FIGURE 4-1 Lignocellulose structure.
SOURCE: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science.
Gasification, Brown explained, is the thermal decomposition of organic matter into flammable gases, using either a bubbling fluidized bed reactor or an entrained flow gasifier to produce syngas. These technologies have been commercialized for coal, but in some respects they work even better with biomass. Though gasification is in theory an equilibrium process at very high temperatures and long residence times, in practice equilibrium is obtained rarely and so the process generates tar, char, and small amounts of contaminants that agronomists would like to claim as nutrients. These contaminants, which include small amounts of alkali metals, sulfur, nitrogen, and chlorine that must be removed before upgrading in order to prevent poisoning of catalysts. Removal of each contaminant requires its own catalyst, adding substantial costs to any gasification process.
A significant advantage of the gasification process is that there is no question of what to do with lignin, as it is turned into syngas, too. Gasification can also handle virtually any feedstock, including waste streams, and produce a uniform intermediate product for upgrading. It can also be used to produce heat, power, fuels, or chemicals and allows for energy integration into biorefinery operations.
Technical challenges are particularly challenging in terms of developing technologies that can cost-effectively remove contaminants from the gas stream (Figure 4-2). Gasification operations must also be integrated with fuel synthesis operations, which is not a simple matter given that fuel synthesis occurs at high pressures and under exacting stoichiometries. From a commercial perspective, gasification only works economically at large scale, which translates into high capital costs that could be as high as $10 per gallon of annual plant capacity. Brown added that biomass gasification must also compete with steam reforming of natural gas, which while not a renewable resource is a domestic resource that would move the country away from imported petroleum and reduce greenhouse gas emissions compared to petroleum.
The other major thermochemical technology is fast pyrolysis, which rapidly heats biomass in the absence of oxygen to produce three products: syngas, charcoal, and a liquid mixture of organic compounds and water known as bio-oil that is recovered from pyrolysis vapors and aerosols (see Figure 4-3). Charcoal, also known as biochar, can be used as a carbon sequestration agent. In combination with the bio-oil, biochar presents an opportunity for producing carbon-negative fuel, said Brown.
Fast pyrolysis, he explained, is characterized by residence times of 0.5 to 2 seconds, a very high rate of heating at moderate temperatures of 400–500°C, and the production of a liquid that looks like petroleum but smells like barbecue sauce. Typically, 60–70 percent of the weight of biomass is converted to bio-oil (see Figure 4-4). The yield of biochar ranges from 13 to 15 percent and that of syngas is in the 13–25 percent range. The syngas can be used as an energy source to support this process.
Pyrolysis chemistry is poorly understood, and Brown stressed the need for chemists and chemical engineers to study this process. Studies at Iowa State have found that pyrolysis converts cellulose into products in a number of competing parallel pathways. This work has also shown that alkali present in biomass acts as a powerful catalyst that produces undesired light oxygenates, and so research is needed to understand how to control or suppress this process. Chemists can play a critical role in this research and help produce a more valuable product in the end. (To see a figure of pyrolsis chemistry, please see Figure 4-5.)