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.