Update on a Mobile Indirect Biomass Liquefaction System
Minnesota’s forestry operations produce 300,000 tons per year of wood waste that is not used in any existing or proposed facility. Through the process of indirect liquefaction, this waste can be converted into liquid fuels that could be transported to remote off-grid sites and reformed to hydrogen to power fuel cells producing electricity.
Using distributed power generation at off-grid sites eliminates the need to build transmission lines at remote sites, which ultimately saves utility ratepayers money. In addition, the wood-to-fuel technology provides a non-fossil fuel, nearly carbon dioxide neutral method to fuel backup generators. Even in areas that are served by the grid, this saves utility ratepayers the cost of maintaining large backup power production systems. Ratepayers may also be able to take advantage of future carbon credits or avoid carbon taxes applied to fossil energy-based power production.
The Energy & Environmental Research Center has developed and tested at small scales much of the technology necessary for distributed indirect liquefaction systems. With funding provided by customers of Xcel Energy through a grant from the Renewable Development Fund, and the U.S. Department of Energy through the EERC Centers for Renewable Energy and Biomass Utilization, the EERC designed and built a mobile, demonstration-sized indirect wood waste liquefaction system. The EERC then operated it in order to determine best construction and operating practices, overall system productivity, and necessary design changes to make the concept more commercially viable. The system was described in this column in the April 2011 issue.
The system uses a unique gasifier to convert the wood waste into synthesis gas, which is cleaned and compressed and flows to a reactor that converts the gas to a liquid. In this program, we focused on the production of methanol, the simplest alcohol, because it can be easily reformed into hydrogen, which can be used to power fuel cells to efficiently make electricity at sites separate from the biomass resource. The gasifier was designed by the EERC to handle wet wood waste with up to 40 percent moisture, thereby eliminating the need to dry the wood before gasification, as most commercial gasification units require.
Two types of wood waste were tested in the system: chipped hybrid poplar and chipped ash. In both cases, the hydrogen content of the gas produced was lower than expected. The methanol production rate was approximately 15 gallons per ton of biomass for both wood types. This initial production rate was relatively low but did serve to validate computer models of the system performance. Using those models, engineers have evaluated several improvements to the system to increase the hydrogen content of the syngas, which should allow production rates as high as 50 gallons per ton with the existing design and as much as 100 gallons per ton with additional hardware.
Demonstrating this technology and using it to validate our engineering models has been an important step toward making use of neglected biomass residues to ultimately provide renewable distributed power generation. But an essential question must be answered: at what cost? The economics of the production of methanol by this technology will be discussed in a future Energy Review column.
Author: John P. Hurley
Senior Research Advisor, EERC