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 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 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. 

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.

Technology to produce high energy biomass briquettes

Briquetting is a process that has the objective of concentrating the sparse energy of vegetable residues such as wood residues, rice husk, bagasse, etc., into high density particles of varied forms and sizes known as “briquettes”. This process eases handling and reduces the costs of transport and storing. When aiming to improve the fuel properties of those, there is a need to eliminate the biomass constituents that are not combustible such as water, extractive, minerals, etc. Among the most used processes are the thermal ones, such as pyrolysis, carbonization, and torrefaction. In this work a discussion about the different technological variants that can be applied for the production of high density energetic and apparent briquettes is discussed. For this reason the experience of the Altyernative Fuels Group of UNICAMP and Bioware, a graduate company from the University incubator is presented.

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. 

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 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. 

Torrefaction is a treatment which serves to improve the properties of biomass in relation to thermochemical processing techniques for energy generation; for example, combustion, co-combustion with coal or gasification. The topic has gathered interest in the past two decades but further understanding is required for optimisation of the process thus enhancing economic efficiency, which is crucial to the success of the treatment commercially and within industry. In particular there is a noticeable gap in current literature regarding the combustion properties of torrefied biomass. This study examines torrefaction in nitrogen of two energy crops, reed canary grass and short rotation willow coppice (SRC), and a residue, wheat straw. Product evolution and mass and energy losses during torrefaction are measured using a range of laboratory scale methods. Experiments at different torrefaction conditions were undertaken to examine optimization of the process for the three fuels. Progress of torrefaction was also followed by chemical analysis (C, H, N, O, ash), and it was seen that the characters of the biomass fuels begin to resemble those of low rank coals in terms of the van Krevelen coal rank parameter. In addition, the results indicate that the volatile component of biomass is both reduced and altered producing a more thermally stable product, but also one that produces greater heats of reaction during combustion. The difference between the mass and energy yield was shown to improve for the higher torrefaction temperatures investigated. The combustion behaviour of raw and torrefied fuels was studied further by differential thermal analysis (DTA) and also, for willow, by suspending individual particles in a methane–air flame and following the progress of combustion by high-speed video. It is shown that both volatile and char combustion of the torrefied sample become more exothermic compared to the raw fuels, and that depending on the severity of the torrefaction conditions, the torrefied fuel can contain up to 96% of the original energy content on a mass basis. Upon exposure to a methane-air flame, torrefied willow ignites more quickly, presumably because its low moisture content means that it heats faster. Torrefied particles also begin char combustion quicker than the raw SRC particles, although char combustion is slower for the torrefied fuel.