Zeroing in on distributed, portable fuels, chemicals production
The biomass industry understands that the high cost of transporting raw materials can ultimately make or break a project. John Hurley, senior research advisor for the Environmental Research Center, explained to attendees at the EERC’s Biomass ’11: Renewable Power, Fuels and Chemicals Conference held in Grand Forks, N.D., how distributed or even portable energy and fuels production is one strategy for alleviating the economic impacts of transportation costs in biomass utilization.
The EERC is in the process of developing a mobile indirect liquefaction system for converting cellulosic biomass materials into biobased chemicals, such as methanol, or even fuels such as ethanol or isobutanol. The work is being funded through the Xcel Energy Renewable Development Fund and the U.S. DOE through the EERC Centers for Renewable Energy and Biomass Utilization. The system is currently under construction, according to Hurley, with parametric testing planned throughout the summer and fall.
The concept of the mobile unit, Hurley explained, originated from a conversation he had with the president of IdaTech LLC, an Oregon-based firm that markets fuel cell systems for converting methanol to heat and power.
“IdaTech was looking for a source of ‘mean methanol’ and so we had developed some technologies at the EERC for converting wood waste into liquid fuels,” Hurley said. “IdaTech said they wanted to use a resource they have in Oregon, which is waste wood that’s left over after a lumber company goes through a forest area and then leaves small wood behind, such as branches and small trees. That can amount to 10 tons per acre.”
Hurley said the EERC’s mobile liquefaction unit makes sense because it can be driven up to the piles of legacy wood and be converted into fuels or chemicals instead of forest service companies having to return to the site on which the waste wood sits, such as by a sawmill where much of it is typically burned.
In the EERC program, the technology will be demonstrated by building and testing a 200-pound-per-hour fixed-bed downdraft biomass gasifier, air-blown and with specialized gas cleaning equipment to produce the syngas. The system will be integrated with 3-meter-long packed-bed catalytic reactors for producing liquid fuels. The unit will also feature sophisticated automation tools for minimizing labor requirements. A design review has confirmed the fixed-bed biomass gasifier selection as the lowest capital cost system for producing methanol. EERC and IdaTech focused on methanol due to it being a good carrier of hydrogen that has high density with a high hydrogen-to-carbon ratio.
A glaring advantage the EERC’s gasification system has over other conventional gasification systems, according to Hurley, is that it can process biomass with high moisture content (“green wood”) with up to 20 percent or more. This reduces cost for drying the wood before gasification, resulting in substantial energy and processing savings.
“Most gasifiers typically operate with about 15 to 20 percent moisture in their [biomass] fuel,” Hurley said. “One thing we wanted to do was come up with a gasifier design that could handle specifically more than 20 percent moisture because if we’re going to handle green, it might have up to 45 to 50 percent moisture in it.”
In fact, the high moisture content creates a syngas with significantly higher hydrogen content than if the moisture were not present, Hurley said. High hydrogen content is especially conducive to making liquid fuel from the gas stream since the hydrogen-to-carbon ratio in a liquid fuel, like methanol, is much greater than that of the wood itself. By increasing the hydrogen content in the gas stream, explained Hurley, higher carbon conversion efficiencies can be achieved.
“Hydrogen is a key when you want to convert this syngas into a liquid fuel because you want a hydrogen-to-carbon monoxide ratio of about 2-to-1,” he said. “Typically, when you gasify, especially dry wood, it’s good to be at like 1-to-1 so you want to push that reaction, especially when you’re dealing with a wet wood.”
The EERC can modify its gasification equipment to either employ a Fischer-Tropsch conversion route or employ gasification with an alcohol synthesis reaction. Hurley said the typical production method for the mobile liquefaction unit using lignocellulosic feedstock is gasified to a syngas at temperatures between 450 and 620 degrees Fahrenheit at fairly high pressures.
Depending on the catalysts used, the system can produce methanol at about 95 percent purity with the remaining 5 percent comprising water and other heavier alcohols. “You can use the methanol to power a fuel cell, but it can also be used in biodiesel operations,” Hurley said. “Converting bio-oil into biodiesel requires about 10 percent methanol by weight and that’s an advantage of methanol.”
Bench scale gas-to-liquid testing indicated that approximately 50 gallons of methanol can be produced per ton of moist wood for a single reactor, and up to 80 gallons per ton for two reactors.
While Hurley admitted that the gasification equipment is unique, the EERC continues to fine-tune and optimize the technology in order to maximize economic and technological feasibility. Among them include: adding a hydrogen separation membrane to a single reactor that could double production to 100 gallons of methanol per ton of wet wood; a water-gas shift reactor that, coupled with hydrogen separation, could triple production to 150 gallons per wet ton; a char conveyor system to clean wastewater in an internal loop; harvest energy of compression from unreacted gas with a turboexpander to produce electricity and cooling for reducing power input requirements; modifying the compressor or a generator to run on excess syngas to produce power; and find funding to test all of these concepts.