Cleanliness of Biomass Fuel and Effects on Combustion

Hygiene is the base of health, that is the proverb or phrase that we often hear. It turns out that hygiene is not only related to health problems, biomass fuel also apparently considered the importance of 'cleanliness' is. Cleanliness in the sense of freedom from a number of impurities or contaminants. Just as in the field of health, impurities will also cause problems in the processing and utilization. The cleaner the biomass fuel the easier it is in the processing and utilization becomes more optimal as well. Biomass fuels typically used by industry include: wood chip, wood pellet, palm shell or PKS (Palm Kernel Shell), and wood briquette. Wood pellets and wood briquette mostly use wood powder like sawdust as its raw material, and if the raw material size is too large then it needs to be reduced to the size of the wood powder. The densification process of wood powder into wood pellet and wood briquette will be easier and the lifespan of the compaction equipments becomes longer when using clean wood powder. For example, a nail or screws in wood powder to the pelletiser will cause severe damage to the roller by grinding the roller grooves within the pelletiser, so that the magnetic separator needs to be installed before the wood powder is fed to the pelletiser. Wood chips and clean palm shells (PKS) are also standard for a number of biomass power plants.

Vibrating Screen For PKS Cleaning; Photo taken from here

Foreign objects in the form of metal from PKS cleaning; Photo taken from here

Manual screen for PKS clearing

The biomass fuel cleaning process becomes an integral part of the fuel production process. A screen is a equipment commonly used for cleaning, and some add a magnet to separate the impurities or foreign objects from metal. Manual screen to mechanical one used for cleaning even to get the maximum level of cleanliness, then the sieve (screen) is made multi-deck. For large capacity mechanical sieves are commonly used, both vibrating and rotary screen. In the process of producing wood pellets and wood briquettes, a commonly used sieve is a rotary screen that functions in addition to separate the impurities as well also to obtain the appropriate particle size. While vibrating screen is commonly used for cleaning palm shells (PKS). Feeding to the screen usually uses a conyeyor. For raw material with large particle size such as palm shell (PKS) and wood chip belt conveyor is commonly used, while for raw material of wood powder such as sawdust, screw conveyor is recommended, for minimizing the dust problem.

Rotary Screen at Sawdust Briquette Factory

Sawdust Briquette to be carbonized

What foreign impurities or objects should be removed from the biomass fuel? Stones, gravel, sand, soil, glass, plastics, fabrics, paper and any number of metals are some impurities that should generally be removed as much as possible. Basically the processing and utilization of biomass fuel will have different levels of tolerance. Biomass power plants operating at high temperatures will require a higher level of cleanliness or a small degree of tolerance for the impurities. Likewise for the production of wood pellets and wood briquette in large capacity, then the quality of raw materials becomes important. This is because at high mechanical pressures during pelleting or briquetting the impurities have the potential to cause severe abrasion so that the equipment will quickly wear out. More specifically for palm kernel shells (PKS), which popular recently as biomass power plant fuel, the presence of fibers and empty fruit bunches is also considered an impurity. This is because the existence of these objects will reduce the heating value and some chemical elements in the ashes are not friendly or cause problems in boiler pipes especially those operating at high temperatures.

Deposit and ash crust on boiler pipes

Formation of deposit in boiler pipe

Factors that affect biomass burning mainly are the calorific value, moisture content, ash content and ash chemistry. The higher the ash content, the calorific value will be smaller. High silica (Si), potassium (K) and chlorine (Cl) contents will cause many problems in high temperature combustion. So the biomass fuel must be chosen according to its use or the type of utilization technology. For example, fibers and empty fruit bunches containing high chlorine (Cl) elements, will corrode boiler pipes so that leakages occur even though they have been operating for less than 10,000 hours. The high content of potassium (K) and silica (Si) will make a deposit or crust on the boiler pipes so it will also disrupt the heat exchange. In addition to impurities or foreign objects containing chemical substances that would be a problem in boilers, impurities or foreign objects that are physically large such as stones, concrete flakes, cement clumps, pieces of iron and so on in addition to disturb combustion also have the potential to interfere handling of the biomass fuel.

EFB Pellets Great Potential in Indonesia and Malaysia Has Not Been Taken

Wood pellets are still occupying the highest grade biomass fuel than biomass pellets fuel from other than wood. This is because its combustion properties are better than others. But as the magnitude of the biomass energy needs,  it is necessary to look for some other alternatives. Palm oil empty fruit bunch (EFB) is a potential raw material for pellets. Currently, Indonesia is the biggest palm oil producing country, especially crude palm oil (CPO) in the world with an area of palm oil plantations of more than 7 million hectares and hundreds of palm oil milss to process the palm oil fruits. Palm trees require warm temperatures, sunshine and high rainfall, so that only some parts of the world can do that, those are Asia, Africa and South America. Palm oil production is dominated by Indonesia and Malaysia with an amount between 85-90% of world palm oil production. Empty fruit bunches itself is a waste of CPO production which are numerous and today generally untapped.

Another factor driving this is when the supply of raw materials by planting some sort of energy plantation perceived longer and require more capital, or when the availability of raw materials such as woody biomass derived from wastes of the wood processing industries has been limited, the empty fruit bunches of palm oil can be a strong candidate. While the other factor come from the palm oil mill itself that many industries were overwhelmed to handle empty fruit bunch waste itself. Those things are more encouraging empty fruit bunch (EFB) processing into EFB pellets.

In terms of combustion properties as EFB pellet is lower quality from wood pellets, so it should also be used technologies that are more appropriate (suitable) in accordance with the character of the EFB pellets product. Type of fluidized bed combustion technology is the best choice because it is able to optimally handle EFB pellets at once able to reduce the content of a number of negative traits. Chlorine is the most avoidable component of mainly pellets made from agricultural wastes such as EFB. The fluidized bed combustion technology has also been developed from along ago so that is supposed to achieve optimum combustion conditions of EFB pellets become easy thing to do.

Fluidized Bed Combustion System Best Match For Agroindustrial Waste Biomass

Fluidized bed combustion (FBC) systems are widely used for power generation. FBC systems can be modified to be circulating FBC (CFBC), where the material is circulated back into the combustion chamber for burning up.  FBC systems are generally limited to a size of less than 300 MW, but there is powerplant  in Lagisza, Poland who designed it to 460 MW and hopes can be scaled up to 600-800 MW in the future. FBC and CFBC system is important because coal power plant that has long operated can be repowered, opening the possibility of cofiring with biomass on a wider scale conditions. The advantages of FBC systems include:  

-Ability to burn fuels with a wide range in moisture content, particle size, and density. And the potential for this type of fuel coal, biomass, tire-derivated-fuel, agricultural residues, and urban wood wastes.

-Heat transfer from combustion  more efficient so that the combustion temperature is lower, which in turn will reduce NOx emissions.

-Lower costs for SO2 capture because limestone can be added directly to the medium fluidization at a lower cost than installing scrubbers after combustion.

Coal power plant to install FBC  the system as part of a retrofit or new plant construction will be very open to the use of biomass (even if without biomass at first). FBC system has many advantages associated with the ability of fuel flexibility. Transport of biomass from feedstock sources to power plants efficiently if compacted (densification) is made to be like pellets. Wood pellets made ​​from woody biomass is increasingly limited its availability unless intensively pursued as create energy plantations, while the agro-industry wastes such as oil palm biomass is widely available and has not been utilized so that the potential for densification as fuel in FBC systems. Aspects of the environment, climate change and globalwarming is  major factors is the use of biomass fuels.