Lignin's Role in Reducing Life-Cycle Carbon Emissions Explored in New Research Paper
Cellulosic biofuels are the focus of intense research aimed at developing transportation fuels that are significantly lower in carbon intensity than those derived from petroleum. Biofuels have the potential to reduce the impact of the transportation sector on the climate—cellulosic ethanol, by some estimates, may reduce the carbon emissions relative to gasoline by up to 80 percent. While researchers have developed technologies capable of converting many components of wood and other plant material into liquid fuels, lignin, a chemical in plants that gives their cells rigidity, has proven difficult to break down.
Current models of the refining process for biomass-to-transportation fuels assume that the lignin component is burned onsite to meet the plant’s process heat and power needs. Onsite combustion offsets some of the plant’s energy costs, and provides the plant with offset credits for greenhouse gas emissions.
Other options exist, including shipping the lignin to nearby coal-fired power plants. Offsetting some of the coal burned in these plants with lignin from biorefining reduces their carbon footprint. What is the most effective way to use lignin such that the positive impacts of reducing energy demand and emissions can be achieved at the lowest capital cost and water demand? The answer to this question interests parties across the energy industry, from policymakers to utilities and operators of generating plants, to the biofuels research & development community. For the first time, research conducted at the Lawrence Berkeley National Laboratory (Berkeley Lab) addresses this question at a national production scale.
A new study, published in the journal Environmental Science & Technology and led by Corinne Scown, uses life cycle analysis modeling (LCA) to answer this question. Scown is in Berkeley Lab’s Environmental Energy Technologies Division (EETD). Scown and her colleagues conducted a life-cycle assessment of four options for using lignin: (1) onsite combustion for heat and power; (2) onsite combustion plus the use of additional gas-fired power generation; (3) export lignin to coal-fired power plants, use natural gas to meet the biorefinery’s heat requirements and a portion of electricity use; (4) export lignin to coal-fired plants, use natural gas to meet all of the biorefinery’s heat and power needs.
In cases 1 and 2, biogas produced at the refinery and a solids boiler for the lignin produce electricity and process heat used in the manufacturing of the biofuels. Cases 3 and 4 eliminate the need for a solids boiler at the refinery site.
The team evaluated these four cases under a U.S.-based cellulosic biofuel production scenario in which corn stover (leaves, stalks, husks and cobs of corn), wheat straw, and the fast-growing tall grasses of the genus Miscanthus are converted to 160 billion liters of ethanol annually. The results are applicable to any biofuel process that cannot breakdown lignin. “As far as we know, this is the first evaluation of lignin use options at the scale of a national biofuels production scenario,” says Scown. “We also know of no other study that has explored the life-cycle water use tradeoffs of such a scenario.”
Using a computer model of an ethanol biorefinery, the research team calculated that the life-cycle greenhouse gas emissions ranges from 4.7 to 61 grams of carbon dioxide per megajoule (g CO2e/MJ). This compares to 95 g CO2e/MJ for gasoline.
Scown adds, “Overall, we found that exporting lignin to coal-fired power plants can reduce GHG emissions at a magnitude comparable to using lignin onsite to provide power in some cases. Export of lignin can reduce life-cycle water consumption by up to 40 percent, and reduce capital costs by up to 63 percent, in part, by eliminating the need for an onsite solids boiler.”
The study also found that nearly half of U.S. coal-fired power plant capacity is expected to be retired by 2050, which will limit the capacity for co-firing with lignin, and double the transportation distances between biorefineries and coal power plants.
The article “The role of lignin in reducing life-cycle carbon emissions, water use, and cost for U.S. cellulosic biofuels,” was authored by Corinne D. Scown, Amit A. Gokhale, Paul A. Willems, Arpad Horvath, and Thomas E. McKone. The research was funded by the Energy Biosciences Institute.