Air Emissions Control for the Biomass Industry
Environmental concerns have prompted government agencies to create rigorous standards for emissions control, and the biomass industry is no exception. Each process requires a different approach, depending on the feedstock, technology, regulations and life-cycle costs.
As a result of environmental concerns, government agencies have established increasingly stringent standards for emissions control. Some of the biomass processes are listed below along with the air emission controls that can be utilized to meet the necessary requirements.
Ethanol Production: Particulate matter (PM), volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) are typically released during fermentation, distillation, the spent grain drying process, and from storage tanks. The contaminants include: acetaldehyde, acrolein, ethanol, formaldehyde, 2-furaldehyde, methanol, acetic acid and lactic acid.
Installations of scrubbers or thermal oxidizers have met the requirements for air pollution abatement. Alternate solutions include the Photo-catalytic Gas Treatment (PGT)1 system, which achieves air emission control without any auxiliary fuel. A comparison to the regenerative thermal oxidizer (RTO) is shown for an ethanol process. (Figure 1)
Wood Pellets: Wood pellets are used as a supplement or replacement for oil and coal. The pellet is pressed or extruded under high pressure, generally after the wood has been dried to less than 15 percent moisture for use with combustion systems such as boilers.
European utilities have already started using wood pellets to burn along with coal in existing power plants.
The wood pellet plants operating in the Southeastern U.S. have installed and/or evaluated wet electrostatic precipitators (WESPs) and RTOs or PGT systems to control the PM, VOC, and HAPs which are primarily generated in the drying process of the green wood chips.
Poultry Litter: Poultry litter is utilized as a biomass fuel source for power generation. More than 500,000 tons of poultry litter annually, as well as other biomass, produce 55 megawatts of power, enough electricity to serve approximately 40,000 homes.
Potential process air emissions are controlled through a semi-dry absorption scrubber (SDA) and baghouse. (Figure 2)
Gasification: Biomass, wood chips, sawdust, char, coal, rubber or similar materials are converted into solid ashes, soot and syngas or producer gas with a gasifier. The gas can then be filtered for tars and soot/ash particles, cooled and directed to an end-user such as a kiln, furnace, boiler, an engine or fuel cell to produce electricity.
The quality of the gas from different gasifiers will vary depending on the system and overall fuel approach.
Pre-drying of wood biomass is essential in some processes in order to achieve the higher Btu syngas necessary for the process.
A wet electrostatic precipitator system, for instance the SonicKleen WESP, with a PGT or RTO system, can provide the necessary air emissions control for the facility. (Figure 3)
Air Emission Control Equipment
New technologies such as Catalytic Gas Treatment/PGT and traditional technologies are capable of meeting the most stringent control requirements of the biomass industry. A variety of control technologies are used in the biomass industry:
CGT or PGT: The CGT or PGT system utilizes a simple liquid process to absorb aldehydes and/or alcohols from industrial process gas streams. The absorption system uses hydrogen peroxide (H2O2), nitric acid (HNO3) or sodium hydroxide (NaOH), and aqueous catalysts to capture and oxidize the alcohols and/or aldehydes.
The lower the concentration of emissions in the process stream, the lower the CGT/PGT operating cost. Other notable features include "instant-on" and no standby costs.
The process gas flows up through a single-stage packed absorption tower, where formaldehyde, methanol, ethanol, and other aldehydes and/or alcohols are absorbed into the counter flow-water solution. The solution is retained for sufficient time in a reaction tank to complete the oxidation process.
The clean solution is recycled back to the top of the absorption tower to repeat the simple, continuous oxidation process. Additional catalysts and hydrogen peroxide are added as necessary to maintain the desired oxidation levels.
The CGT/PGT system is an economical solution to achieve compliance with the EPA maximum achievable control technology (MACT) requirements and eliminate the increasing fuel cost associated with the RTO alternative solution. It also eliminates the greenhouse gas emissions associated with nonrenewable fuel use.
WESP: The WESP system is an ultra-high efficiency mist and particulate eliminator that achieves compliance with the ever-increasing EPA MACT requirements for particulate, fumes, mists and condensibles.
The WESP system eliminates the problems associated with dry electrostatic precipitators, fabric filters and high-energy scrubbers:
Hydroscopic particles, which tend to become sticky and solidify causing hard-to-remove deposits and plugging/blinding
Saturated gas streams, which can plug fabric filters or cause corrosion problems
High pressure drop: 30-inch water column for a scrubber for high efficiency sub-micron particulate removal, 6- to 8-inch water column in a fabric filter
Re-entrainment losses or bleed-through losses
A down-flow WESP design offers unique features not available with an up-flow design. (Figures 4 and 5)
In wood dryer applications, a venturi/cyclonic separator has been extremely successful in process gas conditioning providing removal of larger particulate and fully saturating the process gas prior to the WESP. (Figure 6)
The WESP provides excellent PM control compliance and provides the protection required for downstream equipment for many of the biomass process air emission control systems.
SDA: The SDA system is ideally suited as an efficient acid gas/mist control device upstream of a baghouse or ESP for many types of boiler applications in the biomass industry.
The SDA injects lime slurry into the boiler (process) exhaust through an efficient atomizing nozzle system at the top of a down-flow absorption tower. The droplet size and uniform distribution into the air stream are essential to the performance of the SDA system. Droplet size is generally measured in sauter mean diameters and distributed in a bell curve fashion. Although the mean diameter is an important factor, the maximum droplet size and percentage of maximum droplets need to be considered in sizing the retention time to insure that all the water is evaporated and the bottom of the tower remains dry. Evaporation time for the larger droplets is exponentially longer than for small droplets.
A two-phase water and compressed air nozzle can generate a droplet one-tenth the size of a single-phase, water-only nozzle, thus substantially reducing the retention time and size of the absorption tower.
The lime reagent absorbs and reacts with the acid mist to form calcium sulfite or sulfate, which is carried through to the baghouse where it is collected and removed from the system.
The SDA offers several benefits over other types of control including:
Greater than 90 percent removal of SO2 and greater than 95 percent HCl removal
Utilizes a low-cost reagent, lime
No liquid effluent-all water is evaporated into the air stream
No water treatment required
Simple to operate
Wide turndown ratio of 10 to 1
RTO: Today's RTOs have optimized the benefits of structured media, simplified flow control and compacted the configuration into a smaller prepackaged unit, greatly improving on the original design. Thirty years of design improvements have resulted in a greatly improved RTO. Many of the low-concentration biomass processes still require substantial amounts of auxiliary fuel, however, and have a large carbon footprint.
There are many variations in biomass fuel and energy production that require a variety of customized solutions. Each application must be evaluated in light of the process characteristics, emissions regulations, and life-cycle costs to arrive at an optimal solution and choice of equipment. BIO
1Groupe Conseil PROCD Inc., CGT™ US Patent 7,304,187, PGT Patent-Pending
Rodney L. Pennington is a registered professional engineer with more than 36 years of diverse experience in all phases of research, engineering, design, management, sales and marketing of air pollution control and energy conservation systems. He has more than 20 patents, is a published author and speaker and has served as an expert witness in regenerative technology. Reach him at firstname.lastname@example.org.