Guidelines for small scale biochar production system to optimise carbon sequestration outcome : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Bioprocessing Engineering at Massey University, Palmerston North, New Zealand
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2019
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Massey University
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Abstract
Biochar is made in a 60 kg batch pyrolysis reactor developed by Massey University in both prior work and during this project. This thesis details the design and control features necessary to produce biochar (charcoal) at temperatures ranging from 400-700°C. It also examines the emissions abatement necessary to achieve the best possible carbon footprint by combusting the gases to avoid release to the atmosphere. The feedstock for this work was Pinus radiata without bark.
The biochar reactor is a vertical drum mounted on top of a combustion chamber containing two forced draft LPG burners. The combustion gases pass through an outer annular drum and so heat the biomass through the external wall. Evolving pyrolysis gases then move toward a central perforated core inside the drum, then descend into the combustion chamber where they are partially combusted. The range of highest treatment temperatures (400-700°C) was extended by controlling the partial combustion by varying a secondary air supply into the combustion chamber
(previously only 700°C was achievable).
Effective emissions abatement requires complete combustion. This work reveals that the flammability of the pyrolysis gases is not high enough to self-combust and so does not remove soot and other products of incomplete combustion, such as CO and CH4. Therefore, supplementary fuel is always needed. Here, this was achieved using modulated LPG burners at the flare. This system has the problem of batch pyrolysis reactors, where the release of volatiles from the reactor is uncontrolled, making the design of a variable rate flare system a non-trivial matter. Modifications made to the reactor design in this project include insulating the flare chimney, extending it to provide sufficient residence time, and installing adjustable vents to ensure sufficient air entrainment for complete combustion. This achieved emissions of CO and CxHy
(hydrocarbon, mostly CH4) of 32 and 51 ppm respectively, which were well within the US EPA limits for both suspension and fluidised bed biomass burners(2.400 and 240 ppm respectively).
The net environmental impact was determined for char made at 700°C, through carbon footprint analysis. An efficient system is needed to achieve a net sequestration benefit. Here, even with emissions abatement and the above mentioned very low CO and CxHy emissions, no net benefit was achieved. With the flare working, the net fractional sequestration was -0,14 (kg C sequestered)/(kg C in biomass). Then, when the flare is turned OFF, the net fractional sequestration was -1,2401 (kg C sequestered)/(kg C in biomass). Therefore, another frame of reference for well-operated systems is that the permissible emission should be less than 0.001 (kg C emitted as CO)/(kg C biomass), without considering methane or other GHGs.
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Figures 2.1, 2.2, 2.3, 2.4, 2.8 & 2.9 have been removed for copyright reasons but may be accessed via their source listed in the References. They correspond respectively with Equation 1 (Brownsort, 2009), Fig 1 (Brownsort, 2009), Fig 10 (Cimò, 2014), Fig 4 (Antal & Grønli, 2003), Fig 3 (Antal & Grønli, 2003) & Fig 1 (Neves et al., 2011).
Keywords
Biochar, Pyrolysis, Equipment and supplies, Environmental aspects, Carbon sequestration