained from pyrolysis is critical for the development of specifically engineered biochar, be it for agricultural use, carbon storage, energy generation or combinations of the three.

Strategies for producing biochars with minimum PAH contamination

“…It is reported that the magnitude of secondary (char) reactions is mainly influenced by the intensity and duration of contact of vapours with feedstock / biochar [37,39] as longer vapour residence times increase biochar yields [40]. This means that reduced hot vapour residence time, besides reducing biochar formation [23], should also reduce PAH formation as already speculated in McGrath et al…”

“…In a batch-reactor with no carrier gas flow, the gas-solid residence time for secondary reactions to take place is maximized [23,40]. Higher carrier gas flow decreases the hot vapour residence time which results in decreased PAH formation [35].…”

Roles of Phosphoric Acid in Biochar Formation: Synchronously Improving Carbon Retention and Sorption Capacity

“…Typically, an approximate 40 to 60% of biomass C would be carbonized to biochar during pyrolysis, and the remaining C would be retained in liquid and gaseous coproducts (Crombie et al, 2014; Ling et al, 2013). The initial conversion process requires high energy to ignite the pyrolysis temperature to 400 to 650°C (Crombie and Mašek, 2013; Vardon et al, 2013). Therefore, it is challenging to produce biochar more efficiently for C sequestration than burying biomass in soils.…”

Biochar for Soil Improvement: Evaluation of Biochar from Gasification and Slow Pyrolysis

“…While pyrolysis is a process that excludes the additional input of an oxidant stream and thus exposes the produced biochar to its own product gas, gasification uses a sub-stoichiometric amount of oxidant (commonly air or steam), which has an enormous impact on the biochar characteristics. Hydrothermal carbonization occurs in a liquid environment under pressure, which also results in very different chars compared to the chars created in a gaseous environment, see, e.g., [35,36].…”