Abstract 

Torrefaction of woody biomass is a process for biomass drying and partial pyrolysis. The torrefied wood is an oven-dried fuel with less volatile matter, higher carbon content and higher calorific value compared to un-treated wood. As a means of woody biomass fuel upgrading, torrefaction thus produces a fuel with faster combustion rate, less smoking and hydrophobic characteristics.

Torrefaction technologies available include continuous and fixed-bed batch processes with direct contact with inert gas as heating medium or indirect contact with steam. Typical torrefaction conditions are at temperature ranged 250-270°C and residence time less than 60 min which is determined by particle size. Torrefaction is evaluated to be an extensive drying process with the increase in calorific value reflecting significant moisture reduction but limited fixed carbon increase. Torrefied fuel has an energy efficiency of 84% which is lower than that 89% for the oven dried fuel due to the weight loss and insignificant calorific value increase in torrefaction. Further more, the benefit in heating value and environment obtained by torrefaction may be not able to balance the cost involved for energy and capital requirement. The selling price for a torrefied fuel has to be more than 24% higher than an oven dried fuel.

However, the concept of torrefaction is recommended in the context of densified fuels such as fuel logs and pellets. The use of torrefaction and densification may provide a more economically viable fuel.

Introduction

Torrefaction of woody biomass occurs at temperature between 200-300°C to produce a torrefied wood fuel. The temperature and residence time of torrefaction influence the physical and chemical properties of the torrefied products. Typical torrefaction conditions are at a temperature range of 250-270°C with residence time less than 60 min (Felfli et al 1998). 

The process of woody biomass torrefaction consists of biomass drying and incomplete pyrolysis. At the end of the drying stage, the temperature inside the biomass particle is of the order of 100-150°C and some physical shrinkage may have occurred without any chemical changes (Buekens and Schoeters 1985). When the temperature is over 150°C, biomass pyrolysis will start in the first of the two pathways (Shafizadeh 1985). The reactions involve a reduction in the molecular weight of wood polymers by bond scission; the appearance of free radicals; the elimination of water; the formation of carbonyl, carboxyl and hydro peroxide groups; the evolution of carbon monoxide and carbon dioxide; and finally, the production of a charred residue or torrefied wood. As a result, the torrefied wood is an oven-dried fuel with less volatile matter, higher carbon content and higher calorific value compared to un-treated wood. As a means of woody biomass fuel upgrading, torrefaction thus produces a fuel with faster combustion rate, less smoking and hydrophobic characteristics. 

This report was prepared for ES (Engenius Solutions), Christchurch to evaluate the technical and economic feasibility of biomass torrefaction for domestic and export fuel markets. This work was funded by Technology New Zealand’s TechNet Program. 

Technical Feasibility 

Technologies for producing torrefied wood (TW) have been in existence since the 1930’s in France (General Bioenergy, 2000). The processes include continuous and fixed-bed batch system. A continuous process consists of a biomass grinder, a dryer, a screw type torrefier, a boiler and a heat recovery system.  Steam as a heat transfer fluid circulates within the kiln walls and over the raw material feeder screw. Heat transfer occurs through conduction as wood particles come into contact with the heated surfaces. With this equipment, small wood particle size of less than 10mm in thickness is required. 

A fixed-bed batch process has the same pieces of equipment as the continuous process but with a different type of torrefier. The batch process conducts drying and torrefaction in the presence of an inert gas, whose temperature increases with each stage in the process as it forced through the fixed-bed of material. With this type of process, heat transfer occurs through convection. The drying and torrefaction stages may be processed separately, either in separate kilns or at different times. Batch processes are less sensitive to particle size.

Recently, Transnational Technology, Hawaii, has proposed a new process using superheated steam as a drying and torrefaction medium in one piece of equipment. The steam is re-circulated over an indirect heater and around the wood biomass until the wood is dried and torrefied. Steam generated from wood moisture removed by airless drying is vented from the dryer and is available for energy recovery. Similar to the batch process, particle size will determine the torrefaction time if the temperature is fixed at 270°C. 

As mentioned in the introduction, Torrefaction temperature is usually controlled in the range of 250-270°C and residence time varies with wood particle sizes. Under these conditions, wood biomass undergoes drying and partial pyrolysis. The final torrefied biomass product is characterized by its proximate analysis. Table 1 compares the proximate analysis results and calorific values for untreated wet biomass, torrefied biomass and charcoal. The characteristics of  a torrefied wood can be variable due to the heterogeneous nature of wood variation in constitutes in different species, tree age, growing location and even with the different parts of a tree such as bark, stem wood and branches. 

Table 1 Comparison of fuel characteristics due to different treatments

Note these results are indicative only as different species have been included

(Oven dry base) 

As can be seen, untreated wood usually has moisture content of 47-85% based on an oven dry (od) basis, torrefied wood and charcoal are dry fuels and the moisture content is typically 3-5% due to their hygroscopicity when they are expose to ambient conditions. Torrifaction reduces volatile matter to 72% from 75% for softwood, while carbonization to charcoal reduces volatile matter significantly down to 11%.  Torrefied wood has a fixed carbon of 28% compared to 23% for softwoods. However, in charcoal fixed carbon is more typically 87% due to the loss of volatiles. As a result, torrefied wood has a calorific value of around 24 MJ/kg which is 24% higher than for untreated softwood, but charcoal has a calorific value which is 67% higher than for the untreated softwood. The insignificant reduction in volatile by torrefaction may not contribute too much to the smoking reduction during combustion of the torrefied wood.

The calorific value of fuels is mainly attributable to the moisture content, the fixed carbon content and the carbon and hydrogen content of the volatile matter. The reduction of moisture content due to torrefaction appears to have a more significant effect on fuel calorific value and combustion rate. The increase in calorific value is directly proportional to the amount of moisture reduced. Each kilogram of water in wood reduces the calorific value by 2.3 MJ, which results in the significant calorific value difference between oven-dried wood and wet wood.  The calorific value increase extent is greater due to significant moisture reduction than that due to limited fixed carbon increase by torrefaction. Therefore, torrefaction is considered to be equivalent to an extensive drying process.

Economic Feasibility

The cost of producing torrefied wood fuels in New Zealand will be markedly affected by the nature and purchase price of the raw material feedstock, processing scale and utilization, which will be largely affected by the product market. As a preliminary evaluation report, this section provides a brief economic feasibility assessment based on energy input for processing and energy output of the torrefied wood product without detailed processing equipment and specific production scale. For doing this, some relevant parameters have been assumed.

Total energy consumption for torrefaction of wet wood chips equivalent to 1kg oven dry is estimated at 2.67 MJ including the heat required to raise the temperature of wet wood chips from 15°C to 100°C, the heat to evaporate the moisture present in the wood chips, the heat to bring the temperature of the dried wood chips to 270°C for torrefaction. Considering the energy possessed by the oven dried and torrefied wood chips and the yield of torrefied wood, total energy input to the torrefaction system is calculated at 21.87 MJ and energy output at 18.40 MJ per kilogram oven dried wood. The energy efficiency is 84% for production of torrefied wood. The lower energy output is caused by the weight loss of 23% (77% yield) during torrefaction.

From the technical evaluation, torrefaction is identified to be an extensive drying process. The energy efficiency comparison of torrefaction and drying will provide us a picture of economic feasibility. In an ideal drying process, the energy input of 21.6 MJ/kg (od) and output of 19.2 MJ/kg (od) result in an energy efficiency of 89%, which is higher than that of torrefaction. As we know, drying is an expensive process to produce low-value products such as fuels due to energy and capital equipment requirements. The cost involved is not able to balance the benefit obtained unless the drying can be integrated with other operations by using exhaust heat. With lower energy efficiency than drying, torrefaction is less economic.

From a point view of energy content, torrefied wood has a heating value 24% higher than the oven dry wood, the selling price for torrefied wood should be at least the same percentage higher than the oven dry wood. This extra price only covers the weight loss during torrefaction. Further price increase for torrefied wood will be required to cover the extra energy consumption and capital investment for torrefaction. These factors will increase the price challenge to compete with dry biomass fuels and should be considered before making any decision for investment of torrefaction. 

Recommendations

The above evaluation is based on the torrefaction of wood chips. When comparing with drying, the environmental and heating value benefits gained by torrefaction are not significantly greater than the extra energy consumption and capital investment. However, the torrefaction concept can be used in the production of densified fuels in the form of heat logs and pellets. Key aspects of densification are temperature, applied pressure and residence time. Temperature allows thermoplastic reactions to occur while pressure causes mechanical interlocking of particles with increasing pressure and pressuring time leading to improved fuel quality (Chin and Siddiqui, 2000). A densified fuel from wood residue and bark can be made without binders using pressures of 70-75 MPa and temperatures of 150-180°C (Graham, 1985). A water-resistant pellet fuel can be produced with conventional pelletising processes where the wood residues are pretreated with high-pressure steam for short periods (Shen and Stiasny, 1987).

The use of densification technologies is a well-proven means of producing solid fuels from wood residues. Fuel logs have been used for heating homes, while briquettes and pellets are used commercially and industrially overseas for generating heat and electricity.  Typically such fuels have proven to be clean burning with low gas emissions, require less storage space and can be transported more cost effectively. If the torrefaction temperature of 250-280°C is adopted in the densification processes, a torrefied and densified solid fuel may give a much better combustion performance and better market price.

References 

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