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Low ash content bio-materials for pellet plant

By Jack Huang    Jan 24, 2014
To establish a profitable biomass pellet plant, it’s important to think for your customers. The ash content of the raw materials is a concern, as—at the end-user level—not all boilers can handle high ash-content pellets. More maintenance is required for the utilization of high ash biomass fuel, which is in fact one of the characteristics of pellets composed of agricultural materials. Knowing the potential of the pellets created using various raw materials available helps assess its ultimate demand, and hence the feasibility of establishing a profitable biomass pellet plant.

Although agricultural raw materials vary greatly in their chemical compositions, most contain higher traces of silicon, calcium, potassium, chlorine and sulfur, thereby resulting in a lower melting point of the ash and increased degradation of the boiler. Substantial melting leads the presence of slag, while partial melting results in other undesirable effects such as agglomeration, sintering and deposit formation. The ash’s melting point is raised by silicon and calcium, and is lowered by potassium. Wood-based materials contain far less nitrogen, sulfur and chlorine, which—in combination with its higher caloric value—is why it is the biomass fuel source of choice.
 

Nitrogen, sulfur and chlorine content of various raw materials

Table 1: Nitrogen, sulfur and chlorine content of various raw materials
 

The presence of excessive levels of chlorine in the biomass fuel may lead to the severe metal corrosion. This can be reduced by using additives, similar to the process utilized in the coal industry. Overall, it is recommended that potassium and chlorine levels not exceed 0.2 percent and 0.1 percent, respectively, for biomass materials used in power generation.

By changing the time of harvest it is possible to reduce levels of water-soluble chemicals in the raw material to enhance its combustion properties. The chlorine and potassium content of a number of grass species is reduced by harvesting late in the season or after winter has passed. The same occurs to leached straw, thereby converting yellow to gray straw to reduce the content of water-soluble potassium and chlorine.

Straw ash composition

Table 2: Straw ash composition

High-quality biomass fuel is produced from raw materials with low levels of potassium, chlorine, sodium and sulfur, as these elements lead to particulate emissions during combustion, which is also prefered by large scale biomass pellet plants. Several factors ultimately determine the amount present in the feed, some of which can be controlled. These include: fertilization practices, choice of species, stem thickness, time of harvest and regional precipitation. Biomass-fuelled boilers are typically designed for wood, which is lower in ash and chlorine levels. However, new multi-fuel boilers are being developed and are becoming more widespread. These can burn agricultural and wood biomass more and more effectively. One of the keys to a successful biomass pellet plant occurs at the planning stage, in understanding the needs of end-users, and if these needs are compatible with available raw materials for the prospective operation.