Biomass Featured Articles
Torrefaction of Reed Canary Grass, Wheat Straw and Willow
By T.G. Bridgeman, J.M. Jones, I. Shield and P.T. Williams.Abstract
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. 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Biomass is a unique fuel and has the potential to play a significant role in the future energy mix in the UK. Unlike other renewables, biomass can provide continuous electricity generation, and is the only widespread source of renewable heat. Increased use of biomass as a source of energy (electricity and heat) will contribute to the reduction of CO2 emissions, increase energy security, and support sustainable development and regeneration of rural areas, both through increased agricultural and forestry activity and the provision of small scale localised energy and heat generation schemes.
However, renewables still only account for a minute proportion of energy generation in the UK. In 2005, 4.2% of electricity was generated by renewable sources, 53% of which was from biomass and wastes (including MSW (municipal solid waste), animal waste, forestry residues, industrial and domestic wood, co-fired generation and short rotation coppice). However, in 2004 renewable heat generated from biomass and biofuels accounted for just 1% of the total heat market. Evidently there are a number of obstacles to overcome in order to expand the biomass sector and the use of biofuels. Whilst some of these include bureaucracy, economics and fragmented approaches from regional and national authorities, there are also limitations associated with biomass fuel properties.
Direct comparisons of biomass with coal, still the dominant solid fuel in electricity and heat generation, often reveal inferior properties of biomass. In particular, it has lower energy densities, is a bulkier fuel (with poorer handling and transportation characteristics), more tenacious (its fibrous nature means it is difficult to reduce to small homogeneous particles) and – in most cases – a higher moisture content, resulting in storage complications such as degradation and self heating. These properties can have negative impacts during energy conversion such as lower combustion efficiencies and gasifier design limitations.
Torrefaction is a thermal treatment that occurs in an inert atmosphere. It removes moisture and low weight organic volatile components and depolymerises the long polysaccharide chains, producing a hydrophobic solid product with an increased energy density (on a mass basis) and greatly increased grindability. As a result, significantly lower energy is require to process the torrefied fuel and it no longer requires separate handling facilities when co-fired with coal in existing power stations. (In 2005, 15% of all renewable electricity generation in the UK was from biomass co-fired with coal.) It has also been suggested that the modified fuel can be compacted into high grade pellets with substantially superior properties when compared with standard wood pellets. The process can be incorporated into a combined drying, torrefaction and pelletisation process, with both economic and energy efficiency benefits. Finally, it has been suggested that torrefied biomass is a suitable feedstock for entrained flow gasification, systems previously not considered feasible for raw biomass solid fuels. This is because torrefied biomass forms more spherical shaped particles during grinding or milling. However, the process requires a separate plant, an input of process energy and the production of gaseous and volatile streams, entailing capital costs, operating costs and emission control. The balance between these associated costs and energy consumption and the cost and energy benefits from a more grindable, higher calorific value fuel are therefore critical for the future of torrefaction and require thorough analysis and reliable, extensive data. A evaluation of the torrefaction of wood chips published in 2001 concluded that the environmental and heating value benefits gained by torrefaction are not significantly greater than the extra energy consumption and capital investment, although it did indicate potential for the torrefaction concept in fuel densification. However, a number of other publications are optimistic about the benefits torrefaction has to offer. Furthermore, there are concepts such as retrofitting existing power stations in order to ultilise waste heat to serve a torrefaction process plant which must be considered.
Torrefaction has only received significant interest in the last two decades, and has yet to become a commercial process. The majority of research to-date has focused on the compositional changes that occur in biomass, determined by proximate and ultimate analysis, along with mass and energy yields. Some studies have also investigated the composition of volatile matter released during the process. This previous work has focused primarily on woody biomass, including the comparisons of softwoods and hardwoods. Energy crops have received little attention, and there is still a need for comprehensive understanding of the optimum torrefaction conditions, as well as investigations of the combustion behaviour of these fuels, including the emissions and smoke compositions from torrefied fuels. Work presented in this paper reports on fundamental investigations into torrefaction of an energy grass, and makes comparisons with a woody biomass (willow) and an agricultural bi-product (wheat straw). In the second half of the paper combustion properties of all three fuels are investigated. The results from this paper will contribute to further work involving economic analysis of this pretreatment, which is beyond the scope of this paper.
2. Experimental procedure
2.1. Torrefaction process
2.1.1. Samples
Two energy crops and an agricultural residue, namely willow (short rotation coppice, SRC), reed canary grass and wheat straw respectively, have been torrefied with subsequent analysis of the solid residues. These types of lignocellulose biomass material cover two of the three solid biofuel categories currently being considered as suitable for thermochemical conversion processes in the UK. Standard DD CEN/TS 14961:2005 categorises willow (short rotation coppice) as woody biomass (forest and plantation wood), whilst reed canary grass (grasses) and wheat straw (cereal crops) are classed as herbaceous biomass.
The reed canary grass and wheat straw were grown at the Woburn Experimental Farm in Bedfordshire, UK, part of Rothamsted Research. The SRC willow was grown by Rural Generation in Northern Ireland.
2.1.2. Evolved gas analysis by TGA-FTIR
A Nicolet Magma-IR AEM connected to a Stanton Redcroft Simultaneous Analyser STA-780 Series was used to perform torrefaction experiments at laboratory scale whilst simultaneously analysing the volatile pyrolysis products for corresponding TGA data. Samples weighing between 25 and 35 mg were placed in a nickel crucible. The temperature was ramped to 378 K at 10 K min-1 and held for 5 min in order to remove moisture. The second stage of the regime used the same ramp rate to reach the desired torrefaction temperature in the range 503– 563 K. The inert atmosphere was maintained using nitrogen at STP at a flow rate of 50 ml min-1. It is widely accepted in the literature that the onset of primary pyrolysis occurs at 473 K [e.g. 12,13]. The time period between this temperature being reached and the point of cooling has previously been defined as the ‘torrefaction time’. Each run was carried out so that this time interval was 30 min.
2.1.3. Ultimate analysis
The C, H, N, O, S contents are measured using a CE Instruments Flash EA 1112 Series elemental analyzer. All measurements were repeated in triplicate and a mean value corrected for moisture content is reported. The ash content of the raw samples was measured in a Carbolite OAF 10/1 furnace, adhering to standard DD CEN/TS 14775:2004. It was assumed that no ash will be lost during the pyrolysis stage thus an ash content value has been calculate for each solid residue from overall mass yields.
2.1.4. Cell wall composition
Lignin, cellulose and hemicellulose were determined at the Institute of Grassland and Environmental Research, Aberystwyth, UK, via Klason, Acid Digestible Fibre (ADF) [15], and Neutral Digestible Fibre (NDF) wet chemistry methods. Briefly these procedures entail the following: ADF was determined by detergent incubation under acid (pH 2) conditions. The sample was boiled and filtered, hemicellulose and cell solubles dissolve and are filtered away. The residue left is ADF and consists mainly of cellulose and lignin. Acid detergent lignin (ADL) was measured by further treating ADF with strong acid, which dissolves cellulose. The cellulose content was derived by subtracting ADF values from ADL values.
2.2. Fuel analysis
2.2.1. Calorific value determination
Friedl et al. provide a method using two equations for the calculation of CV based on the carbon, hydrogen and nitrogen content: an ordinary least squares regression (OLS) (Eq. (1)) and a partial least squares regression (PLS) method (Eq. (2)). These were calibrated using 122 different biomass samples and give the higher heating value (HHV) on a dry basis.
HHV(OLS) = 1.87C2 - 144C - 2802H + 63.8CH + 129N + 20147 (1)
HHV(PLS) = 5.22C2 - 319C - 1647H + 38.6CH + 133N + 21028 (2)
where C = carbon, H = hydrogen, and N = nitrogen content expressed on a dry mass percentage basis. Calorific values are reported are calculated using an average of the results from both two methods and further corrected for ash content to give a calorific value on a dry ash free basis.
2.2.2. Thermogravimetric analysis (TGA)
Pyrolysis tests were performed using a TGA analyser (Stanton Redcroft Simultaneous Analyser STA-780 Series), by heating a typical sample mass of 3 mg in a purge of nitrogen (50 ml min-1), at a rate of 20 K min-1. The final temperature was 1173 K, with a holding time of 15 min. Burning profiles of the four samples were performed using a DTA analyser (Stanton Redcroft Simultaneous Analyser STA-780 Series). A typical sample mass of 3 mg was heated at 20 K min-1 in a purge of air (50 ml min-1) to a final temperature of 1173 K. Both thermogravimetric and differential temperature measurements were recorded simultaneously during combustion analysis.
2.2.3. Combustion studies
A Meker-burner was used to conduct combustion studies of stationary willow and torrefied willow particles (prepared using the TGA procedure described above). The Meker-burner flame consists of a number of short blue inner cones and a large single outer flame reaction zone which approximates to a 1D flame. Natural gas was used and the stoichiometry controlled so that the part of the flame in which the particles were placed reached a temperature of ~1500 K. The oxygen content at this location was 1.75 mol%. The particles (2–4 mm in length) were held in place on a steel needle adjacent to an R-type thermocouple in a ceramic housing. A water-cooled probe surrounded both the particle and the thermocouple before entry into the flame. The probe is placed into the flame ensuring that the particle and thermocouple are located centrally above the burner. The water-cooled sleeve is then retracted so that the particle and thermocouple are exposed to the flame. Upon completion of particle combustion, the watercooled sleeve is slid back over the needle and thermocouple and the unit is removed from the flame. A Photo-Sonics Phantom V7 high-speed video system was used to record the images of the combusting particles at a speed of 1000 frames per second (fps). The captured footage was used to observed the combustion behaviour and determine the duration of different combustion stages.
3. Conclusions
Herbaceous biomass undergoes greater alterations in its properties during torrefaction than woody biomass, in terms of mass loss and atomic composition. The overall mass loss in both herbaceous species investigated was similar but wheat straw underwent greater changes in terms of it elemental composition and increases in energy content: carbon content rose by a maximum of 14% to 51 wt%, oxygen content was reduced by 30% to 25% and the energy content rose by 20% to 22.6 MJ kg-1. However, the overall energy yield was slightly lower for wheat straw. In contrast the mass and energy yields of willow were noticeably higher than the two herbaceous species. For example, after treatment at 543 K the mass yield of willow was 80%, whereas the figures for wheat straw and reed canary grass were 72% and 71%, respectively. The energy yields for these conditions were 77%, 78% and 86% for wheat straw, reed canary grass and willow respectively. However, willow torrefied at 563 K only experienced an increase in its overall energy content of 9.5% These observed differences appear to be mostly attributed to differences in the lignocellulose composition of the fuels, specifically the higher hemicellulose content of graminaceous crops.
Observations of combustion behaviour revealed a number of differences between torrefied biomass and raw fuels. For all fuels, volatile combustion was modified and occurred over a shorter temperature range and the herbaceous crops produced heats of combustion that were between 10–65% higher, depending on the treatment temperature and crop. The higher fixed carbon content also meant that torrefied biomass produced greater heats of combustion during char burnout. All these observed behavioural changes were more pronounced for the products of higher torrefaction temperatures. From DTA and the analysis of high speed videos of the combustion of untreated and torrefied willow particles it was observed that the ignition times of both combustible volatiles and char were reduced as a result of torrefaction. These changes in properties are expected to be beneficial during combustion applications.
It is suggested that future work investigates the change in nature of the composition of volatile component of torrefied fuels and should seek to identify any potential benefits for other thermochemical conversion processes.
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May 2008
