Utilization of Torrefied Biomass
During torrefaction, biomass is heated up to a temperature of 200-300 °C in an inert atmosphere. A part of the original biomass is released as volatiles and the remaining char can be used as a fuel. The properties of torrefied biomass are more similar to coal: it absorbs less water, it is resistant to biodegradation, it has higher heating value and its grinding is easier.
Due to the mentioned advantages of torrefied biomass to raw biomass, torrefied biomass can actually be utilized in multi applications. Here we make a brief summary.
Co-firing in Pulverised Coal Power Plants: Pulverized coal combustors are widely installed in coal fired power plants. Nowadays, the designs of these plants are modified in order to use raw biomass as a renewable fuel. Torrefied biomass, the properties of which are more similar to coal, can replace coal or be co-fired without changing the plants’ design. A higher heating value, better grindability and a lower tendency to absorb water result in a better fuel compared to raw biomass for use in pulverized fuel boilers. Thus, huge modification cost of power plant can be avoided.
Gasification: Particle size and moisture content of the fuel are key factors during gasification. Therefore, torrefied biomass, due to its lower moisture content, good grindability and higher heating value, is a premium fuel to be utilized for gasification. A uniform and small particle size of torrefied biomass improves the flow properties of the feedstock and results in higher H2 and CO formation which consequently improves the efficiency of the entrained-flow gasifiers. Torrefier and gasifier can be integrated in a plant so that the released volatiles from the torrefaction can be used in gasifiers. Improved fluidization of torrefied biomass can also increase its usage in fluidized bed gasifiers.
The concentration of alkali metals is higher in the torrefied biomass than raw biomass. Alkali metals catalyze the char gasification reaction, and thus char gasification from ash forming matter perspective might be slightly more rapid.
Pellet Production: Pellet production from milled torrefied biomass has advantages compared to raw biomass . Torrefied biomass can be milled easier, but it requires more energy to produce pellets. This can be attributed to the changes in the lignin during torrefaction which reduces its thermal softening. In order to produce pellets from torrefied biomass, higher temperature of operating condition is needed. A balance needs to be established between torrefaction temperature and pelleting consumption. It’s important to confirm the optimum torrefaction temperature of different biomasses. Then torrefied pellets can be produced without any binder. Overall, producing torrefied pellets is both economical and efficient.
Blast Furnaces in Steel Industry: Torrefied biomass can be considered for the application in blast furnaces, since it is a renewable fuel with lower net emission of CO2. It can be added to coal blends during coke making or can be utilized as injectant into the blast furnace. However, the high content of alkali metals and high amount of volatile matters in torrefied biomass limit its usage in blast furnaces. Therefore, the steel industry’s priority is carbonized biomass, and the application of torrefied biomass seems limited.
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