Biomass Gasification: Any New Environmental Concerns?

By Todd A. Potas | January 10, 2008
Biomass gasification has become a viable commercial-scale option for both conventional dry-grind and cellulosic ethanol facilities. In fact, biomass gasification capacity may be quickly added to existing facilities to take advantage of their infrastructure, assuming proven, dependable technologies exist commercially. A variety of technologies is available to provide renewable power or displace fossil fuel for process steam generation. It's also possible to provide additional ethanol or other alcohol production by thermochemically converting biomass-generated gasifier synthesis gas, or syngas (Figure 1). Thermochemical conversion at cellulosic ethanol plants will help mitigate the higher construction costs associated with these facilities and help diversify corn-based feedstock use.


Figure 1. Gasification provides a wide array of useful end products.
SOURCE: J.SCAHILL, U.S. DOE


Several regulatory issues arise with the addition of any gasification process. This article presents some of these environmental issues related to regulatory applicability, air emissions, water discharge and solid waste (ash) for some typical biomass gasification designs. The discussion includes not only supplementing or replacing natural gas-fired steam and drying energy with synthesis gas combustion, but also issues related to separation and reformation of synthesis gas for production of ammonia, carbon dioxide, sulfur, and alcohols such as methanol, ethanol and/or butanol.

Several types of gasifier designs are available for biomass
(Figure 2).


Figure 2. A variety of gasification designs provide for feedstock flexibility.
SOURCE: J. SCAHILL, U.S. DOE


Updraft/Downdraft Gasifiers
One advantage of the updraft and the downdraft gasifier designs is that larger feed sizes can be used with less concern for incoming moisture content. For example, these systems can use 2-inch or less diameter chipped wood to produce synthesis gas for subsequent combustion to produce steam and/or electric power, similar to PrimeEnergy LLC's updraft gasifier used at Central MN Ethanol Co-op in Little Falls, Minn. The subsequent combustion unit burning the synthesis gas can be designed for use as a thermal oxidizer. This allows for control of volatile organic compound emissions from drying spent grain from an ethanol facility. This type of system can replace natural gas-fired boilers and regenerative thermal oxidizer emission controls used for the spent grain dryer exhaust at existing ethanol plants. The gasifier/thermal oxidizer system can also have natural gas back-up for biomass gasifier down-time.

Several additional environmental compliance issues are related to this type of gasifier solid fuel system. First, additional air emission units are needed for wood handling and storage. Particulate emissions can be generated from the unloading, chipping, handling and storage of the wood or other biomass brought on-site. Wood generates less dust due to its larger size and higher moisture content. Therefore, baghouse or cyclone control may not be required for particulate control.

Second, different gasifier and thermal oxidizer operating parameters are required for proper combustion performance. Due to the reduction conditions of the gasification reaction, many potential combustion pollutants are never generated because key elements remain in the ash. This pertains especially to pollutants formed from sulfur and other potentially hazardous inorganic compounds such as acid gases. Once the synthesis gases are formed and utilized for combustion, the same pollutant formation issues that arise during fossil fuel combustion will occur (e.g., nitrogen oxides, carbon monoxide, particulate matter and VOCs). Controlling NOx formation and carry-over of fine particulates is critical for low-emission performance. Particulate carry-over in the updraft/downdraft systems can be a concern and is often controlled with add-on control technologies.

The quality of the synthesis gas from updraft/downdraft systems is well-suited for combustion applications. However, it may not be particularly good for thermochemical conversion to hydrogen or other compounds like alcohols because the synthesis gas from these units is not as refined as might be needed to conduct thermochemical conversion. A more refined synthesis gas will provide more efficient conversion. Gas membrane separation can be used to separate hydrogen as a product. This process is being conducted at Community Power Corp.'s Biomax 25 downdraft gasifier used at the Natural Resources Research Institute's Coleraine Minerals Research Laboratory in Coleraine, Minn.

Finally, there is the issue of ash characterization, utilization and/or disposal. Updraft and downdraft gasifiers produce both bottom and fly ash. Each of these ash streams has different properties making collection, characterization and utilization more challenging. Due to the larger size of the incoming wood, sandy soils from harvest and storage are more difficult to keep out of the gasifier. This can lead to operational problems in these types of gasifiers, depending on the ash separation systems employed. Because ash is removed at two locations, composite characterization of the ash is difficult. However, this characterization is helpful in determining the potential uses for the ash products. For example, if the level of leachable metals is low, the ash may be sold for land application. However, the alternative may also be true. If the levels are too high it may add costs to the ash disposal. It is important to have the ash characterization early in the project to determine its potential uses and their effect on the economic feasibility of the project.

Fluid Bed and Circulating Fluid Bed Gasifiers
Bubbling and circulating fluid bed gasifiers operate by maintaining a highly turbulent mixing zone for the gasification reactions. Air, oxygen or steam can be used to control the gasification conditions, including the bed velocities. Circulating a fraction of the fluidized materials assists the steady-state conditions in the fluid bed and allows for some bottom-ash removal. The majority of the ash is carried out the top of the gasifier in the circulating systems and all the ash is carried out in bubbling bed systems.

The fluidized nature of these gasifiers requires the size of the feedstock to be much smaller than for fixed-bed gasifiers. For example, wood chips need to be sized to less than three-quarters of an inch. This allows more sands and soil material to be separated prior to being fed to the gasifier, minimizing the carry-over of sandy particulates. Corn stover, especially corn cobs, is projected as a promising feedstock for these types of gasifiers. An example of this system is Frontline BioEnergy LLC's fluid bed gasifier at Chippewa Valley Ethanol Co. in Benson, Minn.

The synthesis gas from fluid bed systems is well-suited for combustion applications, similar to updraft/downdraft units. The synthesis gas, with some clean-up of tars and potentially desulfurization, can be used for thermochemical conversion to alcohols.

Entrained Flow Gasifiers
Entrained flow gasifiers are more complex systems requiring even smaller particle size and, thus, lower moisture content than fixed- or fluid-bed systems. This additional up-front processing produces a higher quality synthesis gas better suited for thermochemical conversion to methanol, ethanol and/or butanol. The gasifier can operate on dried, finely ground corn stover, but also other biomass, such as wood or other crop residues. Particle sizes can range to as low as minus 200 mesh. This biomass converter system is proposed for Range Fuels Inc.'s planned ethanol plant in Treutlen County, Ga.

With the drying and fine milling necessary for these systems, additional emission sources are generated. The drying emissions will need to be controlled or integrated into other process operations to prevent uncontrolled VOCs from being emitted to the atmosphere. Baghouse control will be required for the milling. For optimum milling and gasifier performance, the process will benefit from further sands/soils removal prior to these operations. By producing a higher quality synthesis gas, thermochemical conversion to alcohols is projected to be very efficient at a commercial scale.

Summary
There are many advantages to utilizing gasifiers as part of ethanol production facilities. Biomass feedstock is cleaner, renewable and less expensive than traditional solid fossil fuels, and synthesis gas from gasifiers can be converted into alcohols to supplement corn- or sugar-based ethanol production. However, with each type of gasification process, there are also environmental and regulatory considerations. Additional particulate and VOC controls may need to be integrated into current ethanol plant or new gasification/combustion design to mitigate emissions. The ash byproducts of gasification may prove difficult to collect and characterize, and can cause operational issues. Future research and development will pave the way for biomass gasification to become more environmentally compliant and commercially viable.

Todd A. Potas, PE, QEP, is a principal at Natural Resource Group LLC and manages air quality and environmental services for the ethanol industry. Reach him at (612) 347-6789 or www.nrg-llc.com.