When emissions exceed regulatory standards, plant managers are often faced with the dilemma of cutting back production in order to maintain compliancy or incur stiff penalties. Plants that are forced to run boilers at a lower level to scale back emissions lose productivity and efficiency, which quickly becomes very costly. Fortunately, there are smart equipment options that help maximize efficiency while reducing particulate emissions. From the biomass fuel delivery mechanisms to the boiler combustion air systems to reliable mechanical dust collector operations, proper biomass air and fuel handling techniques can not only minimize emissions, but also increase BTU output from your boiler.
From the time biomass fuels arrive at the facility, there are simple steps that can keep systems running at full capacity while staying well within the bounds of emission standards. First it's critical to look at the boiler biomass feed system. Improperly functioning feed systems are the culprits of many emission overruns. If the feed system is not designed and equipped to operate smoothly, emissions issues will crop up in the later stages of the air and ash handling process.
Unlike coal, which tends to be more uniform and easier to manage with less foreign debris, biomass fuels clump in ways that often stymie conveyer system equipment like metering reclaimers, screw feed bins, chute work and boiler feed spouts. It is paramount that the biomass fuels are properly screened and sized to the boiler size specifications as soon as they arrive at the plant.
Foreign material such as rocks and metal must be removed, and the biomass fuel should be shredded or hogged to the size specification immediately after being unloaded at the plant and prior to being sent to storage or the boiler. If this is done properly and consistently, all of the equipment downstream will perform more reliably, with less power and less maintenance.
At the boiler, biomass metering feed bins that bridge often result in an uneven flow of fuel to the boiler. This is an early indicator that emission problems are likely to occur down the line. The irregular sizes and flow characteristics of biomass material require biomass metering bins to be designed with rugged screws that can continue to operate and feed the fuel in a uniform manner. If the bins are not equipped with the proper mechanisms to minimize bridging of the woody biomass, there will be interruptions in the fuel delivery and improper fuel distribution in the boiler. This will result in lower combustion efficiency and higher emissions at the stack.
Indications that there are problems with the fuel delivery system can easily be seen at the boiler grate on stoker type boilers. If the fuel distribution system is creating uneven mounds of fuel on the boiler grate, the combustion air system will not perform properly. Combustion air will always take a path of least resistance. In this case, the airflow will be around the pile of fuel that needs to burn and to a more open area of the stoker grate where there is no fuel. For combustion, air needs to pass through the biomass and not around it. Therefore the biomass feed system must have the ability to create an even bed of fuel over the stoker grate, and the ability to adjust the fuel levels across the grate area.
In addition to impeding efficient heat production, poor fuel delivery and distribution will entrain excessive amounts of particulate matter from the boiler grate into the exhaust gas draft system. This also will create higher levels of NOX and CO. The net result will be increased emissions levels for particulate matter and greenhouse gases.
Properly designed metering bins and fuel distribution spouts are the solution to keep biomass fuel delivery uninterrupted and at even levels, minimizing excessive particulate matter and harmful gases from getting into the exhaust gases of the boiler. A live bottom screw metering bin located on the boiler feed side wall is an excellent way to accomplish these goals by controlling fuel feed. This is done by accurately metering fuel into the boiler inlet chutes and feed spouts, and acting as a fuel retention device for uninterrupted flow into the boiler.
