U.S. Department of Energy - Energy Efficiency and Renewable Energy

Biomass Program

Large-Scale Gasification

Biomass gasification involves thermally converting biomass to simple chemical building blocks that can be transformed to fuels, products, power and hydrogen. Components include feed preparation, the biomass gasifier, and a gas treatment and cleaning train. The initial syngas contains particulates and other contaminants and must be cleaned and conditioned prior to use in fuels, chemical or power conversion systems (e.g. catalyst beds, or fuel cells).

Biomass gasification provides an efficient approach for producing fuels and products from biomass. The gasification process readily converts all major components of biomass including lignin, which is resistant to biological conversion, to intermediate building blocks. Utilization of the lignin, which is typically 25-30% of the biomass is essential to achieve high efficiencies in the biorefinery. The gasification process is "omnivorous" in this regard and can convert most biomass feedstocks or residues to a clean synthesis gas. Once a clean synthesis gas is obtained, it is possible to access and leverage the process technology developed in the petroleum and chemicals industry to produce a wide range of liquid fuels and chemicals.

There are several widely used process designs for biomass gasification. In staged steam reformation with a fluidized-bed reactor, the biomass is first pyrolyzed in the absence of oxygen, then the pyrolysis vapors subsequently reformed to synthesis gas with steam providing added hydrogen, as well as the proper amount of oxygen and process heat comes from burning the char. With a screw auger reactor, moisture (and oxygen) is introduced at the pyrolysis stage and process heat comes from burning some of the gas produced in the latter. In entrained flow reformation, both external steam and air are introduced in a single-stage gasification reactor. Partial oxidation gasification uses pure oxygen, with no steam, to provide the proper amount of oxygen. (Using air instead of oxygen, as in small modular uses, yields producer gas (including nitrogen oxides) rather than synthesis gas.)

Flow diagram of biomass gasification via staged steam reformation with a fluidized bed gasifier.

Flow diagram of biomass gasification via staged steam reformation with a screw auger gasifier.

Flow diagram of biomass gasification via entrained flow steam reformation.

Flow diagram of biomass gasification via partial oxidation (auto thermal).

Biomass gasification is also important in its role of providing a source of fuel for electricity and heat generation for the integrated biorefinery. Virtually all other conversion processes, whether physical or biological, produce residue that can't be converted to the primary product(s). To avoid a waste stream from the refinery and to maximize the efficiency of the biorefinery, these residues can be used for combined heat and power production (CHP). In existing facilities, these residues are combusted to produce steam for power generation. Gasification offers the potential to utilize higher-efficiency power generation technologies such as combined cycle gas turbines or, in the future, fuel cells. Gas turbine systems offer potential electrical conversion efficiencies approximately double that of steam-cycle processes, with fuels cells being nearly 3 times as efficient.

Status of Gasification Technology

Biomass gasification technologies have been a subject of commercial interest for several decades. Interest in biomass gasification increased substantially in the 1970s because of uncertainties in petroleum supplies, with most of the development occurring in small-scale systems. Low-energy gasifiers are now commercially available, and dozens of small-scale facilities are in operation.

In the 1980s, government and private industry sponsored R&D for gasifier systems primarily to gain a better understanding of reaction chemistry and scale-up issues. In the 1990s combined heat and power was identified as a potential near-term opportunity for biomass gasification because of incentives and favorable power market drivers. R&D concentrated on integrated gasification combined cycle (IGCC) and gasification co-firing demonstrations, which culminated in a number of commercial-scale systems. In the U.S., projects mostly processed very recalcitrant feeds such as bagasse and alfalfa.

Technical Barriers for Gasification

Feed processing and handling

Syngas platforms require a supply of uniform feedstock and reliable feed preparation, storage, and handling systems. Commercial operators must have quality control (QC) procedures to ensure uniformity in biomass feedstocks and for long-term fuel supply contracts. In addition, the history of biomass project development has taught that reliable feeders are key to any successful project or system. There are a number of feed systems that function reliably using feedstocks within a narrow range of physical properties (size, moisture, etc.). However there are few if any feed systems that can function reliably across the range of feed properties likely to be encountered if biorefineries are to become a major element of the U.S. economy. An alternate approach is to develop in-plant feedstock handling that can economically convert a wide range of feedstocks to a consistent form that existing feeders need to function reliably.

Gasification

Biomass and black liquor gasification are technologies that have developed to the point of large-scale demonstration. However, widespread commercial availability of gasifier systems suitable for integration with fuels synthesis or hydrogen separation technologies has not yet been realized. In part this is due to areas of fuels chemistry that require additional investigation to support the commercial demonstration programs and facilitate the development and scale-up of advanced gasifiers and gas clean-up systems.

Syngas cleanup and conditioning

The raw gases from biomass systems do not meet strict quality standards for downstream fuel or chemical synthesis catalysts nor those for some power technologies (fuel cells or fuel cell/turbine hybrids), and will require gas cleaning and conditioning to remove contaminants such as tar, particulates, alkali, ammonia, chlorine, and sulfur. Available cleanup technologies do not meet the needed cost, performance or environmental criteria needed to achieve the Program goals or commercial implementation.

Sensors and Controls

Effective process control will be needed to maintain plant performance and emissions at target levels with varying load, fuel properties, and atmospheric conditions. Development of new sensors and analytical instruments is needed to optimize control systems for thermochemical systems.

Process Integration

As with all new process technologies demonstrating sustained integrated performance that meets technical, environmental, and safety requirements at sufficiently large scale is essential to supporting commercialization. Black liquor mill integration has the added complexity of being attached to an existing commercial process where the unit operations associated with steam production, power, pulping, and chemical recovery must all be integrated.

Containment (materials of construction)

Experience with black liquor gasifiers indicates that the reactions are difficult to contain and that long term and economically acceptable approaches are yet to be developed. Solutions involve metals used for reactor shells and internals, refractory materials to line containment vessels, vessel design, and increased knowledge of bed behavior and agglomeration.

For Further Reading