The Production of Engineered Biochars in a Vertical Auger Pyrolysis Reactor for Carbon Sequestration

Patrick Brassard,Michèle Grenier,Stéphane Godbout,Joahnn Palacios,Vijaya Raghavan,Dan Zegan

doi:10.3390/en10030288

Abstract

Biomass pyrolysis and the valorization of co-products (biochar, bio-oil, syngas) could be a sustainable management solution for agricultural and forest residues. Depending on its properties, biochar amended to soil could improve fertility. Moreover, biochar is expected to mitigate climate change by reducing soil greenhouse gas emissions, if its C/N ratio is lower than 30, and sequestrating carbon if its O/Corg and H/Corg ratios are lower than 0.2 and 0.7, respectively. However, the yield and properties of biochar are influenced by biomass feedstock and pyrolysis operating parameters. The objective of this research study was to validate an approach based on the response surface methodology, to identify the optimal pyrolysis operating parameters (temperature, solid residence time, and carrier gas flowrate), in order to produce engineered biochars for carbon sequestration. The pyrolysis of forest residues, switchgrass, and the solid fraction of pig manure, was carried out in a vertical auger reactor following a Box-Behnken design, in order to develop response surface models. The optimal pyrolysis operating parameters were estimated to obtain biochar with the lowest H/Corg and O/Corg ratios. Validation pyrolysis experiments confirmed that the selected approach can be used to accurately predict the optimal operating parameters for producing biochar with the desired properties to sequester carbon.

Highlights

In 2014, the Intergovernmental Panel on Climate Change reported that “global emissions of greenhouse gas (GHG) have risen to unprecedented levels despite a growing number of policies to reduce climate change” [1]
Results from this study demonstrated that the response surface methodology approach can be Results from this study demonstrated that the response surface methodology approach can be used to accurately predict the optimal operating parameters of a vertical auger reactor (temperature, used to accurately predict the optimal operating parameters of a vertical auger reactor, required to produce engineered biochars with specific solid residence time, and nitrogen flowrate), required to produce engineered biochars with specific characteristics for C sequestration
Biochar characteristics highly depend on the pyrolysis operating conditions and biomass feedstock

Summary

Introduction

In 2014, the Intergovernmental Panel on Climate Change reported that “global emissions of greenhouse gas (GHG) have risen to unprecedented levels despite a growing number of policies to reduce climate change” [1]. GHG emissions need to be lowered by 40% to 70% compared to the 2010 values by mid-century, and to near-zero by the end of the century, if we are to limit the increase in global mean temperature to two degrees Celcius [1]. The thermochemical decomposition of biomass under oxygen-limiting conditions at temperatures between 300 and 700 ◦ C, can be a sustainable management solution for agricultural and forest biomasses, and is proposed as a strategy to mitigate climate change. The resulting co-products of pyrolysis are: a liquid bio-oil, non-condensable gases, and a solid biochar. Non-condensable gases are generally used to heat the pyrolysis unit. Bio-oils have heating values of 40%–50% of that of hydrocarbon fuels [2], and could be used to replace fossil heating oil

Methods

Results

Conclusion