Economics and Greenhouse Gas Emissions in the Synthesis of Biochar by Pyrolysis.

Biomass Carbon Storage

Biochar is gaining importance as a potential CO2 sequestering agent, produced by the pyrolysis of biomass to give a solid material, which is called charcoal, or can be tailored more specifically to make biochar.  Biochar is char derived from the thermal conversion of biomass which is used for non-energy purposes.  It may however have an alternative use as an energy carrier. Biochar distinguishes itself from charcoal and similar materials by the fact that biochar is produced with the intent to be applied to soil as a means to improve soil health, to filter and retain nutrients from percolating soil water, and to provide biomass carbon storage.

This article summarises an analysis approach, described in more detail in (1) that assessed the economic and GHG consequences of the production of biochar by slow and fast pyrolysis using a crop residue case, namely corn stover [the residues from the production of corn left in the field] as a specific example.  The case was assessed where the residue was transported to a large fast or slow pyrolysis facility yielding both energy products [heat and electricity] at a range of outputs and biochar with the biochar applied to the original cropland.  Specifically the assessment considered :

• Cost of feedstock harvest, hauling, storage and use along with implications for nutrient replacement and tillage alteration.
• Value of energy production and the costs of associated processes. 
• Value of biochar application and subsequent implications for crop production.

GHG related accounts involving:

• Offsets for displaced fossil fuels,
• Emissions saved and increased from fossil fuels and manufactured agricultural inputs employed in the farm-to-pyrolysis facility-to-farm process,
• Sequestration enhancements and losses involved with residue recovery and biochar application.

In examining these factors it became clear that many items are uncertain and would allow us to develop only a preliminary case study on net economic benefits and a simultaneous GHG life cycle assessment. 

Production Of Biochar

Results – Biochar Production from Corn Stover

Table 1.    Estimated GHG offsets (in CO2e t-1 of feedstock) for fast and slow pyrolysis

The results in Tables 1 and 2 show that pyrolysis for the production of biochar, whether in a slow or fast pyrolysis process can lead to significant CO2 sequestration in terms of reducing GHG emissions by 0.8 and 1.1 CO2e/t of corn stover converted for fast and slow pyrolysis respectively.  This amounts to 108% of the coal equivalent emissions for the electricity generated under the fast plant and 595% for the slow meaning the offset efficiency is greater than the power offset due to the sequestration and nutrient offset elements.

Table 2 summarises the calculations yielding a total estimate of value.  This indicates for the numerous assumptions made that the fast and slow pyrolysis power plants are both unprofitable under current conditions in the USA with the slow pyrolysis plant being less so largely due to its higher value energy sales with the biochar value also making a difference to some extent.

Table 2.    Economic assumption and results summary with economic results reported per tonne of feedstock [Results in US$, mid 2008]

Biochar Application

There are a wide range of experimental findings on the yield implications of biochar application.  We assumed that biochar application increased crop yield by 5% on fields to which it was applied and only led to gains once. Under a more substantial increase of 43% slow pyrolysis becomes profitable.  Fast pyrolysis gains at a much slower rate requiring a 193% yield increase to become more profitable. 

If biochar prices are high then the value particularly of slow pyrolysis increases.  Namely when the biochar value exceeds $246 t-1 slow pyrolysis becomes profitable.  Fast pyrolysis requires a value in excess of $1047 per ton.

It is possible that feedstocks will be available that can be obtained for tipping fees or under other arrangements.  If we reduce the feedstock costs both pyrolysis options become more profitable.  For fast pyrolysis they would have to get the feedstock for $14 or less to be profitable.  In the slow case cases this alone cannot make the prospect profitable.  Rather a $11 t-1 fee (a subsidy for operations) would be needed to be profitable.

This is only one possible scenario – the use of other feedstocks and the technology selection, coupled with operation at other plant capacities will yield different value for the amount of CO2e/t sequestered.

(1) B.A. McCarl, C. Peacocke, C.-C. Kung, G. Cornforth, R. D. Sands and R. Chrisman, ''Economics of Biochar Production, Utilisation and Greenhouse Gas Offsets", Biochar for Environmental Management: Science and Technology, Johannes Lehmann and Stephen Joseph (eds.), 2009, Earthscan, London, p. 341-357.