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United States Department of Agriculture

Agricultural Research Service

Research Project: SUSTAINABLE CROPPING SYSTEMS FOR IRRIGATED SPECIALTY CROPS AND BIOFUELS

Location: Vegetable and Forage Crops Production Research

Title: Use of Biochar from the Pyrolysis of Waste Organic Material as a Soil Amendment

Authors
item Granatstein, C -
item Kruger, C -
item Collins, Harold
item Galinato, S -
item Garcia-Perez, M -
item Yoder, J -

Submitted to: Extension Reports
Publication Type: Other
Publication Acceptance Date: August 30, 2009
Publication Date: N/A

Interpretive Summary: Biochar is a charcoal-like material produced by the thermochemical pyrolysis of biomass materials. It is being considered as a potentially significant means of storing carbon for long periods to mitigate greenhouse gases. Much of the interest comes from studies of Amazonian soils that appear to have been amended with biochar which led to significant improvements in soil quality and large increases in crop yields. These changes have persisted for hundreds, if not thousands, of years. What is not known is how long it takes for biochar to integrate with the soil and thus express its benefits. In this study, biochars from several different feedstocks were evaluated for their characteristics and their fate in five different Washington State soils. A bench-scale pyrolysis reactor was built at Washington State University (WSU) to provide biochar from different underutilized biomass feedstocks in the state, and at different process temperatures using slow pyrolysis.An economic analysis was done to determine the cost of production of biochar, using biomass from forest thinning done for wildfire risk reduction as a case study. If pyrolysis of biomass expands due to changing energy prices and policies, the economic optimization of biochar and bio-oil developed in this study will be a valuable tool. Pyrolysis can address two challenges – the need for renewable energy, in particular, liquid fuels; and alternatives to burning as a disposal method for waste biomass. The controlled thermochemical conversion of pyrolysis can wring valuable products from biomass that would otherwise only contribute air pollutants. The 168 p report describing the specifics of the study can be found at the Washington State University Extension Energy Program website, www.pacificbiomass.org.

Technical Abstract: Biochar is a charcoal-like material produced by the thermochemical pyrolysis of biomass materials. It is being considered as a potentially significant means of storing carbon for long periods to mitigate greenhouse gases. Much of the interest comes from studies of Amazonian soils that appear to have been amended with biochar which led to significant improvements in soil quality and large increases in crop yields. These changes have persisted for hundreds, if not thousands, of years. What is not known is how long it takes for biochar to integrate with the soil and thus express its benefits. In this study, biochars from several different feedstocks were evaluated for their characteristics and their fate in five different Washington State soils. A bench-scale pyrolysis reactor was built at Washington State University (WSU) to provide biochar from different underutilized biomass feedstocks in the state, and at different process temperatures using slow pyrolysis. An economic analysis was done to determine the cost of production of biochar, using biomass from forest thinning done for wildfire risk reduction as a case study. If pyrolysis of biomass expands due to changing energy prices and policies, the economic optimization of biochar and bio-oil developed in this study will be a valuable tool. Key findings include: Biochar yield decreases (at a decreasing rate) with increasing slow pyrolysis temperature. The thermodynamics of the pyrolysis process do not support formation of dioxins and PAHs, as confirmed by a literature search, qualitative analysis, and quantitative analysis. The only compound found above the limit of detection was phenanthrene, and its environmental hazard was far below standards set by Ecology. All dioxin levels were similar to background levels found in Washington State soils. Biochar carbon concentration and pH increase with increasing pyrolysis temperature, while N and S concentration generally remain unchanged. Woody feedstocks lead to higher biochar C concentration than herbaceous feedstocks, but they are of lower pH and liming value.Higher temperatures (up to a point) lead to more bio-oil and less biochar, as does fast pyrolysis versus slow pyrolysis. The relative prices of bio-oil and biochar influence the economic trade-off between production of bio-oil and biochar. If producers can receive high prices for bio-oil but low prices for biochar, they can increase sales revenue by choosing a temperature and heating rate that yields more bio-oil at the expense of biochar (i.e., high temperature, fast pyrolysis). In contrast, if biochar prices are high and bio-oil prices are low, there is an incentive to choose a lower final temperature and slow pyrolysis that will provide more biochar and less bio-oil.For slow pyrolysis the optimal estimated yield and price for biochar are 26% and $0.077/kg, and for bio-oil optimal yield and price are 38% and $0.192/kg. Maximum revenue for slow pyrolysis is $0.09296/kg of forest-based feedstock (assuming both products are used solely for their energy value). Biochar represents an offset of about 2.93 MT CO2 per MT biochar applied to the soil.If pyrolysis of biomass expands due to changing energy prices and policies, the economic optimization of biochar and bio-oil developed in this study will be a valuable tool.

Last Modified: 4/20/2014
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