Upgrade PKS with Torrefaction

Application of the tax on PKS (Palm Kernel Shell) based on the Minister of Finance Indonesia No 67/2010 on stipulation of export goods subject duties (Bea Keluar/ BK) has made the selling price in the international market or the export market to be high. The amount of export duties and export taxes which value is currently around US $ 15 / ton, making a number of parties thought to do the processing of the PKS. Enforcement of the above regulations is also intended to encourage the growth of domestic industry. PKS categorized as raw materials, thus further processing into a solution to it. The selling price of PKS from Indonesia less competitive, when compared before, because it becomes much more expensive.

The use of palm shells (PKS) today especially for fuel of large scale power plant. And in particular Japan and Korea are much in need of the PKS. The need also tends greater for the next few years. Japan, especially in the next 1-2 years the need is predicted to increase sharply as the operation of power plants using biomass and part of PKS as a mixture (co-firing) with coal.

 Torrefaction is biomass processing to increase the calorific value and improve its characteristics. With torrefaction the energy content in biomass increased by 20% and conversion rate reached 70%, or nearly three times the carbonization conversion were an average of 25%.  In addition to the torrefaction then torrefied PKS product will be hydrophobic so that it does not absorb water / moisture.  This will make it easier in terms of handling and storage. Furthermore, by compaction (densification) into pellets torrified PKS, the density also will increase, as well as the energy content per volume of it. Pelleting torrified PKS will save significant transportation costs from the manufacturer to the user. JFBC technology is able to perform continuous torrefaction process, with a production capacity up to 140 tonnes per day of torrified PKS.

Huge Demand of Torrified Biomass For Energy Application

Biomass ranks fourth as energy resource on global basis. Biomass is CO2 neutral and contains very little sulfur, hence it does not contribute greatly to acid-rain problems. Biomass have unique role on a renewable energy source.While the growing need for sustainable electric power can be met by other renewables, biomass is our only renewable source of carbon-based fuels and chemicals. Bioenergy is the word used for energy associated to biomass, and biofuel is the bioenergy carrier, transporting solar energy stored as chemical energy. Biofuels can be considered a renewable source of energy as long as they based on sustainable biomass production.

As Europe is very much the center of the global wood fuel market in general and the wood pellet/briquette market in particular, it comes as no surprise that vast majority of big wood fuel producers  of many countries have European countries as their final destination. With the goal set by the European Union to achieve a 20% share of renewable energy in the energy mix and a 20% decrease in greenhouse gas emissions by 2020 (DIRECTIVE 2009/28/EC, 2009) it is likely that the increase in EU demand for bioenergy will accelerate. However, it is also likely that a large share of future use of bioenergy in Europe will be from biomass of non-European origin as the resources are unlikely to be cost cost-competitive compared to biomass to biomass imported from other parts of the world.  

Trading wood fuel is always complex due to the biomass itself being both low in value per volume unit as well as difficult to store and transport as a result of it being a “living material” and hence susceptible to degradation from biological processes. Torrefaction is a technology to improve the quality of the biomass fuel and is followed by densification (pelleting / briquetting) will save transportation costs. Torrefaction has many advantages that overcome some problems in the wood fuel in general.

The quantities of biomass co-fired in large coal fired and other fossil fuel-fired power plant boiler have increased fairly dramatically over the past few years, particularly in Northern Europe but also elsewhere in the world. The level of co-firing activity worldwide, and the co-firing ratios at specific plants, are likely to increase further over the next few years.

Biomass materials have significant levels of inorganic matter as impurities, and many of the practical problems encountered with the combustion of biomass materials, or the co-combustion of biomass materials with coal and other fossil fuel, are associated with the nature and behaviour of the biomass ash and the other inorganic constituents. In practical terms, the ash-related problems in biomass combustors and boilers, and in plants co-firing biomass with more conventional fossil fuels, have commonly been associated with:

-The formation of fused or partly fused ash agglomerates and slag deposits at high temperature within furnaces;

-The formation of bonded ash deposits at lower gas temperatures on the heat exchange surfaces in the boiler convective sections and elsewhere;

-The accelerated metal wastage of boiler components due to gas-side corrosion and erosion;

-The formation and emmision of sub-micron aerosols and fumes; and

-The handling and utilization/disposal of ash residues from biomass combustion plants, and of the mixed ash residues from the co-firing of biomass in coal-fired boilers.

In very general terms, the nature of the problems and the impact on plant perfomance depend both on the characteristics of the biomass fuel, i.e. principally on the ash content and the ash chemistry, and on the design and operation of the combustion equipment and the boiler. Raw material have significant role of the densified (pellet/briquette) torrefied biomass quality. We will choose raw material with low ash content and a high ash melting temperature.

The peat and coal have the higher ash contents, but only a relatively small portion of the mineral material is in the water and acetate soluble fractions and is considered to contribute to the formation of the fine ash/aerosol material. In the case of the biomass materials, the total mineral contents are lower, but a much higher proportion of the mineral material is considered to contribute to the formation of the fine ash/aerosol fraction. The ash residue is normally weighed to provide an estimate of the ash content of the fuel, and then analysed for the ten major elements present in coal ashes, i.e. SiO2, Al2O3, Fe2O3, CaO, MgO, TiO, Na2O, K2O3, P2O5 dan SO3.

Usually slagging takes place with biomass fuels containing more than 4% ash and non-slagging fuels with ash content less than 4%. The ash content of different types of biomass is an indicator of slagging behaviour of the biomass. Generally, the greater the ash content, the greater the slagging behaviour. But this does not mean that biomass with lower ash content will not show any slagging behaviour. The temperature of combustion temperature, the mineral compostion of ash and their percentage combined determine the slagging behaviour. If conditions are favorable, the the degree of slagging will be greater. Minerals like SiO2, Na2O and K2O3 are more trouble some.

The selection of raw materials is an important factor for the production of torrified biomass. High quality torrified biomass need to be produced to meet a variety of industrial and domestic needs. Chemical treatment of raw materials can be made ​​to increasing the quality of raw materials, but it will do if the quality of raw materials is not sufficient anymore. Finally a reliable technology for the production of  torrified biomass absolutely necessary to meet those needs.

Which one is better, pelletization of torrefied biomass or torrefaction of pelletized wood?

Torrefaction, a process different from carbonization, is a mild pyrolysis process carried out in a temperature range of 230 to 300 °C in the absence of oxygen.  This thermal pretreatment of biomass improves its energy density, reduces its  oxygen-to-carbon (O/C) ratio, and reduces its hygroscopic nature. During this process the biomass dries and partially devolatilizes, decreasing its mass while largely preserving its energy content. The torrefaction process removes H2O and CO2 from the biomass. As a result, both the O/C and the H/C ratios of the biomass decrease. But Torrefaction will increases the relative carbon content of the biomass. The properties of a torrefied wood depends on torrefaction temperature, time, and on the type of wood feed. Torrefaction also modifies the structure of the biomass, making it more friable or brittle. This is caused by the depolymerization of hemicellulose. This makes it easier to co-fire biomass in a pulverized-coal fired boiler or gasify it in an entrained-flow reactor. There is a 29 to 33% increase in energy density (energy per unit mass) of the biomass through torrefaction. This increases its higher heating value (HHV) to about 20 MJ/kg.  To know more advantages of the torrefaction, please click here.

In biomass, hemicellulose is like the cement in reinforced concrete, and cellulose is like the steel rods. The strands of microfibrils (cellulose) are supported by the hemicellulose. Decomposition of hemicellulose during torrefaction is like the melting away of the cement from the reinforced concrete. Thus, the size reduction of biomass consumes less energy after torrefaction.

During torrefaction the weight loss of biomass comes primarily from the decomposition of its hemicellulose constituents. Hemicellulose decomposes mostly within the temperature range 150 to 280 °C, which is the temperature window of torrefaction. As we can see from  Figure below, the hemicellulose component undergoes the greatest amount of degradation within the 200 to 300 °C temperature window. Lignin, the binder component of biomass, starts softening above its glass-softening temperature (~130 °C), which helps densification (pelletization) of torrefied biomass. Unlike hemicellulose, cellulose shows limited devolatilzation and carbonization and that too does not start below 250 °C.

Weight loss in wood cellulose, hemicellulose, and lignin during torrefaction

Thus, hemicellulose decomposition is the primary mechanism of torrefaction. At lower temperatures (< 160 °C), as biomass dries it releases H2O and CO2. Water and carbon dioxide, which make no contribution to the energy in the product gas, constitute a dominant portion of the weight loss during  torrefaction. Above 180 °C, the reaction becomes exothermic, releasing gas  with small heating values. The initial stage (< 250 °C) involves hemicellulose depolymerization, leading to an altered and rearranged polysugar structures (Bergman et al., 2005a). At higher temperatures (250–300 °C) these form chars, CO, CO2, and H2O. The hygroscopic property of biomass is partly lost in torrefaction because of the destruction of OH groups through dehydration, which prevents the formation of hydrogen bonds.

A typical reaction time is about 30 minutes. The properties of torrefied wood depend on (1) the type of wood, (2) the reaction temperature, and (3) the reaction time. Pelletization may not increase the energy density on a mass basis, but it can increase the energy content of the fuel on a volume basis. Pelletization of torrefied biomass is better than torrefaction  of pelletized wood from the standpoint of process energy consumption and  product stability.This is because :

a. Torrefied biomass (torrefied wood), for example using sawdust as feedstock, so the torrefaction process will consume less energy due to the smaller particle size than the pelletized wood (wood pellets).  Surface material can be in contact with the process of torrefaction is also larger in general when the particle size is smaller, so that better product quality (product stability). Normally before entering the torrefaction process feedstock will be diminished to the size of a certain size and drying up to a certain moisture content.
b. Physical form of pelletized wood (wood pellets) will be damaged due to torrefaction so irregular and will tend to shrink. While torrefied biomass has no problem with it because the physical form of the final product after pelletization.