Fluidized bed gasification has been successfully used to convert biomass wastes (i.e. wood wastes, bark, and agricultural wastes) into a clean fuel gas that can be used to fire various types of industrial equipment. Past applications have included gasification of wastes to provide gas for dryers previously fired on natural gas. Fluidized bed gasifiers also have applications in the lumber and plywood industries where they can be used to fire small boilers and direct fired dryers and kilns that are currently fired on expensive natural gas or oil.
- Reduced cost of boiler or dryer/kiln operation by using wood and or bark wastes rather than gas or oil.
- Reduced cost for additional steaming capacity compared to new wood and or bark fired boilers.
- Reduced dependency on external fuel sources for propane, natural gas and oil.
Fluidized Bed Technology
To understand a fluidized bed, refer to figure 1, and imagine a container filled with fine sand having a porous bottom. If air is forced through the bottom, it seeps through the sand as shown in figure 1-A. If the airflow is increased a point is reached where the sand particles separate to let the air through, and bubbles form which mix and churn the sand. The sand bed takes on a fluid-like character, and the bed is violently mixed and agitated. This is a fluidized bed.
Fluidized Bed Gasification
In fluidized bed gasification, the sand bed is preheated to a temperature of 1000oF. Solid or liquid waste are injected into the bed, and when they are mixed with the hot sand, the wastes are quickly decomposed into a combustible gas. The flow of air is controlled so that only about 25% of the incoming wastes are "burned" in the bed to raise and maintain the temperature at 1500oF. The remaining material is decomposed into gas. Once the bed is preheated, no more fossil fuels are needed. The wastes supply all required heat. The process is shown in figure 2.
The fluidized bed system is made in such a way that the ash particles are carried out of the bed with the gas stream.
This ash is then removed from the gas stream by a special ash removal system. The hot gas is then directed to a special burner that can burn the hot gas while producing low emissions. Any gravel or stones in the incoming fuel can be removed by periodically extracting some of the sand from the bottom of the bed while it is operating, screening to separate stones from the sand, and then reinjecting the sand into the bed. Because the primary product of the process is a gaseous fuel and not simply heat, the process is called "fluidized bed gasification".
BENEFITS OF FLUIDIZED BED GASIFICATION
The fluidized bed gasification process offers several substantial benefits compared to simple burning processes, and other forms of gasification.
The overall thermal efficiency of fluid bed gasifiers is typically in the range of 75% to over 90%, depending on the ash and moisture content of the fuel.
Tolerates Many Feedstocks
Unlike some burners (such as suspension burners) or old style fixed bed gasifiers, the fluid bed gasifiers can operate satisfactorily with highly variable feed materials ranging from coal, shredded wood and bark to sawdust fines, or lump wood with particle sizes of less than 1 1/2 – 2 inches. In contrast, other types of gasifiers or burners require either dry pellets, nuggets of cleanwood, or uniformily dry sanderdust. Thus the various types of fuels generally available around lumber mills can be used in fluid bed gasifiers with good results.
The fluid bed gasifier does not have moving grates or other moving parts in the high temperature regions of the bed. Where there are moving parts, heavy duty industrial components proven in lumber and pulp mill operations are used. Reliability is thus high.
The size of energy conversion systems is generally dictated by their air flow. Because fluid bed gasifiers use comparatively small amounts of air, the equipment is comparatively small and compact (see Figure 3). This permits systems to be completely shop fabricated and assembled on skids thereby reducing purchase price and installed costs.
Because the process produces a fuel gas rather than just quantities of heat, it can be easily applied to a variety of industrial processes including boilers, dry kilns, veneer dryers, or several pieces of equipment at once.
Operation with wood/bark fuels results in very low emissions, including low NOx, carbon monoxide, and particulate emissions. No "tail end" exhaust cleanup devices are required.