By the gasification of woody or grassy materials a synthesis gas is obtained. The so-called Fischer-Tropsch (F-T) process converts this gas into very clean liquid fuels; for instance diesel. F-T diesel can be used without any problem in the existing infrastructure and in cars. It causes less environmental problems compared with diesel from fossil fuels. In a recent study it is shown that F-T liquids can be produced for 40 Euro cents /liter. A price for wood of Euro 4 /GJ or Euro 75 /ton dm has been used. An analysis has been made of possibilities for the improvement of the efficiency. Especially tri-generation (production of liquid fuels as well as electricity) seems attractive. Also the process of torrefaction in combination with Entrained Flow gasification and heat integration seem promising. The result of these improvements is an increase in the efficiency. The overall efficiency will be between 60 and 70%. If the electricity produced is expressed in "F-T liquids equivalent", the efficiency will be about 80%. The cost of a kWh has been fixed at 0.06 Euro cents. In combination with an expected cost reduction of 25%, the final cost of a liter of F-T liquid will be between 25 and 30 Euro cent. These last numbers are in accordance with present day price for clean diesel (without tax) based on fossil fuels with a barrel price of $ 30.

Torrefaction for Entrained-Flow Gasification of Biomass
Energy Research Centre of the Netherlands

Improvement of grindability and fluidization properties of biomass through torrefaction is seen as a promising pretreatment option to implement large-scale (entrained-flow) gasification of biomass. The present work aimed for knowledge built-up and generation of design data to support this option. Torrefaction is a thermal treatment of biomass in a temperature range of 200-300 C. Especially the decomposition of hemicellulose in this range is believed to improve the relevant properties. Compared to untreated biomass, the energy requirements of size reduction can be reduced with 50-85% and capacity expansion of a factor 2 to 6.5 is obtainable after torrefaction. As the fibrous structure of biomass is partly destructed, small particles of 30-400 m have less needled shape, which improves its fluidization characteristics. The energy retained in the torrefied wood amounts to 83% to 97% (LHVdaf) and strong property improvements can be achieved at reaction times of less than 10 min.

Torrefaction for Biomass Upgrading
Energy Research Centre of the Netherlands  Unit ECN Biomass  

Torrefaction is a mild pre-treatment of biomass at a temperature between 200-300 °C. During torrefaction the biomass its properties are changed to obtain a much better fuel quality for combustion and gasification applications. In combination with pelletisation, torrefaction also aids the logistic issues that exist for untreated biomass.  This paper treats the principles of torrefaction and production technology that is under development at ECN (TOP technology for the production of TOP pellets from biomass).  Attention is also paid to the process its economics and its influence on the economics of a biomass-to-energy production chain. Torrefaction of biomass is an effective method to improve the grindability of biomass to enable more efficient co-firing in existing power stations or entrained-flow gasification for the production of chemicals and transportation fuels. Torrefaction by means of the TOP process leads to a very energy dense fuel pellet of 15-18.5 GJ/m3. Typically, the process has a thermal efficiency of 96% and the total production costs amount 40-50 €/ton of TOP pellets. The logistic costs amount 50%-66% the costs involved with wood pellets.  

Torrefaction for biomass co-firing in existing coal-fired power stations “BIOCOAL”
Energy Research Centre of the Netherlands

Torrefaction is considered to be a pre-treatment technology to make biomass more suitable for co-firing applications. Especially, the improved grindability of biomass after torrefaction may enable higher co-firing rates in the near future. Torrefaction is, however, a technology that is not commercially available yet. This work contributed to the development of torrefaction by means of extensive parametric research, process simulations and process design. Experimental work has revealed that torrefied biomass can be produced with a grindability comparable to coal and with a combustion reactivity comparable to wood. The process can be operated at high process energy efficiency; typically 96% thermal efficiency and 92% net process efficiency (including the internal electricity consumption). Reactor technology based on moving-bed principles is identified very promising reactor for torrefaction. A production plant of 60-kton/a product requires a capital investment of 5.2 to 6.3 M€. The total production costs amount in the range of 40 to 56 €/ton product (excluding feedstock costs), which is 2 to 2.8 €/GJ. It is recommended to further develop moving-bed technology for torrefaction by means of pilot-scale testing (prototype) and to investigate other important product properties such as hydrophobic nature and leeching behaviour in relation to operating conditions. 

Wood briquette torrefaction

Several torrefaction experiments using wood briquettes are reported in this paper. The torrefied briquettes weight yield lies between 43 and 94 %, and energy yields ranged from 50 to 97 % depending on the operating parameters. After torrefaction the briquettes showed an increase of approximately 15 % in heating value, and a decrease of approximately 73 % in equilibrium moisture.  It was shown that torrefied briquettes achieved hydrophobic character and remained unaffected when Immersed in water. This research also provides information on proximate and elemental analysis, showing that temperature has more influence than residence time. The aforementioned data indicate that torrefaction is a feasible alternative to improve energy properties of ordinary briquettes and prevent moisture absorption during storage.

Torrefaction of agricultural and forest residues  
D. Tito Ferro, V. Vigouroux, A. Grimm and R. Zanzi
Cubasolar 2004, April 12-16, 2004, Guantánamo , Cuba  

This paper presents a work on biomass torrefaction performed in a laboratory unit with a reactor tube of a length of 0.5 m and an inner diameter of 0.04 m). The experiments are conducted pine, lucern, sugar cane bagasse, wood pellets and straw pellets. The reactor was heated to the selected temperature (230°C, 250°C or 280°C) and kept at the final temperature for a period of 1, 2 or 3 hours. The effect of the raw material, temperature, residence time and nitrogen flow on the properties of the torrefied products is studied. The torrefied biomass products are characterized with elemental composition, energy content, moisture content, ash content, volatile fraction. The gaseous products are also analysed. The type of biomass influenced the product distribution. During torrefaction biomass undergoes changes in physical and chemical properties. The fixed carbon content and energy density increase with both time and temperature of torrefaction, while the yield on a weight basis decreases. The torrefied biomass has hydrophobic properties and a higher calorific value than the raw material. 

More efficient biomass gasification via torrefaction
Mark J. Prins_, Krzysztof J. Ptasinski, Frans J.J.G. Janssen
Eindhoven University of Technology, Environmental Technology Group, 
Department of Chemical Engineering and Chemistry, The Netherlands    

Wood torrefaction is a mild pyrolysis process that improves the fuel properties of wood. At temperatures between 230 and 300 C, the hemicellulose fraction of the wood decomposes, so that torrefied wood and volatiles are formed. Mass and energy balances for torrefaction experiments at 250 and 300 C are presented. Advantages of torrefaction as a pretreatment prior to gasification are demonstrated. Three concepts are compared: air-blown gasification of wood, air-blown gasification of torrefied wood (both at a temperature of 950 C in a circulating fluidized bed) and oxygen-blown gasification of torrefied wood (at a temperature of 1200 C in an entrained flow gasifier), all at atmospheric pressure. The overall exergetic efficiency of air-blown gasification of torrefied wood was found to be lower than that of wood, because the volatiles produced in the torrefaction step are not utilized. For the entrained flow gasifier, the volatiles can be introduced into the hot product gas stream as a ‘chemical quench’. The overall efficiency of such a process scheme is comparable to direct gasification of wood, but more exergy is conserved in as chemical exergy in the product gas (72.6% versus 68.6%). This novel method to improve the efficiency of biomass gasification is promising; therefore, practical demonstration is recommended.

Torrefaction of wood: Part 1. Weight loss kinetics
Mark J. Prins, Krzysztof J. Ptasinski, Frans J.J.G. Janssen  
Eindhoven University of Technology, Environmental Technology Group, 
Department of Chemical Engineering and Chemistry, The Netherlands  

Torrefaction is a thermal treatment step in the relatively low temperature range of 225–300 C, which aims to produce a fuel with increased energy density by decomposing the reactive hemicellulose fraction. The weight loss kinetics for torrefaction of willow, a deciduous wood type, was studied by isothermal thermogravimetry. A two-step reaction in series model was found to give an accurate description. The first reaction step has a high solid yield (70–88 wt%, decreasing with temperature), whereas less mass is conserved in the second step (41 wt%). The fast initial step may be representative for hemicellulose decomposition, whereas the slower subsequent reaction represents cellulose decomposition and secondary charring of hemicellulose fragments. The kinetic model is applied to give recommendations for industrial torrefaction process conditions, notably operating temperature, residence time and particle size.