Traditionally, torrefaction is a thermal process for roasting biomass operated at 200° to 300° and for a relatively long residence time (30 to 60 minutes) under an inert atmospheric condition. The name torrefaction is adapted from the process used to roast coffee beans, which is performed at lower temperatures

Summary of Advantages and Disadvantages of Various Biomass Pretreatments

Source: Modified from Shah and Gardner, in press. Biomass Torrefaction: Applications in Renewable Energy and Fuels. In Encylopedia of Chemical Processes, Boca Raton, FL: CRC Press.

and in the presence of air. Nevertheless, the important mechanical effect of torrefaction on biomass is similar to the effect on coffee beans. In the open literature, torrefaction is also referred to as roasting, slow-and-mild pyroly­sis, wood cooking, and high-temperature drying [18-25]. The drying and grinding of biomass is not as easy as torrefaction and grinding due to the physical nature of biomass.

The process of torrefaction dehydrates and depolymerizes the long poly­saccharide chains of biomass. This results in a product that is hydrophobic and has a higher energy density and improved grinding and combusting capabilities [26-32]. This process is best illustrated through the Van Krevelen plot shown in Figure 7.1 [18-24]. The figure illustrates that torrefaction results in the reduction of oxygen content and increased heating value of the bio­mass. Generally, during torrefaction an increase in both mass and energy density occurs because about 30% (by weight) of the biomass is transformed

TABLE 7.7

Aspects of Torrefied Biomass for Gasification and Other Applications

Torrefied Product

Has lower moisture content and higher heating value Is easy to store and transport

Is hydrophobic, does not gain humidity in storage and transportation Is less susceptible to fungal attack Is easy to burn, forms less smoke and ignites faster Significantly conserves the chemical energy in biomass

Has heating value (11,000 BTU/lb) that compares well with coal (12,000 BTU/lb)

Generates electricity with a similar efficiency to that of coal (35% fuel to electricity) and considerably higher than that of untreated biomass (23% fuel to electricity)

Has grindability similar to that of coal

Requires grinding energy 7.5 to 15 times less than that for untreated biomass for the same particle size

Has mill capacity 2 to 6.5 times higher compare to untreated biomass Possesses better fluidization properties in the gasifiers

Is suitable for various applications in heating, fuel, steel and new materials manufacturing industries

Source: Bergman and Kiel, 2005. Torrefaction for biomass upgrading. Proceedings of the Fourteenth European Biomass Conference and Exhibition," Paris, October, pp. 17-21. Bergman al. 2004. Torrefaction for entrained flow gasification of biomass. In: W. P.M. Van Swaaij, T. Fjallstrom, P. T. Helm, and P. Grassi (Eds.), Proceedings of the Second World Biomass Conference on Biomass for Energy, Industry, and Climate Protection, Rome, Italy, May 10-14, pp. 679-682, Energy Research Centre of the Netherlands (ECN), Petten, The Netherlands, Report No. ECN-RX—04-046; Bergman. 2005. Combined Torrefaction and Pelletization: The TOP Process. Energy Research Centre of the Netherlands (ECN), Petten, The Netherlands, Report No. ECN-C-05-073; and Bergman et al., 2004. Torrefaction for Entrained Flow Gasification of Biomass. Energy Research Centre of the Netherlands (ECN), Petten, the Netherlands, Report No. ECN-C-05-067.

into volatile gases. These gases carry only 10% of original biomass energy content [29, 30, 33-35]. This implies that during torrefaction, a substantial amount of chemical energy is transformed from the raw material to the product resulting in the enhanced fuel properties of the torrefied biomass. Mild pyrolysis of biomass results in gases such as H2,CO, CO2, CH4,CxHy; the liquids such as toluene, benzene, H2O, sugars, polysugars, acids, alcohol, furans, ketones, terpenes, phenols, fatty acids, waxes, and tannins and solids that contain char and ash.

As shown in Table 7.7, torrefied biomass possesses very valuable properties. It has lower moisture content and therefore higher heating value compared to untreated biomass. The storage and transportation capabilities of torrefied biomass are superior to those of untreated or only dried biomass. Torrefied biomass is hydrophobic and does not gain humidity in storage and trans­portation. It shows little water uptake on immersion (7-20% of mass) and is more stable and more resistant to fungal attack compared to charcoal and

an untreated biomass. Pelletization, by itself, produces biomass with higher mass density; however, the pellets are not hydrophobic and are susceptible to fungal attack. Torrefied biomass significantly conserves the chemical energy present in the biomass. The heating value of torrefied wood is approximately 11,000 BTU/lb and is nearly equal to that of a high volatile bituminous coal which is 12,000 BTU/lb. It generates electricity with an efficiency comparable to that of coal of approximately 35%, on a fuel to electricity basis [36, 37], and much higher than that of untreated biomass which has an efficiency of 23%, on a fuel to electricity basis [36, 37]. Bergmann et al. showed that torrefied biomass has better fluidization properties than that of untreated biomass, but similar to that of coal [38-41].

Untreated biomass requires many times the grinding energy (by a factor of 7.5 to 15) to achieve a similar particle size compared to torrefied biomass. This energy difference is significantly larger than the energy loss of biomass and energy supplied during torrefaction. The grindability of torrefied bio­mass versus that of untreated biomass is compared in Figure 7.2. The mill capacity of the torrefied biomass can also be as high as 6.5 times that of the untreated one. Finally, torrefied biomass is suitable for various applications such as working fuel, residential heating, new materials for the manufacture of fuel pellets, reducer in the steel smelting industry, the manufacture of charcoal and active carbon and gasification, and co-firing with other fuels in gasifiers, boilers, and so on [38-41]. Such wide usefulness makes torrefied biomass a valuable and marketable product.

Ratafia-Brown et al. [6] identified various feed preparation techniques needed for an entrained bed gasifier based on the nature of biomass and the nature of the feeding mechanism. For wet feeding, they proposed a pyrolysis process that produces a bioslurry which can be fed into the entrained bed gasifier by a feeder or an injector. This technique is analogous to the coal slurry feed used in the GE gasifier, and it is most appropriate for strawlike crops. For dry feeding they proposed three alternatives. For woody biomass, the feed can be milled (and cut) and broken down to size of 1 mm particles and fed to the entrained bed gasifier by either a screw feeder or a piston compressor. The process of milling can also be preceded by torrefaction. The torrefied biomass in this case can be fed to the gasifier by a pneumatic feeder, a screw feeder, or a piston compressor. For all other types of biomass they presented two options. One option is to follow the path of torrefaction, mill­ing, and feeding just like the one for woody biomass. The other is to gasify the biomass in a pressurized fluidized bed to generate gas product and char and feed these materials to the entrained bed gasifier for the production of biosyngas.