Use of Palm Kernel Shells in Circulating Fluidized Bed Power Plants

Palm kernel shells are widely used in fluidized bed combustion-based power plants in Japan and South Korea. The key advantages of fluidized bed combustion (FBC) technology are higher fuel flexibility, high efficiency and relatively low combustion temperature. FBC technology, which can either be bubbling fluidized bed (BFB) or circulating fluidized bed (CFB), is suitable for plant capacities above 20 MW. Palm kernel shells (PKS) is more suitable for CFB-based power plant because its size is less than 4 cm.

With relatively low operating temperature of around 650 – 900 oC, the ash problem can be minimized. Certain biomass fuels have high ash levels and ash-forming materials that can potentially damage these generating units. In addition, the fuel cleanliness factor is also important as certain impurities, such as metals, can block the air pores on the perforated plate of FBC unit. It is to be noted that air, especially oxygen, is essential for the biomass combustion process and for keeping the fuel bed in fluidized condition.

The requirements for clean fuel must be met by the provider or seller of the biomass fuel. Usually the purchasers require an acceptable amount of impurities (contaminants) of less than 1%. Cleaning of PKS is done by sifting (screening) which may either be manual or mechanical.

In addition to PKS, biomass pellets from agricultural wastes or agro-industrial wastes, such as EFB pellets which have a high ash content and low melting point, can also be used in CFB-based power plants. More specifically, CFBs are more efficient and emit less flue gas than BFBs.

The disadvantages of CFB power plant is the high concentration of the flue gas which demands high degree of efficiency of the dust precipitator and the boiler cleaning system. In addition, the bed material is lost alongwith ash and has to be replenished regularly.

A large-scale biomass power plant in Japan

The commonly used bed materials are silica sand and dolomite. To reduce operating costs, bed material is usually reused after separation of ash. The technique is that the ash mixture is separated from a large size material with fine particles and silica sand in a water classifier. Next the fine material is returned to the bed.

Currently power plants in Japan that have an efficiency of more than 41% are only based on ultra supercritical pulverized coal. Modification of power plants can also be done to improve the efficiency, which require more investments. The existing CFB power plants are driving up the need to use more and more PKS in Japan for biomass power generation without significant plant modifications.

Utilization of Date Palm Biomass

Date palm trees produce huge amount of agricultural wastes in the form of dry leaves, stems, pits, seeds etc. A typical date tree can generate as much as 20 kilograms of dry leaves per annum while date pits account for almost 10 percent of date fruits.

date-wastes

Date palm biomass is found in large quantities across the Middle East

Date palm is considered a renewable natural resource because it can be replaced in a relatively short period of time. It takes 4 to 8 years for date palms to bear fruit after planting, and 7 to 10 years to produce viable yields for commercial harvest. Usually date palm wastes are burned in farms or disposed in landfills which cause environmental pollution in dates-producing nations.

The major constituents of date palm biomass are cellulose, hemicelluloses and lignin. In addition, date palm has high volatile solids content and low moisture content. These factors make date palm residues an excellent biomass resource in date-palm producing nations.

Date palm biomass is an excellent resource for charcoal production in Middle East

A wide range of physico-chemical, thermal and biochemical technologies exists for sustainable utilization of date palm biomass. Apart from charcoal production and energy conversion (using technologies like combustion and gasification), below are few ways for utilization of date palm wastes:

Conversion into fuel pellets or briquettes

Biomass pellets are a popular type of alternative fuel (analogous to coal), generally made from wood wastes and agricultural biomass. The biomass pelletization process consists of multiple steps including pre-treatment, pelletization and post-treatment of biomass wastes. Biomass pellets can be used as a coal replacement in power plant, industries and other application.

Conversion into energy-rich products

Biomass pyrolysis is the thermal decomposition of date palm biomass occurring in the absence of oxygen. The products of biomass pyrolysis include biochar, bio-oil and gases including methane, hydrogen, carbon monoxide, and carbon dioxide.

Depending on the thermal environment and the final temperature, pyrolysis will yield mainly biochar at low temperatures, less than 450 0C, when the heating rate is quite slow, and mainly gases at high temperatures, greater than 800 0C, with rapid heating rates. At an intermediate temperature and under relatively high heating rates, the main product is bio-oil.

Bio-oil can be upgraded to either a special engine fuel or through gasification processes to a syngas which can then be processed into biofuels. Bio-oil is particularly attractive for co-firing because it can be more readily handled and burned than solid fuel and is cheaper to transport and store.

Conversion into biofertilizer

Composting is the most popular method for biological decomposition of organic wastes. Date palm waste has around 80% organic content which makes it very well-suited for the composting process. Commercial-scale composting of date palm wastes can be carried out by using the traditional windrow method or a more advanced method like vermicomposting.