Energy & Fuels 2009, 23, 951–957
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Effects of the Composting and the Heating Rate on Biomass Gasification Agustı´n Garcı´a Barneto,*,† Jose´ Ariza Carmona,† Araceli Ga´lvez,‡ and Juan A. Conesa‡ Chemical Engineering Department, HuelVa UniVersity, Campus El Carmen, 21071 HuelVa, Spain, and Chemical Engineering Department, UniVersity of Alicante, P.O. Box 99, 03080 Alicante, Spain ReceiVed July 23, 2008. ReVised Manuscript ReceiVed NoVember 12, 2008
In biomass gasification facilities different biomasses are used, with different proportions of hemicellulose, cellulose, and lignin. Several pretreatments are performed in these installations in an effort to change physical characteristics of the feedstock such as drying, size reduction, size fractionation, and leaching with water. Taking into account that lignin gasification produces more hydrogen than other components of the biomass, it could be of interest as a pretreatment that improves lignin content. Composting is a biological process that modifies biomass composition, increasing lignin content. Without going into any economic details, the present work studies the effect of biomass composting (biological pretreatment) on hydrogen production in air gasification. This laboratory-scale experience was completed, studying the effect of the heating rate on the gasification process. In this study, two different biomasses were used: Leucaena (Leucaena Leucocephala) and Tagasaste (Chamaecytisus palmensis). The experimental results show that combining slow heating rate and biomass composting, hydrogen production increases 20% (into studied range). This result can be related with feedstock composition (inorganic matter and lignin contents) and char properties.
1. Introduction Biomass can contribute significantly to reduce the CO2 emissions. According to Maniatis,1 energy from biomass based on short rotation forestry and other energy crops can contribute significantly toward the objectives of the Kyoto Agreement in reducing the greenhouse gases emissions and to the problems related to climate change. A number of biomass technologies are available for converting biomass to energy. These processes can change raw biomass into a variety of gaseous, liquid, or solid materials that can then be used for energy generation. This conversion can be done in three ways: thermochemical (break down biomass under high temperature), biochemical (break down biomass under microorganism or enzymatic processes), and chemical (oils from biomass can be chemically converted into a liquid fuel).2 In recent years, there have been many developments in the science and technology of thermochemical biomass conversions. Among the established and best available thermochemical technologies, incineration, gasification, and pyrolysis conversion are found.1,2 These thermal processes provide an efficient, environmentally acceptable and cost-effective method of providing a sustainable energy source.3 Gasification is the thermochemical conversion of a solid biomass to a gaseous fuel by heating with a gasification agent * To whom correspondence should be addressed. Telephone: (+34) 959219982. Fax (+34) 959219983. E-mail:
[email protected]. † Huelva University. ‡ University of Alicante. (1) Maniatis, K. Progress in Biomass Gasification: An Overview. In Progress in Thermochemical Biomass ConVersion; Bridgewater, A. V., Ed.; Blackwell Scientific Publications: Oxford, UK, 2001; pp 1-32. (2) Faaij, A. Modern Biomass Conversion Technologies. Mitigation and Adaptation Strategies for Global Change 2006, 11, 343–375. (3) Kwant, K. W.; Knoef, H. Status of Biomass Gasification in Countries Participating in the IEA and GasNet Activity August 2004; EU GasNet & IEA Bioenergy Gasification. Netherlands COUNTRY REPORT, 2004; http:// energytech.at/pdf/status_of_gasification_08_2004.pdf.
(i.e., air, oxygen, steam, hydrogen, CO2, or mixtures of these gases). The obtained gas is easier and more versatile to use than the original biomass, with possible uses in power gas engines and gas turbines, or to produce liquid fuels.4-7 Modern integrated gas-steam cycles (IGCC) are capable of achieving high thermodynamic efficiencies.8 The raw product gas exiting a biomass gasifier contains particulates, tars, and other constituents that may interfere with downstream utilization technologies. The concentrations of these constituents will depend on the reactor design and characteristics of the biomass feedstock, and in certain cases gas cleaning is necessary, which affects the economic investment.9,10 Usually, in biomass gasification facilities, feedstock pretreatments consist of drying, size reduction, size fractionation, and leaching with water.8 Although many studies show that chemical pretreatments have an influence on thermal properties of biomass,11-13 these treatments have an influence on physical characteristics of the (4) Reed, T. B.; Daas, A. Handbook of Biomass Downdraft Gasifier Engine Systems; Biomass Energy Foundation Press: Golden, CO, 1998. (5) Klass, D. L. Biomass for Renewable Energy, Fuels, and Chemicals; Academic Press: San Diego, CA, 1998. (6) Norbeck, J. M.; Johnson, K. The Hynol Process: A Promising Pathway for Renewable Production of Methanol. 2000. http://localenergy.org/pdfs/Document%20Library/MethanolfrombiomassHynolexecsum.pdf. (7) Mitsubishi Heavy Industries. Biomass Gasification Methanol Synthesis System. http://www.mhi.co.jp/power/e_power/techno/biomass/. (8) McKendry, P. Energy Production from Biomass (Part 3): Gasification Technologies. Bioresour. Technol. 2002, 83, 55–63. (9) Brandberg, A. R. L.; Ekbom, T. Methanol and Other Energy Carriers from Renewable Resources; International Symposium on Alcohol Fuels, ISAF XIII, Part 1: Implementing the Transition to a Sustainable Transport System, 2000; http://www.ecotraffic.se/pdf/isafxii.pdf. (10) Hamelinck, C. N.; Faaij, A. P. Future Prospects for Production of Methanol and Hydrogen from Biomass. J. Power Sources 2002, 111 (1), 1–22. (11) Szabo, P.; Varhegyi, G.; Till, F.; Faix, O. Thermogravimetric/Mass Spectrometric Characterization of Two Energy Crops, Arundo donax and Miscanthus sinensis. J. Anal. Appl. Pyrolisis 1996, 36, 179–190.
10.1021/ef8005806 CCC: $40.75 2009 American Chemical Society Published on Web 12/30/2008
952 Energy & Fuels, Vol. 23, 2009
Barneto et al. Table 1. Proximate and Ultimate Analysis of the Samples Proximate Analysis (Dry Basis) (% wt)
Leucaena biomass Leucaena compost Tagasaste biomass Tagasaste compost
Ultimate Analysis (Dry and Ash Free) (% wt)
moisture (%)
ash
carbon fixed
volatile matter
C
H
N
O
4.5 5.5 4.2 3.8
3.7 4.2 1.1 2.3
14.4 17.6 17.8 18.2
81.9 78.2 81.1 79.5
44.7 45.6 46.9 48.0
6.0 4.4 6.4 6.2
1.8 2.2 2.1 2.4
44.7 44.2 42.9 41.6
biomass but do not affect its chemical properties (except inorganic reduction). The main components of the biomass are hemicellulose, cellulose, and lignin. Thermal degradation of these substances produces several volatiles: carbon dioxide, carbon monoxide, water, methane, hydrogen, and hydrocarbons, in different proportions. According to Yang et al.,14 lignin produces four times more hydrogen than cellulose and almost three times more than hemicellulose. For this reason, to increase hydrogen concentration in raw gas, it is reasonable to study pretreatments that modify the chemical composition of the biomass feedstock. For example, composting is a treatment that increases lignin content in biomass.15,16 Composting is a natural biological process of decomposition of organic materials in an aerobic environment. During the process, bacteria, fungi, and other microorganisms break down organic materials to a stable mixture called compost while consuming oxygen and releasing heat, water, and carbon dioxide.15 Under this process, degradation and changes in the chemical nature of the organic components of the biomass have been described in literature.18,19 Particularly, it was found that the new components in the stable organic matter (compost) have different thermal behavior from that of original biomass.20,21 On the other hand, the effect of heating rate on biomass and coal pyrolysis is known, particularly on the yield of volatile matter and char reactivity.22,23 According Fushimi et al.,22 higher heating rate causes a rapid evolution of volatiles, producing a porous char that increases final conversion of biomass and (12) Meszaros, E.; Jakab, E.; Varhegyi, G.; Szepesvary, P.; Marosvolgyi, B. Comparative Study of the Thermal Behaviour of Wood and Bark of Young Shoots Obtained from an Energy Plantation. J. Anal. Appl. Pyrolysis 2004, 72, 317–328. (13) Gomez, C.; Meszaros, E.; Jakab, E.; Velo, E.; Puigjaner, L. Thermogravimetry/Mass Spectrometry Study of Woody Residues and an Herbaceous Biomass Crop Using PCA Techniques. J. Anal. Appl. Pyrolysis 2007, 80, 416–426. (14) Yang, H.; Yan, R.; Chen, R.; Ho, D.; Zheng, Ch. Characteristics of Hemicellulose, Cellulose and Lignin Pyrolysis. Fuel 2007, 86, 1781– 1788. (15) Vargas-Garcı´a, M. C.; Sua´rez-Estrella, F.; Lo´pez, M. J.; Moreno, J. Effect of Innoculation in Composting Processes: Modifications in Lignocellulosic Fraction. Waste Management 2007, 27, 1099–1107. (16) Shi, J. G.; Zeng, G. M.; Yuan, X. Z.; Dai, F.; Liu, J.; Wu, X. H. The Stimulatory Effects of Surfactants on Composting of Waste Rich in Cellulose. World J. Microbiol. Biotecnol. 2006, 22, 1121–1127. (17) Haug, R. T. The Practical Handbook of Compost Engineering; Lewis Publishers: London, 1993. (18) Ciavatta, C.; Govi, M.; Pasotti, L.; Sequi, P. Changes in Organic Matter during Stabilization of Compost from Municipal Solid Wastes. Bioresour. Technol. 1993, 43, 141–145. (19) Ha¨nninen, K.; Kovalainen, J.; Korvola, J. Carbohydrates as Chemical Constituents of Biowaste Composts and Their Humic and Fulvic Acids. Compost Science and Utilization, Autumn 1995, 51–68, 1995. (20) Blanco, M. J.; Almendros, G. Maturity Assessment of Wheat Straw Compost by Thermogravimetric Analysis. J. Agric. Food Chem. 1994, 42, 2454–2459. (21) Garcı´a, A.; Ariza, J.; Conesa, J. A.; Blanco, M. J. Submitted for publication. (22) Fushimi, C.; Araki, K.; Yamaguchi, Y.; Tsutsumi, A. Effect of Heating Rate on Steam Gasification of Biomass. 1. Reactivity of Char. Ind. Eng. Chem. Res. 2003, 42 (17), 3922–3928. (23) Sathe, C.; Pang, Y.; Li, C. Effects of the Heating Rate and IonExchangeable Cations on the Pyrolysis Yields from a Victorian Brown Coal. Energy Fuels 1999, 13 (3), 748–755.
increases the reaction rate in steam gasification. Roberts et al.24 studied coal pyrolysis and related heating rate effects with structural and superficial char properties. Mermoud et al.,25 working with large wood char particles, conclude that a lower heating rate produces a dense charcoal and a minor amount of volatiles. Moreover, the charcoal obtained at different heating rates exhibits very different gasification kinetics. The gasification rate in dense char is smaller than that of low-density char. The most used gasification agent is air.26-29 In this case, the nitrogen introduced with the air dilutes the product gas, giving gas with low hydrogen content (5-16%)30 and a net calorific value8 between 4 and 6 MJ/Nm3. The hypothetical use of composting as a pretreatment trying to improve these results depends on a future economic study that takes into account its benefits (hydrogen production) and its difficulties (i.e., mass loss close to 30%). In this context, the main objective of the present work is to highlight the positive effects of composting on hydrogen production in a laboratory-scale experience. Using a semicontinuous reactor, several samples of biomasses and composts have been gasified using air as gasification agent. At the same time, the effect of the heating rate is studied. Introducing the samples into a furnace with different rates, the combined effect of the composting and heating rate on the raw gas that exits the gasifier was studied. 2. Materials and Methods Lignocellulosic feedstocks used in this work are considered energetic crops useful in soil restoration. Leucaena (Leucaena leucocephala) is species of leguminous tree that grows quickly in arid spaces and is used as firewood or for livestock. Tagasaste (Chamaecytisus palmensis) is a small spreading evergreen shrub used as fodder in arid regions. Blends of stalks and leaves from these species can be easily composted, resulting in a material (compost) habitually used as a soil amendment. In this work, the composts have been obtained after 35 days of composting under controlled aerobic conditions. The elemental analysis of the major elements (C, H, N, O) was performed with a PerkinElmer CHNS analyzer (Table 1). The TG runs were carried out with a Setaram 92-12.18 Model TGA on (24) Roberts, D.; Harris, D.; Wall, T. On the Effects of High Pressure and Heating Rate during Coal Pyrolysis on Char Gasification Reactivity. Energy Fuels 2003, 17 (4), 887–895. (25) Mermoud, F.; Salvador, S.; Van de Steene, L.; Golfier, F. Influence of the Pyrolysis Heating Rate on the Steam Gasification Rate of Large Wood Char Particles. Fuel 2006, 85, 1473–1482. (26) Stahl, K.; Neergaard, M. IGCC Power Plant for Biomass Utilisation, Varnamo, Sweden. Biomass and Bioenergy 1998, 15 (3), 205–211. (27) Nieminen, J.; Kivela, M. Biomass CFB Gasifier Connected to a 350 MWth Steam Boiler Fired with Coal and Natural Gas-THERMIE Demonstration Project in Lahti in Finland. Biomass and Bioenergy 1998, 15 (3), 251–257. (28) Van der Drift, A.; van Doorn, J.; Vermeulen, J. W. Ten Residual Biomass Fuels for Circulating Fluidized-Bed Gasification. Biomass and Bioenergy 2001, 20 (1), 45–56. (29) Li, X. T.; Grace, J. R.; Lim, C. J.; Watkinson, A. P.; Chen, H. P.; Kim, J. R. Biomass Gasification in a Circulating Fluidized Bed. Biomass and Bioenergy 2004, 26 (2), 171–193. (30) Narvaez, I.; Orio, A.; Aznar, M. P.; Corella, J. Biomass Gasification with Air in an Atmosphere Bubbling Fluidized Bed. Effect of Six Operational Variables on the Quality of the Produced Raw Gas. Ind. Eng. Chem. Res. 1996, 35 (7), 2110–2120.
Biomass Gasification
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in the pipe. The stoichiometric index (λ) has been calculated as a function of speed of the pipe, V, and length occupied by the sample in the pipe, L. In that form, it is possible to establish a relationship between this index and the elementary composition of the sample.
Figure 1. Schematic diagram for gasification.
samples of around 5 mg. Pyrolysis runs were carried out in a nitrogen atmosphere and combustion runs in synthetic air (N2/O2 4:1). Three heating rates (5, 10, and 20 °C/min) have been used from 25 to 900 °C. 2.1. Composting. Twenty kilograms of biomass (Leucaena and Tagasaste), collected from a research farm in Huelva (Spain), was chopped (trunks, branches, and leaves) to 3 cm, moistened to 60 wt % of water holding capacity at atmospheric pressure, and composted in a polyethylene container in a controlled environment room (at 24 °C). At the bottom of the container, a drilled polyamide pipe ensures continuous aeration of the samples. Four temperature sensors were introduced in different points of the compost pile. The composting material was rotated every 4 days (transferred to an equivalent container), and homogeneous replicated samples were taken by picking up samples from different points. After determining the moisture of the samples, additional water was sprayed on the remaining pile, if required. The compost samples were air-dried, grounded in a rotary knife-mill to a final particle size
