05-05-2012, 05:19 PM
Biomass for Steam and Power
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Introduction
This paper explores the use of equipment to fire biomass to replace fossil fuel in plant operations. Biomass is a low cost fuel that also reduces our carbon footprint. We’ll discuss the technology and economics of fuel feed equipment, stoker boilers, bubbling fluidized bed boilers, circulating fluidized bed boilers, gasifiers, combustion control systems, emission controls, revenue and capital costs. Most types of organic material can be burned for steam and power. In the final analysis, the choice of feedstock comes down to a matter of economics. The technology is currently available and new innovations are remarkable. Various types of wood waste have been burned in the forest products and pulp and paper industry for decades, if not centuries. Some locations are using agricultural wastes and products such as rice straw, rice hulls, corn stover, distiller grains, animal bedding waste, manure, bagasse etc. as boiler fuel. Although applications exist to convert biomass to biofuels and chemicals, the intent of this discussion is to convert biomass for steam and power. Biomass can be burned directly in a boiler, or a gasifier can be utilized to produce syngas that can be used for a substitute fuel. Fuel Handling
A robust material handling feed system is required to ensure the efficient operation of any of the combustion devices. Biomass is one of the more difficult materials to handle, and is especially obedient to Newton’s 1st and 3rd Laws. I. “Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.” There are multiple external forces that prevent biomass from staying in motion. III. “For every action there is an equal and opposite reaction.” Once in place, biomass tends to stay in place!
Biomass presents a variety of interesting challenges and takes special handling. Wood chips or wood waste materials tend to hang-up and bridge. After the bridge forms, biomass is difficult to move. Compaction makes the material stronger and must be avoided to ensure uninterrupted operation. As a part of a good boiler or gasifier operation, getting fuel to the plant reliability is key. Below is an example illustrating methods of handling biomass material.
Fuel Storage Silo with Traveling Screw It is important to employ good fuel feed and conditioning systems. There should be enough “take away” capacity in order that fuel handling systems do not jam and plug. Materials must be screened adequately to remove sticks, oversize material, stones and tramp metal. Non-magnetic materials need to be detected to prevent damage to downstream systems. Equipment must be accessible for lubrication, inspection and maintenance programs. Appropriate cleanouts should be designed where systems might plug. Storage vessels should be designed to prevent bridging. Bins may be tapered slightly outward to avoid compaction and help prevent bridging. Also, the outlet screw discharge rotates around the bin to ensure uniform flow through the bin and first-in/first-out operation.
Screw conveyor design is critical in handling biomass. Material handling is significantly different for biomass versus coal, gravel and ash applications. Biomass will compress,
compact, and gain strength in compression. Screws are tapered to improve flow characteristics.
Boilers & Gasifiers
Stoker fed units, bubbling fluidized bed boilers, circulating fluidized bed boilers and gasifiers are available for burning various biofuels. Let’s review their operating characteristics to understand which applications are most appropriate for biomass.
Stokers
Stoker fed units were one of the first technologies to fire biomass. Still used today, they are reliable and efficient and can use a variety of fuels, including wood waste, municipal solid waste, and agricultural materials such as corn stover, straw, and animal waste. The stoker can also handle sludge and combination fuels, including coal, and tire derived fuel.