Article pubs.acs.org/EF
Fluidized Bed Gasification of Torrefied and Raw Grassy Biomass (Miscanthus × gigantenus). The Effect of Operating Conditions on Process Performance Marzena Kwapinska,† Gang Xue,‡ Alen Horvat,‡ Luc P. L. M. Rabou,§ Stephen Dooley,‡ Witold Kwapinski,‡,∥ and James J. Leahy*,‡,∥ †
Technology Centre for Biorefining & Biofuels, University of Limerick, Limerick, Ireland Carbolea, University of Limerick, Limerick, Ireland § Biomass & Energy Efficiency, Energy Research Centre of The Netherlands (ECN), 1755 Petten, Netherlands ∥ Materials and Surface Science Institute, University of Limerick, Limerick, Ireland ‡
ABSTRACT: Torrefaction is suggested to be an effective method to reduce the cost of biomass provision and improve the fuel properties. In this study, both raw and torrefied Miscanthus × giganteus (M×G) were gasified in an externally heated air-blown bubbling fluidized bed (BFB) gasifier using olivine as the bed material. The effects of equivalence ratio (ER) (0.18−0.32) and bed temperature (660−850 °C) on the gasification performance were investigated. Torrefied M×G has higher energy density primarily due to a higher ratio of lignin to cellulose and hemicellulose; it has lower bulk density, smaller particle diameter and lower reactivity than the original biomass. These properties affect its performance during gasification. The cold gas efficiency was on average 12% lower for torrefied than for raw M×G for the range of operating conditions studied. Within the same temperature range the carbon conversion was about 10% higher for raw than for torrefied biomass. The hydrogen conversion was higher for torrefied M×G since gasification of this feedstock results in higher yields of methane and ethane and lower yields of unreacted process water. The carbon loss with char elutriated from the gasifier for torrefied M×G was significantly higher than that of raw (5% vs 3%) and was driven by physical properties of torrefied M×G. The results obtained suggest that chemical composition expressed as lignin to cellulose and hemicellulose ratio has a pronounced effect on carbon conversion efficiency and tar production. of C in the fly ash and agglomeration which leads to extensive bed extraction with unburned carbon and sensible heat being carried by the extracted material,3 (b) high O/C ratios in the feedstock, and overoxidation of biomass in the gasifier in order to evaporate moisture, and (c) syngas cleaning concerns arising from to the production of carbon particulates and heavy hydrocarbon compounds. The formation of carbonaceous material (char or particle fines) and tars are directly correlated to the fuel’s physical structure and chemical composition. Torrefaction which involves heating biomass to temperatures of between 200 and 300 °C under an inert atmosphere is suggested to be an effective method to reduce the cost of biomass provision and improve its fuel properties. During torrefaction biomass is partially decomposed with the physicochemical changes including drying, devolatalization of hemicellulose and to a lesser extent depolymerization and devolatalization of cellulose and depolymerization and softening of the lignin. The moisture and some low molecular weight organic volatiles are released during this process4,5 and the hygroscopic biomass is rendered hydrophobic,6,7 which makes it more convenient for transport and long-term storage.8 The decomposition of the hemicellulose and to a lesser extent of cellulose, lignin, and extractives results in a decrease in mass,9,10
1. INTRODUCTION With growing evidence linking increases in anthrophogenic greenhouse gas (GHG) emissions to climate change there is a need to develop additional mitigation practices to offset the risk of irreversible effects. This will include more efficient processes for thermochemical conversion of biomass such as gasification and a broadening of the current range of potential biomass feedstocks to include nonwoody materials such as grasses and wastes. Biomass gasification is a less mature technology when compared to coal gasification.1 However, its development has become established practice in countries with large woody biomass resources or with strong support for renewables. The possibility of converting all forms of low energy density organic materials to energy carriers will open the space for gasification to play a role in a future sustainable energy industry. There are three main problems with biomass gasification: (1) bulky feedstock wherein biomass gasification project tends to be small because of the high cost of transporting large amount of biomass to a single point of use,2 (2) tar which limits conversion efficiency and the application of the product gas to combustion in boilers, and (3) agglomeration due to reactive mineral components. The advantages of biomass compared to coal include the following: higher reactivity and lower temperature required for conversion. Biomass gasification is not yet widely deployed commercially because of (a) conversion efficiency losses due to high amounts © 2015 American Chemical Society
Received: June 29, 2015 Revised: October 2, 2015 Published: October 20, 2015 7290
DOI: 10.1021/acs.energyfuels.5b01144 Energy Fuels 2015, 29, 7290−7300
Article
Energy & Fuels while the original energy content is only slightly reduced.11 The energy density of the torrefied biomass is enhanced due to the reduced O/C and H/C in the product.4,12,13 Fuel chemical composition influences the efficiency of gasifiers and gasification systems; according to Prins and co-authors,14 who based their conclusions on theoretical thermodynamic equilibrium calculations, in order to gasify fuels at atmospheric pressure with high thermodynamic efficiency, the O/C ratio of the feedstocks should ideally be lower than 0.4. The higher energy density of torrefied biomass is primarily due to its higher lignin content. Lignin moieties are linked by a multitude of interunit bonds including ether and carbon− carbon linkage.15,16 The energy contained in the C−C bond is higher than C−O or C−H bonds and this directly results in a higher calorific value.12,17 Mass loss in the form of volatiles during torrefaction causes the biomass to become more porous with higher surface area;10 this results in a significant reduction in the volumetric density of torrefied biomass.18 During the torrefaction process the biomass tends to shrink and become more fragile/brittle losing its mechanical strength, making it easier to grind/reduce the particle size.19,8,12 The reduced quantity of chemically bound oxygen in the biomass following torrefaction is suggested to improve the performance of the material when gasified. Research has been carried out to investigate the potential effects of torrefaction on the gasification process; e.g., the decrease in moisture content of biomass after torrefaction can increase the energy efficiency of the gasification processes, as less energy is needed to evaporate the moisture and maintain the appropriate temperature in the gasifier.20 Prins et al.17 compared three gasification scenarios from the point of view of mass and energy balances and reported that torrefaction prior to gasification is a promising pretreatment to achieve more efficient gasification of wood in an oxygen-blown entrained flow gasifier. This conclusion was based on process simulation by chemical equilibrium modeling using dry feedstock. Publications reporting gasification of torrefied biomass in actual gasifiers are rare, and most of them refer to entrained flow gasifiers. Couhert et al.21 gasified raw and torrefied wood samples in a high temperature entrained flow reactor with 20 vol % steam in N2. They concluded that torrefied wood produced more H2 and CO than raw wood at 1400 °C. Chen and co-workers22 numerically compared the performance of raw and torrefied bamboo and high volatile content bituminous coal in an entrained flow gasifier using O2 as the gasification medium and concluded that torrefaction enabled the gasification behavior of biomass to approach that of coal. Moreover, the cold gasification efficiency of torrefied bamboo improved to 49.8% from 29.0% for the raw material. van der Stelt et al.20 concluded that torrefaction is the most cost-effective and environmentally friendly technology for a biomass-to-liquid (BTL) plant located in The Netherlands with a capacity of 1000 MWth of synthesis gas. Berrueco et al.23 reported experimental results in relation to O2/steam gasification of torrefied biomass (spruce and forest residue tops and branches) in a pressurized fluidized bed reactor. The main trend observed for both biomasses was an increase in gas yield with pressure and extent of torrefaction. Additionally it was noticed that tar yield increased with the experimental pressure while the yield of char fell. During torrefaction, permanent gases such as CO2, CO, and CH424,25 are released but not utilized in the gasification process which lowers the overall efficiency of air-blown gasification since the energy contained in the volatiles is not used.17,23
Li et al.26 investigated the devolatilization of palm kernel shell and found that torrefaction results in a decrease in the reactivity of the biomass feedstock. Fisher et al.27 reported that the combustion and gasification (using H2O/N2 as the gasifying medium) reactivities of chars produced from torrefied willow are lower than those of raw willow. The CO2 gasification of torrefied biomasses was also experimentally investigated. Xue et al.10 studied the effect of torrefaction on the CO2 gasification of a grassy biomass Miscanthus × giganteus (M×G) and found that torrefied M×G showed a higher conversion rate and instantaneous reactivity at 850 °C than at 750 °C, suggesting that torrefied material requires higher gasification temperature in order to obtain conversion ratios similar to raw biomass. In a separate study by Wang et al.28 a significant difference in the reactivity (CO2 gasification) between untreated and torrefied wood (birch and spruce) was interpreted as being a result of secondary char-forming reactions (condensation of tar vapor on material surface) when tars were swept away less efficiently during torrefaction and thermal deactivation resulting in rearrangement of the char into a more stable chemical structure at elevated temperatures.29 There are relatively few reports in the scientific and engineering literature providing detailed investigation of process conditions for laboratory scale gasification studies of torrefied biomass. The objectives of this study are thus to investigate the influence of torrefaction on the product gas composition resulting from gasification in an air-blown bubbling fluidized bed gasifier. We report the influence of equivalence ratio (ER) and process temperature on the performance of a torrefied grassy biomass, Miscanthus × giganteus (M×G) and compare its gasification characteristics to those of the raw material.
2. METHODS 2.1. Biomass Torrefaction, Characterization, and Bed Material. The biomass feedstock used in this study, M×G, was supplied by JHM Crops, Limerick, Ireland as pellets, and subsequently crushed to particles of less than 5 mm. The crushed biomass was torrefied in a cylindrical container 250 mm in height and 120 mm in diameter with holes (3 mm) on the caps and along the reactor walls which served to release any vapors generated during the process. The container was tightly packed with biomass (approximately 1.3 kg), to eliminate any O2 present, then placed into a muffle furnace, heated from room temperature to 250 °C at 20 °C/min, and maintained at this temperature for 4 h. Subsequently heating was terminated, and the reactor was allowed to cool to ambient temperature while still in the furnace. Although there was no flow of sweeping N2 in the furnace, vapors released from biomass during torrefaction force the remaining O2 out of the reactor through the holes. The torrefied M×G samples differed slightly from batch to batch, but the average mass yield for all batches was around 76 wt % of initial feedstock. The torrefied M×G was milled in a ball mill prior to characterization and testing to ensure sample homogeneity. The bulk density, particle size distribution, and average particle size of both biomass feedstocks are presented in Table 1. The proximate and ultimate analysis of the raw and torrefied M×G are presented in Table 2. The elemental composition was determined using a Vario EL cube elemental analyzer, while higher heating value was measured using an Isoperibol calorimeter 6200 (Parr Instrument). Chlorine content was determined according to CEN/TS 15408:2006. Fixed carbon content was calculated by subtracting the moisture, ash, and VM content from 100%. The chemical compositions of torrefied and raw M×G were determined according to procedures described in Xue et. al.10 (Table 2). The bed material used in this study was noncalcined olivine (250−500 μm; mean particle diameter, 375 μm) 7291
DOI: 10.1021/acs.energyfuels.5b01144 Energy Fuels 2015, 29, 7290−7300
Article
Energy & Fuels
located after the hot filter. After leaving the analysis section the product gas was oxidized in the afterburner. Residual char in the gasifier reactor was also oxidized on a daily basis before removal of the bed material. Detailed information about the process operating conditions is presented in Table 4.
Table 1. Particle Size Distribution of Raw (R) and Torrefied (T) M×G % of total weight sieve size, mm >2 1−2 0.5−1 0.25−0.5
