Biomass torrefaction: understanding greenhouse gas emissions and potential financial opportunities
||Start Date: 09/01/2011
|Project Number: RS-0028-12
||End Date: 08/30/2012
|Amount Awarded: $69,978
Vance Morey Bioproducts and Biosystems Engineering, CFANS (UMNTC)
Doug Tiffany Applied Economics, CFANS (UMNTC)
Torrefaction is a pre‐commercial process that roasts woody and herbaceous biomass to produce biocoal and process heat. The biocoal produced from a variety of biomass sources typically has 130% of the energy per unit of mass compared to dry untorrified biomass, so it has energy content similar to fossil coal and more favorable logistical and grinding characteristics than unprocessed biomass. This effort extends previous work related to this process with detailed analysis of greenhouse gas (GHG) emissions of this technology as well as economic analysis to model the returns for two or more business entities that might choose to collaborate in a torrefaction enterprise. Improved understanding of GHG emissions for this process using several biomass sources offers a basis to understand commercial opportunities to market torrefied biomass to various markets, especially coal‐fired utilities that are targeted for CO2 taxes and mandated reductions. Use of volatile gases emitted in the roasting process affect the total benefits that might accrue from this process that transforms biomass into a more homogenous and compatible fuel for blending with coal. Additional economic and financial analysis will focus on identifying and modeling situations that may be attractive for co‐located facilities such as fuel ethanol plants, coal‐fired power plants, communities with district heating, or other businesses that have reliable demand for thermal energy.
FINAL REPORT 1/29/2013
Torrefaction is a thermo-chemical treatment (roasting) of biomass at 200 to 320 °C (390 to 600 °F) in the absence of oxygen at atmospheric conditions. During the torrefaction process, the water contained in the biomass as well as superfluous volatiles are removed, and the biopolymers (cellulose, hemicellulose and lignin) partly decompose giving off various types of volatiles. The final product is the remaining solid, dry, blackened material which is referred to as “torrefied biomass” or “biocoal”. With its high energy density and resistance to degradation and absorption of water, torrefied biomass should be quite compatible with the logistics system serving coal in the U.S. and around the world. The volatile gases driven off during torrefaction may be combusted for other beneficial uses beyond providing the necessary heat to sustain the torrefaction reaction and dry the biomass entering the torrefaction chamber.
A life cycle assessment (LCA) study was conducted to understand and assess potential greenhouse gas (GHG) emissions reduction benefits of a biomass torrefaction business integrated with other industrial businesses for the use of the excess heat from the torrefaction off-gas volatiles and biocoal. A torrefaction plant processing 30.3 t/h (33.4 ton/h) of corn stover at 17% wet basis (w.b.) moisture content was modeled. The excess heat from the torrefaction plant met about 42.8% of the process steam needs (excluding the co-products dryer heat demand) of a 379 million liter per year (100 million gallon per year) natural gas-fueled dry-grind corn ethanol plant, which corresponds to about 40% reduction in life-cycle GHG emissions for corn ethanol compared to gasoline. A sensitivity analysis showed that adding a combined heat and power (CHP) system at the torrefaction plant to meet 100% electricity demand of the torrefaction plant (i.e., 2.5 MWe) would result in lower GHG emissions for biocoal, corn ethanol, and co-fired electricity than for the case where the torrefaction plant purchased electricity from the grid.
Torrefaction economics are examined by modeling torrefaction plants producing biocoal from corn stover, with and without, utilization of off-gases that are produced in the torrefaction process. The utilization of biocoal blends in existing coal-fired power plants is modeled to determine the demand for this fuel in the context of emerging policies regulating emissions from coal. Blending of biocoal at existing coal-fired power plants, especially those using bituminous coal, may be a gradual way to improve the environmental performance of coal-fired power plants while extending their economic lives for several decades. Opportunities to co-locate torrefaction facilities adjacent to corn ethanol plants and coal-fired power plants are explored as a means to improve economics for collaborating businesses.
Independent torrefaction plants can produce a ton of biocoal for $42 per ton of biocoal in overhead and operating costs, while co-located torrefaction plants that can sell steam generated from off-gasses operate with net costs of just $17 per ton of biocoal produced. This differential represents a significant competitive advantage to torrefaction businesses when co-location opportunities occur. Ethanol plant returns on equity are substantially improved when sold steam costing $5.00 per 1,000 pounds of steam from co-located plants converting corn stover to biocoal, especially if future standards of advanced biofuels include the use of steam generated from torrefaction off-gases to replace natural gas and/or if carbon taxes are charged on greenhouse gas emissions. Opportunities for biocoal co-firing with bituminous coal are most advantageous in states with high prices of bituminous coal and strong standards favoring production of renewable electricity. Projected federal policies charging carbon taxes on coal plant CO2 emissions as well as estimated allowances for SO2 and NOx emissions will improve economic opportunities to blend biocoal with bituminous coal. However, the practice of biocoal blending will need additional incentives to become economically viable based on delivered costs per ton of corn stover and biocoal at $70 and $150, respectively.
Identified opportunities to improve economic attractiveness of torrefaction facilities to produce biocoal by co-locating them with corn ethanol plants.
Quantified life – cycle greenhouse gas emission savings for the ethanol produced and the electricity produced from the resulting biocoal.
Identified that opportunities for biocoal co-firing with bituminous coal are most advantageous in states with high prices of bituminous coal and strong standards favoring production of renewable electricity.
Identified that projected federal policies charging carbon taxes on coal plant CO2 emissions as well as estimated allowances for SO2 and NOx emissions will improve economic opportunities to blend biocoal with bituminous coal.
- Kaliyan, N; Morey, R; Tiffany, D; Lee, W. Life cycle assessment of corn stover torrefaction plant integrated with a corn ethanol plant and a coal fired power plant. Biomass and Bioenergy. 2012, (submitted).
- Tiffany, D. Torrefied Biomass: Production Costs and Value to Power Utilities. Presentation to North Central Farm Management Association at their meeting at the University of Minnesota. May 19, 2011
- Tiffany, D. Considering Greater Use of Biomass in Japan: a U.S. Perspective. Presentation to the U.S.-Japan Institute at their meeting in Washington, D.C. March 5, 2012