Improving carbon product yields in biocarbon production by combining pyrolysis and anaerobic digestion

Abstract

Solid carbon is an important raw material in industrial processes. Most of the charcoal produced today is via conventional carbonization, which suffers from huge carbon losses due to system inefficiency. Intermediate pyrolysis is principally similar to conventional carbonization and produces biocarbon while capturing the off gasses; among these off gasses is aqueous condensate, which is difficult to utilize due to the high water content and low energy content. This fraction can contain up to 25% of the carbon from feedstock, so utilization of this fraction is important for good overall carbon balance. Anaerobic digestion can be a promising tool for utilizing the carbon in the aqueous condensate by converting it into biomethane. Here, birch and spruce wood were pyrolyzed and the biomethane potential for the aqueous condensates was tested. The mass and carbon balances of the pyrolysis products of birch and spruce at two pyrolysis temperatures were performed, and biocarbon carbon yields ranging from 42% to 54% were obtained. Anaerobic digestion of the aqueous phases collected from the pyrolysis process was performed, with carbon recovery yields between 44% and 59%. A total carbon recovery of 77.8% to 85.7% was obtained, and the primary carbon losses were identified.

Full Article

Improving Carbon Product Yields in Biocarbon Production by Combining Pyrolysis and Anaerobic Digestion

Cornelis van der Wijst,^a,* Nirmal Ghimire,^b Wenche Hennie Bergland,^b Kai Toven,^a Rune Bakke,^b and Øyvind Eriksen ^a

Solid carbon is an important raw material in industrial processes. Most of the charcoal produced today is via conventional carbonization, which suffers from huge carbon losses due to system inefficiency. Intermediate pyrolysis is principally similar to conventional carbonization and produces biocarbon while capturing the off gasses; among these off gasses is aqueous condensate, which is difficult to utilize due to the high water content and low energy content. This fraction can contain up to 25% of the carbon from feedstock, so utilization of this fraction is important for good overall carbon balance. Anaerobic digestion can be a promising tool for utilizing the carbon in the aqueous condensate by converting it into biomethane. Here, birch and spruce wood were pyrolyzed and the biomethane potential for the aqueous condensates was tested. The mass and carbon balances of the pyrolysis products of birch and spruce at two pyrolysis temperatures were performed, and biocarbon carbon yields ranging from 42% to 54% were obtained. Anaerobic digestion of the aqueous phases collected from the pyrolysis process was performed, with carbon recovery yields between 44% and 59%. A total carbon recovery of 77.8% to 85.7% was obtained, and the primary carbon losses were identified.

Keywords: Biocarbon; Pyrolysis-anaerobic digestion combination; Increased carbon yield

Contact information: a: RISE PFI AS, Høgskoleringen 6B, NO-7491 Trondheim, Norway; b: Department of Process, Energy and Environmental Technology, University of South-Eastern Norway, Kjølnes ring 34, NO 3918 Porsgrunn, Norway; *Corresponding author: cornelis.vanderwijst@rise-pfi.no

INTRODUCTION

In order to reduce consumption of fossil resources, efforts should be made towards providing new and renewable alternatives. The metallurgical industry requires a carbon material to act as a reducing agent and energy source, and a transition from fossil coal to renewable carbon would cause a huge reduction in global fossil CO₂ emissions. Although many industrial processes already use charcoal, the majority of the charcoal produced today is still produced in traditional kilns, e.g., earth mound kilns in sub-Saharan Africa and “hot tail” kilns in Brazil (Pennise et al. 2001; Bailis et al. 2013). Usually, these traditional kilns do not have off gas utilization or recovery, resulting in large emissions of incomplete combustion products into the atmosphere, which has a larger global warming impact than the molar CO₂ equivalent of the complete combustion products of the off gasses (Bailis 2009). In addition, these emissions are harmful to humans and can increase mortality and respiratory disease rates for populations close to the points of emission (Bailis et al. 2005).

Brazil is the largest charcoal producer in the world, and the main type of kiln used is the “hot tail” kiln (Bailis et al. 2013). Although they are more efficient than most earth-mound kilns used in sub-Saharan Africa, “hot tail” kilns have a reported maximum charcoal mass yield of 34.1% and a charcoal carbon yield of 52.1% (Pennise et al. 2001). This results in an approximate 65% and 50% loss in mass and carbon, respectively, due to system inefficiency by the venting of off gasses.

Modern pyrolysis, i.e., thermal decomposition without oxygen, is a simple yet powerful primary conversion technique and is fundamentally similar to charcoal production. Pyrolysis is used for a large variety of feedstocks; it has been a promising route for biomass utilization for a long time but has struggled to find commercial feasibility (Maschio et al. 1992). During the last few decades, the research focus on biomass pyrolysis has predominantly been on fast pyrolysis, with the aim of optimizing the bio-oil yield and quality (Bridgwater 2012). In recent years, emphasis on the co-production of bio-oil and biocarbon has increased, as the numerous applications and considerable environmental benefits of biocarbon have been recognized (Laird et al. 2009; Cha et al. 2016).

Intermediate pyrolysis is a relatively new genre within pyrolysis that balances the yield of biocarbon and bio-oil. Typically, 30 wt% of biocarbon is obtained from intermediate pyrolysis, which is in the upper range of traditional charcoal kiln yields, as opposed to the 12 wt% biocarbon yield with fast pyrolysis. The increased biocarbon yield is a result of the decrease in heating rates and increased reaction time when compared to the fast pyrolysis process. Fast pyrolysis processes its feedstock within seconds, while intermediate pyrolysis is usually completed within 30 min to 90 min; “hot tail” kilns have a reported run time of 40 to 50 h (Pennise et al. 2001).

The bio-oil from intermediate pyrolysis usually phase separates into an organic condensate phase and an aqueous condensate phase, most likely due to secondary cracking of the vapours before condensation (Yang et al. 2014). This improves the viscosity and heating value of the organic condensate compared to the oil fast pyrolysis produces and can be used as an energy carrier. However, the aqueous condensate contains a considerable amount of water and water-soluble components, has low calorific value, and there is no direct area of application. This condensate fraction can still contain up to 25% of the carbon from the feedstock; thus it is important to utilise this fraction to ensure efficient carbon utilization and prevent the discharge of polluted water. A promising route for the utilization of the carbon in the aqueous condensate from intermediate pyrolysis (also called aqueous pyrolysis liquid (APL)) is biomethane production via anaerobic digestion (Hübner and Mumme 2015; Fabbri and Torri 2016; Feng and Lin 2017).

Anaerobic digestion (AD) is a biological process in which a consortium of microorganisms breaks down organic compounds to produce biogas (typically consisting of 50% to 75% CH₄ and 25% to 50% CO₂) in the absence of free oxygen. It is a mature, well-established, and robust technology in which mixed communities of organisms synergistically break down various easily degradable organic compounds, but it can also digest more complex, recalcitrant, and inhibiting compounds in low concentrations after some adaption time (Benjamin et al. 1984; Vasco-Correa et al. 2018). Biogas production via the anaerobic digestion of organic wastes is regarded as an effective waste treatment method as well as an energy production technology (Appels et al. 2008; Khalid et al. 2011). However, the anaerobic digestion of raw lignocellulosic biomass has proven difficult due to the recalcitrant nature of lignocellulosic biomass (Yang et al. 2015). It is nevertheless a promising technique for carbon recovery from aqueous side streams, e.g. APL, from intermediate pyrolysis. Although APL is a complex substrate with hundreds of compounds, with a few of these compounds considered toxic to the AD microorganisms, they are able to adapt to a wide range of chemical substance, which can be exploited to overcome the complexity of APL (Torri and Fabbri 2014). Moreover, the production of biomethane is a clean energy source that can be used as drop-in fuel after purification. This can be lucrative and is already available as a viable alternative as a transportation fuel (Appels et al. 2011).

Research on the AD of APL is still in its infancy. However, there is increased interest in pyrolysis as the research focuses on its use as a measure to handle the aqueous side stream (Hübner and Mumme 2015; Feng and Lin 2017). The application of APL in other fields has been hampered because of its low calorific value, acidity, chemical and thermal instability, and presence of complex and inhibitory compounds (Kan et al. 2017; Zhou et al. 2019). While the AD of APL from the pyrolysis of agricultural residues have been examined to some extent, little research has been done with wood as the feedstock where the pyrolysis process is focused on biochar production and quality.

The purpose of this work is to compare the carbon utilization of commercial charcoal production with the biocarbon production via intermediate pyrolysis combined with anaerobic digestion. The “hot tail” kiln process reported by Pennise et al. (2001) was chosen to be the benchmark process for commercial charcoal production, since it is the most common charcoal production method in Brazil. The pyrolysis of two different feedstocks at two different temperatures was performed along with the study of the biomethane potential of the corresponding aqueous condensates.