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EFFECT OF RICE HUSK TORREFACTION ON SYNGAS PRODUCTION AND QUALITY Filomena Pinto, Jorge Gominho, Rui Manuel Neto Andre, David Braz Gonçalves, Miguel Miranda, Francisco Varela, Diogo Neves, João Duarte Navalho Santos, Ana Lourenço, and Helena Pereira Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00259 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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EFFECT OF RICE HUSK TORREFACTION ON SYNGAS PRODUCTION AND QUALITY

Filomena Pinto*1, Jorge Gominho2, Rui Neto André1, David Gonçalves1, Miguel Miranda1, Francisco Varela1, Diogo Neves1, João Santos1, Ana Lourenço2, Helena Pereira2

1 2

LNEG, Estrada do Paço do Lumiar, 22, 1649-038, Lisboa, Portugal

Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal

*Corresponding author: e-mail: [email protected], Phone: +351 210924787 ABSTRACT: The aim of this study was the production of a good quality gasification gas (syngas) with rice husk residues. This material was torrefied before gasification with the aim of improving this last process and the effect of torrefaction time and temperature was investigated. The variation of temperature from 200ºC to 300ºC decreased moisture and volatile matter and, consequently increased HHV. The increase of reaction time from 30 to 60 min at 250ºC or 300ºC had a milder effect than the rise of torrefaction temperature. The optimum conditions for the rice husk torrefaction prior to gasification were 250ºC and 30 min, as the torrefied material presented HHV of 22.1 MJ/kg daf an ash content of 17.7 % The torrefaction increased the rice husk in extractives 14.8 % (vs. 12.6% in the raw material), mainly in non-polar extractives and lignin (36.4 % vs. 28.9 %). Py-GC/MS analyses determined that raw rice husk presented a monomeric composition of lignin (H:G:S) of 4:12:1 and ratio carbohydrates/lignin of 3.3 while the torrefied material showed respectively 3:9:2 and 3.9. Compared with the raw rice husks, the gasification of torrefied rice husk at 250 ºC presented a syngas poorer in CO2 (39% vs. 24 %) and richer in H2 (31% vs. 36 %) and CO (17% vs. 24%) and produced less tar (7.6 g/m3 vs. 3.5 g/m3) but released more pollutants (186 vs. 92ppmv of H2S and 1284 vs. 734ppmv of NH3).

Keywords: torrefaction; gasification; rice husk; tar; H2S and NH3 in syngas

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1. INTRODUCTION Many agricultural and agro-industrial residues can be used as feedstock for energy production such as skins, straws, or other crop and seed components. One example is rice husks that are obtained in huge quantities by the rice industries that process worldwide around 540 million metric tons.1 The rice husk is the outer cover of the grain and represents 14-35% of its weight basis.2 Most of the rice husk residues are burned in open fields since transportation is not economically viable, due to the low energy density. However, in some situations (e.g. short distance and high availability) it is possible to accumulate this residue as feedstock for energy production. The low energy content of rice husks (lower heating value of 13-16 MJ/kg)2 is one disadvantage that can be overcome by a torrefaction pretreatment. Torrefaction is a thermochemical process conducted at 200-300ºC under an inert atmosphere by which the biomass loses moisture, the O/C ratio decreases and the energy content increases.3 The torrefied biomass becomes more hydrophobic, resistant to microbial degradation, and friable, which simplifies decomposition processes.4-9 Torrefaction of biomass involves mainly the decomposition of hemicelluloses that occur from 180ºC to 270ºC with consumption of their structural oxygen, while cellulose and lignin are only slightly decomposed.4,7 The degree of torrefaction and the chosen temperature are key issues to counterbalance the improvements in biomass characteristics and the mass and energy losses.4 Torrefaction can be used as pretreatment to improve biomass performance for production of a gaseous bio-fuel (syngas) through gasification. Syngas, whose main components are hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO) and methane (CH4), can be used directly as a gas biofuel for district heating and electricity production, or for the production of liquid biofuels, such as methanol and ethanol by chemical syntheses. The use of torrefied biomass maximizes the gasification yield and may improve syngas quality and cold gas efficiency (CGE).10,11 For instance, torrefaction of biomass at 300ºC increased biomass to syngas overall cold gas efficiency from 63% to 86% (LHVdry) when torrefaction integrated with entrained flow gasification was used instead of external torrefaction.12 The conversion of rice husks in energy by combustion or gasification was the subject of different studies.2,13-15 Washing and torrefaction before combustion or gasification improved the conversion of these processes; in combustion they increased the efficiency by approximately 80% and in gasification they produced a syngas with sufficient quality to be used in internal combustion engines. 2,16


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The main goal of the present work was to study the gasification of rice husks and the effect of torrefaction at various conditions of temperature and time on improving gasification performance and syngas quality. The raw and the torrefied rice husks were chemically characterized by summative, elemental and approximate analysis and PyGC/MS analysis was used to study the lignin and carbohydrates degradation. The effect of torrefaction conditions was analyzed in relation to volatile matter, moisture, ash, mass yield, energy yield and HHV. Gasification gas produced by torrefied material was compared with that obtained with raw rice husks, considering gaseous main components, tar, H2S and NH3 contents, gas yields and gas HHV. Though there are few publications about rice husks gasification, there is not enough information about the eventual advantages of gasifying torrefied husks instead of the raw material. This paper also addresses the effect of using torrefied husks on the release of tar, H2S and NH3 into gasification gas, which is a new approach.

2. EXPERIMENTAL SECTIONS 2.1 Material. The rice husk used in this study was collected from a rice processing plant in Portugal. Table 1 shows the ultimate and proximate analyses of raw rice husks and of torrefied material.

2.2 Chemical composition and Py-GC/MS profile. The chemical analysis was performed using the 40-60 mesh granulometric fraction according to TAPPI (Technical Association of the Pulp and Paper Industry) standard methods; ash (T 211 om-02), extractives (modified from T 12 os-75), lignin (Klason lignin according to T 222 om-02 and acid soluble lignin to UM T 250). The methodology and equipment used are presented in more detailed in Gominho et al.17 The determination of monosaccharides, glucuronic acid, galacturonic acid and acetic acid is described in Neiva et al.18 Lignin and carbohydrates were characterized by analytical pyrolysis (Py-GC/MS). The extractive-free samples were powdered in a Retsch MM200 mixer ball, a sample of around 0.10 mg was pyrolysed at 550 ºC (10 s) in a 5150 CDS apparatus linked to an Agilent GC 7890B with a mass detector system 5977B, using He as carrier gas (flow 1 mL/min). The parameters for GC/MS operation were: injector temperature: 270ºC; capillary column: ZB-1701 (60 m x 0.25 mm i.d. x 0.25 µm film thickness); split ratio: 1:22. Oven temperature started at 40ºC (held for 4 min), then increased to 70ºC at a rate of 10ºC min-1, then to 100ºC at a rate of 5ºC min-1, it rose to 265ºC at 3ºC min-1 (held for 3 min), and increased to 270ºC at a rate of 5ºC min-1 (held for 9 min). The GC/MS interface temperature was 280ºC; the mass spectrometer was EI mode 70 eV.


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The compounds were identified comparing their mass spectra with Wiley, NIST2014 libraries, and literature.19,20 The percentage of each compound identified was calculated using total area of the chromatogram. The carbohydrates and lignin derived compounds were separately summed and the C/L ratio was calculated. The H:G:S is the relation beteween the different lignin monomers founded in samples: p– hydroxyphenyl (H), guaiacyl (G) and syringyl (S); while the G/S is the ratio between the G and S monomers.

2.3 Torrefaction A Perkin Elmer TGA 7 Thermogravimetric Analyzer was used for torrefaction preliminary studies to evaluate temperature and time in torrefaction process. These assays were done under nitrogen atmosphere, at temperatures varying from 200 to 300ºC (at rate of 5ºC/min); and testing different times (30, 45 and 60 minutes). The results obtained with the thermogravimetric analyzer were checked in a bench scale tubular reactor (internal diameter 0.08 m x height of 1.5 m) operating in batch mode under nitrogen atmosphere of around 7L/min. This reactor was placed inside an oven that supplied external electrical heating. The amount of feedstock used was around 200g. The results obtained with the bench scale reactor confirmed those obtained in the thermogravimetric analyzer, and the best parameters chosen were 250ºC and 30 min, according to the final characteristics of the torrefied material and energy and mass yields. Mass yield was calculated as the ratio between the mass obtained after torrefaction and the initial mass of the feedstock. A similar procedure was followed for energy yield, defined as the ratio between the energetic content of the feedstock after torrefaction and the corresponding initial value. The biomass torrefied at these conditions was gasified in a bench-scale gasification installation.

2.4 Gasification The material used was raw rice husk and torrefied rice husk at 250ºC and 300 ºC. Though the parameters chosen for torrefaction were 250ºC and 30 min, some gasification tests were also done with torrefied husks at 300ºC also during 30 minutes to make sure if the best parameters chosen for torrefaction were also the best option for gasification. Rice husks were used without further reduction of particle sizes.


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Biomass gasification tests were done in a bench-scale bubbling fluidized bed gasification shown in Figure 1 and previously described by Abelha et al.21 The gasifier was a bubbling fluidized bed reactor, with an inside diameter of 80 mm and with a height of 1 500 mm. The feeding system was water cooled to avoid clogging, caused by pyrolysis of the feedstock, and a nitrogen flow was used to help feeding and to avoid gas back flow. Steam and oxygen were used as gasifying and fluidizing agents; they were introduced through a gas distributor, placed at the base of the reactor. Previous results led to the selection of the following gasification conditions22,23: mass flow rate of 5 g daf/min, oxygen flow rate of about 1L/min, mass ratio between steam and fuel of around 1 g/g daf (dry and ash free), and equivalent ratio (ER) defined as the ratio between the used oxygen and the stoichiometric amount of oxygen required for complete fuel conversion) of 0.2. ER was kept constant in all experiments. The gasification of the raw material was done at different temperatures (750, 800, 850 and 890ºC). The results obtained led to the selection of the temperature of 850ºC, because it led to the highest CGE, as it will be discussed later. Thus, the torrefied material was gasified at 850ºC. It was possible to work the gasifier at auto-thermal conditions, however, external electrical heating, supplied by the oven external to the gasifier, was used to ensure more constant gasification temperature and to keep it at the selected value. The gasification gas passed through a cyclone to retain particulates and then went through a quenching system to remove tars and condensable liquids. The filtered gas was injected into CO and CO2 on-line analyzers and afterwards collected in bags to be analyzed by GC (gas chromatography) for CO, CO2, H2, CH4, N2, O2 and other heavier gaseous hydrocarbons (from C2 to C4), mentioned as CnHm. Two columns in series (Porapak Q and molecular sieve) and two detectors (TCD and FID) were used for the gas analyses. Sampling and analysis of tars and particulates were determined using CEN/TS 15439:2006 Standard. Gasification gas was sampled before the quenching system. Isopropanol (2-propanol or isopropyl alcohol) was the solvent used for tars collection and for rinsing the sampling line, to guarantee that all the tars were collected. Part of the homogeneous liquid sample was evaporated and residue was weighed to calculate the amount of tars in g/Nm3. The solids retained inside the bed, containing silica sand, ashes and unconverted carbon were collected for analysis, after each experiment. Method 11 of EPA (Environmental Protection Agency) was used for sampling and analysis of sulfur released in the form of H2S. An absorbing solution of CdSO4 was used to retain sulfide, which was then analyzed by iodometry.


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The NH3 content was determined according to CTM-027 of EPA where the ammonia was retained in an acidic absorbing solution (H2SO4 0.1 N) and then analyzed potentiometrically. NH3 condensed in the gas quenching system was also determined potentiometrically. Usually two replicates were done for each experimental test, but if deviations were higher than 5%, typically due to oscillations in feeding of solid material, more tests were done to confirm the results obtained and to assure reliability. Gas composition is presented on dry basis and gas yield was calculated based on inert-free gas production per weight of dry-ash-free feedstock, without water. Gas higher heating value (HHV) is the gross calorific value of the inert-dry-free gas on a volumetric basis. CGE (%) was calculated as the ratio between the energy present in the produced gas and the energy contained in the initial rice husks, even when the torrefied husks were gasified. Mass and energy balances enabled to evaluate the effect of each experimental parameter on the gasification process.

3. RESULTS AND DISCUSSION 3.1 Torrefaction Figures 2 and 3 present the effect of temperature and time on rice husk torrefaction. Both parameters had impact on the torrefied products. For example, at 250ºC the increase of reaction time from 30 to 60 min did not alter volatile matter, but induced a slight increase on ash content, around 11%, probably due to the decrease of moisture, and a small decrease of about 10% on both mass yield and energy yield. Similar trends were obtained for the torrefaction at 300ºC, though the torrefied materials presented higher ash contents and lower volatile matter than the values obtained for torrefaction at 250ºC, because at 300ºC there was a higher mass loss, as more volatiles were released. At 300ºC the volatile matter decreased around 23%, while ash contents increased 20% when torrefaction time rose from 30 to 60 min, while energy and mass yields only decreased 7% and 6%, respectively (Figure 2), because of the high decreases achieved at 30 min, the rise of torrefaction time from 30 to 60 min led to lower variations in mass yields than those observed at 250ºC. Hence, for torrefaction at 300ºC mass yields were around 45% lower and energy yields about 37% lower than the values for torrefaction at 250ºC. The variation of temperature from 200ºC to 300ºC (Figure 3) led to great alterations on torrefied materials, as a decrease on moisture content and on volatile matter around 13% and 40%, respectively, were observed Figure 3 (A). Thus, HHV of torrefied husk was also observed to increase, Figure 3 (B). Consequently, high decreases on mass


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and of energy yields, respectively of 45 and 42% were also detected, Figure 3 (C). it is also observed in Figure 3 (A) that the ash content of torrefied rice husk at 300ºC more than doubled the value obtained at 200ºC, due to the concentration of the inorganic material. The results shown in Figure 3 (A and B) generally agree with those found in literature, as shown in Table 2. To account for changes due to the use of raw materials with different compositions, values represent variations in percentage of HHV and of the contents of moisture, volatile matter and ash. In general there is a good agreement between experimental results and those reported by Chen et al.26 that also torrefied rice husk at different temperatures. Though, Sarkar et al.5 studied switchgrass torrefaction, the variations reported in the mentioned parameters fairly agree with those presented in Figure 3 (A and B). Tumurulu et al.4 also observed an increase in HHV with the rise of torrefaction temperature from 250º to 300ºC for both rice straw and rape stalk. During torrefaction the biomass partly decomposes, most of the water and some light volatiles are removed, contributing for the decrease of the mass and energy yields. The energy yield gives information about the energy content still present in the torrefied material in relation to the original energy of the raw biomass. It is important to notice that, for example at the torrefaction temperature of 250ºC, great improvements on biomass were achieved, as a significant increase on HHV was obtained in detriment of losses of only 9% of the original energy and 10% of the original mass. This energy densification was probably due to the removal of oxygen atoms present in the raw biomass, through small molecules like H2O and CO2 that traditionally decrease the energy content of the biomass. Matali et al.24 reported elemental oxygen and hydrogen decreases around 28% and 34%, respectively, in torrefied biomass, while carbon content increased about 37%. The energy densification continued to increase when the torrefaction temperature of rice husk torrefaction rose from 250 to 300ºC, but then at the expense of too much original mass and energy loss, as at 300ºC the mass and energy yields of torrefied rice husk were only 48.1% and 55.6%, respectively, Figure 3 (C). The effect of torrefaction on rice husk was also stated by Chen et al.25,26; mass loss attributed to the release of water and volatile matter was also reported, and for torrefaction at 290 ºC the yield dropped to 67%. One of the key aspects when studying biomass torrefaction processes is to establish a degree of torrefaction where the benefits of increasing biomass heating value offset the


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energy and mass loss. Based on Figure 2 and 3 assays, 250ºC and 30 minutes were chosen as the optimum conditions for the torrefaction of rice husk. By applying these conditions, less energy was consumed in the torrefaction process, and the torrefied material preserved more energy than when higher temperatures and times were tested. Therefore, the degree of torrefaction chosen ensured that the benefits of increasing biomass heating value counterbalanced the mass loss. Chen et al. 26 also selected for torrefaction conditions 260ºC and 30 min, because there was an improvement in HHV, and on grinding and hydrophobic conditions, with low losses in mass and energy yields.

3.2 Chemical characterization Table 3 shows the summative analysis of the rice husks before and after torrefaction (250 ºC; 30 min). Raw rice husks presented a high ash content of 16.6 % and a total extractives content of 12.6 %, where dichloromethane extracts represented 56 % of the total. The total lignin was 28.9 % and the monosaccharides fraction accounted for 50.6 %. This chemical composition is in agreement with values found by Kumar et al.27; cellulose 31%, hemicelluloses 22%, lignin 22% and ash 14%. In the torrefied rice husk, the extractives content increased to 14.8 %, mainly due to the non-polar extractives (9.1%). The original extractives presented in husk rice are partially volatilized during torrefaction, however the thermal treatment changes and disrupts the structural cell wall, promoting the formation of compounds that can be removed by solvents during the soxhlet extraction and therefore are quantified as extractives in the chemical summative analysis. The total lignin content (ash free) increased with torrefaction, as can be seen in Table 3 (28.9 to 36.4%) and the monosaccharides content decreased to 40.0 % where xylose decreased 34 % and glucose 12 %. As the value of lignin content increased, the corresponding HHV also increased. The C/L ratio obtained with Py-GC/MS must be looked carefully since the values are similar. In fact, pyrolysis associated at MS is more adequate for quantitative analysis and the quantification is less accurate. Chen et al.

26

stated that during rice husk torrefaction occurred the decomposition of

hemicellulose and the partial depolymerization of lignin and cellulose, thus leading to the increase of lignin content. Similar results were also described for biomass torrefaction by Esteves et al.

28

for E.

globulus wood treated during 24 h at 190 ºC the extractives content increased from 9%


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to 19% and lignin from 25% to 34%. The variation of monosaccharides content in thermally treated wood (220 ºC) was described by Andrade et al. 29 where glucose rose from 41% to 44% and xylose decreased from 4.8% to 2.5%. These chemical variations are important for the gasification process since it is known that lignin and extractives increase the calorific values, while polysaccharides diminish the heat content due to their higher level of oxidation.26 The relation between the carbohydrates and lignin (C/L ratio) determined by Py-GC/MS was 3.3 in the raw rice husk. The main carbohydrates identified are presented in Figure 4, where levoglucosan (LG, 8) represented 13.5% in raw rice, followed by hydroxyacetaldehyde (7.1%), while others such as: 2-oxo-propanal (6), 4-hydroxy-5,6-dihydro-(2H)-pyran-2-one (5) or acetic acid (4) represented less than 5% of total area. In the torrefied rice husk sample (250 ºC; 30 min), the C/L increased to 3.9, as a result of the increase in LG and 2hydroxymethyl-5-hydroxy-2,3-dihydro-(4H)-pyran-4-one (3: 2.4% vs. 3.7%), while the other compounds decreased (Figure 4). The characterization of torrefied rice husk presented by Chen et al.26 and Zhang et al.16 also showed an increase in LG, and a decrease in acetic acid, hydroxyacetaldehyde and furfural in accordance with our results. The raw rice husk lignin was characterized by a H:G:S of 3:9:1 and a G/S ratio of 9.4. The main lignin derived compounds were from G-units: 4-vinylguaiacol (3.7%) and vanillin (1.0%), the other identified compounds were less than 1.0% (Figure 5). The Sunits such as 4-vinylsyringol and trans-4-propenylsyringol were the main compounds, but with values of 0.2% and 0.3% respectively; the only compounds derived from Hunits where phenol (0.3%) and 2,3-dihydrobenzofuran (3.7%). The torrefied rice husk presented a H:G:S of 2:6:1, and as can be seen in Figure 5, the torrefaction induced to a decrease in some G-lignin units such as 4-vinylguiacol (6: 3.7 vs. 2.7%) and vanillin (5: 1.0 vs. 0.6%), but 4-methylguaiacol (2: 0.8% vs. 1.4%) and guaiacol (3: 0.9% vs. 1.2%) increased; while the S-units also slightly increased (1.3 vs. 1.8%). Overall, the G/S ratio was to some extent lower reaching 5.5. As also mentioned by Chen et al.26 and Zhang et al.16 the decrease of 4-vinylguaiacol and the increase of 4-methylguaiacol and guaiacol is a common feature of the pyrolysis of torrefied material.

3.3 Gasification and gas composition Raw rice husks were gasified at different temperatures and the profile of the syngas composition is presented in Figure 6 (A). The rise of gasification temperature from 750ºC to 890ºC decreased the content in CO (24%), CO2 (11%), and CnHm (63%) and


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increased H2 (50%) and CH4 (15%). The rise of temperature favoured steam reforming reactions (1) and (2), and also CO2 reforming reactions (3) and (4) leading to the release of CO and H2. As water gas shift reaction (5) was also promoted, some CO was converted into H2 and CO2, which could explain the high increase in H2 content and the reduction in CO. The decrease in CO and the increase of H2 was also reported by Zhao et al.15 when studying gasification of rice husk. CH4 + H2O ⇆ CO + 3 H2

(1)

CnHm + n H2O ⇆ n CO + (n + m/2) H2

(2)

CH4 + CO2⇆ 2 CO + 2 H2

(3)

CnHm + n CO2⇆ 2n CO + m/2 H2

(4)

CO + H2O ⇆ CO2 + H2

(5)

p CnHm ⇆ q CxHy + CzHu + r H2 (x, z < n and y, u < m)

(6)

The decrease in heavier gaseous hydrocarbon content, CnHm is explained by cracking reactions (6) and by reforming reactions (2) and (4), that are favoured by the higher gasification temperatures and consequently explains the high H2 content increase.21,15 Some of the heavier gaseous hydrocarbon might have been converted into CH4 by reaction (6), which would explain its increase. In fact, CH4 was the hydrocarbon more difficult to destroy and to convert into H2 and CO, by reforming reactions, even in the presence of catalysts. The results obtained fairly agree with others already reported about other types of feedstocks. 22,23,30 Compared with the raw rice husks, the gasification of torrefied rice husk at 250 ºC and 300º C presented a syngas poorer in CO2 and richer in H2 and CO, as can be observed in Figure 6 (B). The highest variation were observed between the gasification of raw rice husks and torrefied husks at 250ºC, CO2 (39% vs. 24 %), H2 (32% vs. 36 %) and CO (17% vs. 23 %). The rise of torrefaction temperature from 250º to 300ºC was less significant, as minor changes were obtained in comparison to raw material, with the exception of CO2 content. The gasification gas obtained with torrefied feedstock at 850ºC had higher H2 and CO contents than those obtained for the syngas produced with raw rice husk. These values were even higher than those obtained for the gasification gas at 890ºC with raw rice husk. These trends were also reported by Sarkar et al.5 for the syngas obtained from torrefied switchgrass in relation to the gas produced by raw switchgrass. The torrefaction process increased the formation of pores, cracks and fissures in rice husk, which would favour the gasification process, allowing a syngas with higher contents of H2, CO and


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CH4. 14 Nevertheless, for the torrefaction temperature of 300ºC, the effect of the higher amount of volatiles already released may be important to explain the decreases of H2 and CO, as well as the increase of CH4.

3.4 Tar content The tar contents of the syngas produced by gasification of the raw rice husks at different temperatures are presented in Figure 7 (A). The tar content increased from 750 ºC to 800 ºC; probably because gasification process was favoured by the use of a higher temperature. This agrees with the lower formation of char and the higher gas yield for the temperature of 800ºC, as discussed next. However, this temperature was probably not enough to promote the conversion of some tars initially formed, as observed for higher temperatures. The further increase of temperature promoted cracking and reforming reactions of hydrocarbons initially formed and thus the conversion of tars into smaller molecules. Mineral matter content of rice husks might also have favoured tar destruction, thus leading to tar reduction.31 This is in accordance with the increase in gas yield observed for the all range of gasification temperatures tested. The tar content in raw and torrefied rice husk is presented in Figure 7 (B). The raw rice husk produced more tar during gasification, when compared to the torrefied material at both temperatures (250 and 300 ºC). When rice husks torrefied at 300ºC were gasified, the release of tar further decreased. In this case the tar released was slighter lower than that obtained for raw material gasified at a higher temperature (890ºC). When instead of raw material was gasified torrefied one at 250ºC the reduction in tar content was around 54%. The rise of torrefied temperature to 300ºC led to a milder change in the release of tar, as this reduction led to a further tar reduction of about 20%. As mentioned before, the reduction in tar content in gasification gas was more important when instead of raw material, torrefied rice husk was gasified. Tar decrease during gasification of torrefied materials was reported by several authors. 30-35

As an example, Sweeney35 mentioned that while the gasification of raw biomass

led to tar contents of 26.7 g/Nm3, tars contents of 20.5 g/Nm3 and 9.2 g/Nm3 were obtained, respectively for medium torrefied material and for dark torrefied material, which corresponds to tar reductions of around 23% and 66%, respectively. Besides the good results in relation to tar reduction, the increase of ashes in torrefied material may increase the tendency to bed agglomeration during gasification. Raw rice husks usually present some tendency to bed agglomeration, especially due to the contents of silica and alkali metals. After torrefaction the contents of these elements


Energy & Fuels

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increased even further, which may increase agglomeration problems. This subject should be further studied.

3.5 Gas yield, HHV and CGE The variation of gas yield and HHV content at different gasification temperatures is presented in Figure 8 (A). Gas yield is presented in NL/g daf of feedstock, NL means liter at normal conditions of temperature and pressure. Temperature increase favoured gas yield, because it promoted gasification reactions and tar conversion into gaseous compounds, as mentioned before, which might explain CH4 content in syngas. The destruction of hydrocarbons promoted by higher temperatures also contributed to H2 and CO formation, thus leading to more syngas, as already discussed in Figure 6, CO could further react by water gas shift reaction. Though no great differences were obtained for syngas HHV, the general tendency is the increase with the rise of gasification temperature, except for the highest temperature (890ºC), where HHV decreased due to the reduction of CH4 and CnHm, Figure 6 (A). Till 850ºC the decrease in CnHm, was compensated by the increase in CH4, which explains the increasing tendency in HHV. The rise of gasification temperature led to an increase in both gas yield and HHV, with the exception of the highest temperature tested 890ºC, which agrees with CGE evolution, as presented in Figure 8 (A). In Figure 8 (B) are presented the results obtained for gasification at 850ºC of the raw and torrefied rice husks (at 250 and 300ºC). It was observed an important decrease in gas yield produced with torrefied rice husk comparatively to the raw material, which is in agreement with the release of volatile matter during torrefaction. A further reduction in gas yield was obtained when using a 300ºC torrefied feedstock, because this material contained the lowest volatile matter, Figure 2 (A). The gas produced with torrefied rice husk at 250ºC had a HHV higher (around 20%) than the value obtained with raw feedstock. This agrees with the syngas composition, Figure 6 (B), as the presence of higher contents of H2, CO and CH4 when torrefied rice husk was gasified led to an increase in HHV, Figure 8 (B). Nevertheless, no important changes were observed in gas HHV when torrefied husks at 300ºC were gasified instead of torrefied feedstock at 250ºC. The results reported fairly agree with those in literature. Gasification of torrefied switchgrass led to a similar increase of HHV syngas.5 The gasification of 230ºC torrefied switchgrass led to a gas with higher HHV than that of the gas produced by


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270ºC torrefied switchgrass.

5

It is possible that the torrefaction process might have

caused different alterations in the structure of rice husk and switchgrass. In addition, the range of torrefaction temperatures used by Sarkar et al. [2] was different from that reported in the present study. The rise of gasification temperature of raw material led to an increase in CGE of around 66% till 850ºC, however, for the highest temperature tested, 890ºC, there was a reduction in CGE, as gas yield remained constant and HHV decreased. The gasification of the torrefied rice husk at 250ºC led to a decrease in the CGE of around 15%, because, torrefied rice husk led to a lower production of gasification gas, though the gas had a HHV 20% higher than that obtained with raw material, as this increase was not enough to compensate the reduction in gas yield. As expected, the gasification of the torrefied feedstock at 300ºC further decreased CGE, because of the lower production of gasification gas. This showed that such high temperature was not advisable for the torrefaction of rice husk. Thus, the calculation of CGE might be a good approach to help the selection of torrefaction conditions. Sarkar et al.

5

also observed a decrease in CGE when torrefied switchgrass was

gasified instead of raw material. Prins et al.36 also stated that the overall efficiency of torrefied wood gasification was lower than that of raw wood, mainly for torrefaction at 300º instead of 250ºC, because of the energy contained in the volatiles that was not used in the gasification process. CGE was lower for torrefied material, due to the lower amount of gasification gas produced, as some gas was already released during the torrefaction process, but the gasification gas had a higher energetic content. Thus, torrefaction is an interesting process, especially because the transportation costs are decreased, as only dry biomass and with a high energetic content is transported. Another important issue is the selection of gasification temperature, high values of this parameter should be avoided to ensure high CGE values. On the other hand, the integration of torrefaction with gasification might improve CGE12, if volatiles released during torrefaction were introduced inside the gasifier.

3.6 Pollutant precursor compounds The presence of pollutant precursor compounds, such as NH3 and H2S in syngas must be quantified since they have negative environmental impact, being converted into nitrogen and sulfur oxides during gas utilization for energy proposes. The presence of NH3 and H2S may also poison the catalysts used in gas treatment and upgrading.


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Figure 9 (A) illustrates the content of H2S and NH3 on syngas produced with raw husks at different gasification temperatures. Only the contents of H2S and NH3 are presented, though other nitrogen and sulfur compounds are formed during gasification, namely: COS, sulfur organic compounds, HCN and nitrogen organic compounds, because H2S and NH3 account for most of the sulfur and nitrogen released to syngas (90-97%). The increase of gasification temperature favored the release of H2S and NH3 in the syngas obtained with raw rice husk, because the rise of temperature promoted gasification process and the release of N-fuel and S-fuel. These results are in agreement with observation made by Paterson et al. et al.

38

37

and Pinto

. However, due to the low contents of N and S in the rice husk, the gasification

released lower contents of NH3 and H2S, comparatively to those usually obtained for coal gasification and for coal and some biomass wastes co-gasification. 38 The gasification of torrefied rice husks increased the release of H2S and NH3, Figure 9 (B), in particular, the 250ºC torrefied rice husks led to increases of H2S and NH3 of about 100% and 75%, respectively, in relation to raw biomass. There was an increase in N and S contents in torrefied husks at 250ºC (Table 1), due to the release of volatile matter, including H2O and CH4. On the other hand, the torrefaction process improved the overall gasification, leading to higher gas yields and lower char amounts, which favoured the release of N-fuel and S-fuel and thus the formation of H2S and NH3 during gasification process. Chen et al.11 also observed S and N contents increases in the torrefied sawdust in relation to raw feedstock. Hilten et al.39 also stated important increases in N contents in torrefied pine chips. When torrefied husks at 300ºC were gasified instead of torrefied material at 250ºC there was a reduction in the release of H2S and NH3 contents, though these concentrations were higher than those obtained with raw husks gasification. It was observed some decrease in S content with the rise of torrefaction temperature to 300ºC (0.04 vs. 0.07 for 250ºC), though there was an increase in N content (1.0 vs. 0.08 for 250ºC). It is possible that during torrefaction at 300ºC some sulfur might have been released. Chen et al.26 also observed an increase in N content of torrefied rice husk with the rise of temperature, while no changes in S contents were observed. On the other hand, Daniyanto et al.40 reported that the increase of torrefaction temperature of sugarcane bagasse from 250º to 275ºC did not change significantly the S and N contents in torrefied material. Rice husk ash content increased after torrefaction and the contents of undesirable mineral components with tendency to bed agglomeration also increased. All these aspects need to be considered before the selection of the best option for rice husk. It would be interesting to check and discuss the ash behavior of raw and of torrefied


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Energy & Fuels

material and possible ashes trend towards agglomeration. However, this subject is out of the scope of the present work and it may be the focus of a future paper. Only the technic and economic analysis of the two options, gasification of raw material or gasification of torrefied one, would allow the selection of the best option. Hence, this subject study needs to be carried on.

4. CONCLUSIONS Rice husks residues can be used as feedstock in gasification processes for obtaining a good quality syngas. The low energy density of rice husks residues is improved by torrefaction and the results obtained showed that temperature had a greater influence than time on torrefaction process. Torrefaction reduced moisture and volatile contents and consequently increased ash content and HHV. Mass and energy yields also decreased. The selected conditions for rice husk torrefaction were 250ºC and 30 min, because the decreases in mass and energy yields were counterbalance by the higher HHV of the torrefied material. Torrefaction increased extractives content, mainly the non-polar fractions, and the total lignin content, and decreased polysaccharides content. The lignin monomeric composition did not change greatly with the torrefaction: the H:G:S varied from 3:9:1 to 2:6:1 in raw and rice husk torrefied. Torrefied rice husks improved gasification and produced a syngas with lower CO2 and richer in H2, CH4 and CO. Syngas produced by rice husks torrefied at 250ºC contained 36 % of H2, 24% of CO and 16% of total hydrocarbons, being 13% of CH4.Syngas had a higher HHV and lower tar content in comparison to the raw material. However, higher H2S and NH3 contents were detected in syngas produced by torrefied rice husks in relation to raw feedstock. The torrefaction at 300ºC allowed to further decrease the release of tar, the contents of H2S and NH3 were also lower, but CGE was reduced.


Energy & Fuels

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ACKNOWLEDGEMENT This research was sponsored by National Funds through FCT – Foundation for Science and Technology by supporting the project PTDC/AAG - REC/3477/2012 RICEVALOR - Energetic valorization of wastes obtained during rice production in Portugal, FCOMP-01-0124-FEDER-027827, a project sponsored by FCT/MTCES, QREN, COMPETE and FEDER. Centro de Estudos Florestais is a research unit funded by FCT (AGR/UID00239/2013). Ana Lourenço was funded by FCT through a post-doctoral grant (SFRH/BPD/95385/2013).

REFERENCES (1) Maiti, S.; Dey, S.; Purakayastha, S.; Ghosh, B. Bioresource Technol, 2006, 97, 2065-2070. (2) Natarajan, E.; Nordin, A.; Rao, A. N. Biomass Bioenerg, 1998, (14), 5/6, 533-546. (3) Cao, L.; Yuan, X.; Li, H.; Li, C.; Xiao, z.; Jiang, L.; Huang, B.; Xiao, Z.; Jiang, L.; Huang, B.; Xiao, Z.; Chen, X.; Wang, H.; Zeng, G. Bioresource Technol, 2015, 185, 254-262. (4) Tumuluru, J. S.; Wright, C. T.; Sokhansanj, S. Review on Biomass Torrefaction Process and Product Properties and Design of Moving Bed Torrefaction System Model Development. Annual ASABE International Meeting, Louisville, Kentucky; 2011. (5) Sarkar, M.; Kumar, A.; Tumuluru, J. S.; Patil, K.N.; Bellmer, D. D. Appl Energy 2014,127,194-201. (6) Campoy, M.; Gómez-Barea, A.; Ollero, P.; Nilsson, S. Fuel Process Technol 2014,121, 63–69. (7)

INE, Statistical Yearbook of Portugal 2012, 2013, ISBN 978-989-25-0219-9.

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Tran, K. -Q.; Luo, X.; Seisenbaeva, G.; Jirjis, R. Appl Energ 2013,112, 539-546.

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Medic, D.; Darr, M.; Shah, A.; Rahn, S. Energy&Fuels 2012, 26, 2386–2393.

(10) Kuo, P. -C.; Wu, W. Energ Conv Manag 2016, 111,15–26. (11) Chen, Q.; Zhou, J. S.; Liu, B.; Mei, Q. F.; Luo, Z. Y. Chinese Sci Bull 2011, 56,1449–1456. (12) Clausen, L. R. Energy 2014, 77, 597–607. (13) Mansaray, K. G.; Ghaly, A. E.; Al-Taweel, A. M.; Hamdullahpur, F.; Ugursal, V. I. Biomass Bioenerg, 1999, 17, 315-332.


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(14) Lin, S. K.; Wang, P. H.; Lin, C. –J.; Juch, C, -J. Fuel Process Technol 1998, 55, 185-192. (15) Zhao, Y.; Sun, S.; Tian, H.; Qian, J.; Su, F.; Ling, F. Biores Technol 2009, 100: 6040-6044. (16) Zhang, S.; Dong, Q.; Zhang, L.; Xiong, Y. Biores Technol 2016, 199, 352-361 (17) Gominho, J.; Lourenço, A.; Miranda, I.; Pereira, H. Ind. Crops Prod, 2012, 39,1216. (18) Neiva, D. M.; Gominho, J.; Fernandes, L.; Lourenço, A.; Chemetova, C.; Simões, R. M. S.; Pereira, H. J Wood Chem Tecnol, 2016, 36, 383-392. (19) Ralph, J.; Hatfield, R.D. J Agric Food Chem 1991, 39, 1426–1437. (20) Faix, O.; Fortman, I.; Bremer, J.; Meier, D. Holz als Roh-und Werkstoff 49, 1991, 299–304. (21) Abelha, P.; Franco, C.; Pinto, F.; Lopes, H.; Gulyurtlu, I.; Gominho, J.; Lourenço, A.; Pereira, H. Energy&Fuel, 2013, 27, (11), 6725-6737. (22) Pinto, F.; André, R. N.; Lopes, H.; Neves, D.; Varela, F.; Santos, J.; Miranda, M. Chem Eng Trans, 2015, 43, 2227–2232. (23) Pinto, F.; André, R. N.; Lopes, H.; Neves, D.; Varela, F.; Santos, J.; Miranda, M. Chem Eng Trans, 2015, 43, 2449-2454. (24) Matali, S.; Rahman, N.A.; Idris, S.S.; Yaacob, N.; Alias A.B.Procedia Engineering 2016, 148, 671-678. (25) Chen, D.; Zhou, J.; Zhang, Q. Energy& Fuels, 2014, 28, 5857-5863. (26) Chen, D.; Zhou, J.; Zhang, Q.; Zhu, X.; Lu, Q. Bioresources, 2014, 9, 5893-5905. (27) Kumar, P.S.; Ramakrishnan, K.; Kirupha, S.D.; Sivanesan, S. Braz J Chem Eng 2010. 27, (2), 347-355. (28) Esteves, B.; Graça, J.; Pereira, H. Holzforshung, 2008. 62, (1), 344-351 (29) Andrade, P.I.; Araújo, S.O.; Neiva, D.M.; Vital, B.R.; Carneiro, A. C. O.; Gominho, J.; Pereira, H. Holzforshung, 2016, 70, (5), 467-474 (30) Pinto, F.; André, R.N.; Carolino, C.; Miranda, M.; Abelha, P.; Direito, D.; Perdikaris, N.; Boukis, I.; Fuel, 2014, 117, 1034–1044 (31) Raut, M. K. Studies into the effect of torrefaction on gasification of biomass. Dalhousie University; 2014. (32) Wannapeera, J.; Fungtammasan, B.; Worasuwannarak, N. J Anal Appl Pyrol, 2011,92, 99-105. (33) Chew, J.J; Doshi, V. Renew Sust Energ Rev 2011,15, 4212-4222.


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(34) Kwapinska, M.; Xue, G.; Horvat, A.; Rabou, L.P.L.M.; Dooley, S.; Kwapinski, W.; Leahy, J.J. Energy&Fuels 2015, 29, 7290-7300. (35) Sweeney, D. J. Performance of a pilot scale, steam blown, pressurized fluidized bed biomass gasifier. Department of Mechanical Engineering, Vol. Doctor, University of Utah, 2012. (36) Prins, M.J.; Ptasinski, K.J.; Janssen, F.J.J.G. Energy 2006,31,3458–3470. (37) Paterson, N.; Zhuo, Y.; Reed, G.P.; Dugwell, D. R.; Kandiyoti, R. Water Environ. J. 2007,18,90-95. (38) Pinto, F.; André, R. N.; Franco, C.; Lopes, H.; Carolino, C.; Miranda, M.; Galhetas, M.; Gulyurtlu, I. Fuel 2012, 97, 770-782. (39) Hilten, R.N.; Speir, R. A; Kastner, J. R; Mani, S.; Das K. C. Energy&Fuels, 2013, 27, 830-843. (40) Daniyanto, S.; Deendarlianto, B. Energ Procedia 2015, 68,157-166.


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Table 1. Elemental, proximate analysis and mineral composition of rice husk Elemental analysis (% daf a) C H N S Proximate analysis (% as received) Ash Moisture Volatile Matter HHV (MJ/kg daf a) a = (daf - dry and ash free basis)

Raw rice husk 49.2 2.2 0.4 0.06

Rice husk torrefied at 250ºC 53.4 6.4 0.8 0.07

16.6 9.5 67.6 19.8

17.7 2.9 78.0 22.1

Table 2 Comparison of torrefaction experimental results with those found in literature. Values represent variations in percentage in relation to raw materials or between different torrefaction temperatures. Ash Moisture Volatile Matter (wet basis) (dry basis) (dry basis)

Variations (%) Raw to 275ºC Figure 3 results Raw to 300ºC (rice husk) 200 to 300ºC

HHV (MJ/kg)

-68

-7

41

10

-68

-31

81

11

-27

-39

99

6

Chen et al.26 (rice husk)

Raw to 290ºC

-38

46

18

200 to 290ºC

-38

48

8

Sarkar et al.5 (switchgrass)

Raw to 270ºC

-16

42

32

-79


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Table 3. Chemical composition (% of oven dry mass) of the raw and torrefied rice husk (torrefaction conditions: 250°C and 30 min).

Raw rice husk Extractives Dichloromethane Ethanol Water Lignin Klason Soluble Monosaccharid es Ramnose Arabinose Galactose Glucose Xylose Mannose Galacturonic acid Glucuronic acid Acetic acid

Torrefied rice husk at 250ºC

12.6

14.8

7.1

9.1

2.4 3.1

2.4 3.3

28.9 28.2 0.7

36.4 35.7 0.7

50.6

40.0

0.0 1.4 0.7 29.9 16.8

0.0 0.9 0.5 26.6 11.2

n.d.

n.d.

0.3 0.2 1.3

0.1 0.0 0.8

n.d. - not determined


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Energy & Fuels

Figure 1 Scheme of the fluidized bed gasification installation.


Energy & Fuels

(A) 250ºC Volatile 250ºC Ash

300ºC Volatile 300ºC Ash

60

40 70

20

50

30

Ash content (%)

Volatile Matter (%)

90

0 25

35 45 55 Torrefaction Time (minutes)

(B) 250ºC HHV

300ºC HHV

HHV (MJ / kg)

18

17

16

15 25

35 45 55 Torrefaction Time (minutes)

(C) 300ºC Mass Yield 300ºC Energy Yield

80

90

60

70

40

Energy Yield (%)

250ºC Mass Yield 250ºC Energy Yield

Mass Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 25

35

45

55

Torrefaction Time (minutes)

Figure 2. Effect of torrefaction temperatures (250ºC and 300ºC) along torrefaction time on volatile matter and ash content (A), HHV (B) and mass and energy yields (C). ACS Paragon Plus Environment

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(A) Moisture

Volatile Matter

Ash

100 80

(%)

60 40 20 0 175

200 225 250 275 Torrefaction Temperatue (ºC)

300

325

(B)

HHV (MJ / kg)

18

17

16

15 175

200

225

250

275

300

325

Torrefaction Temperature (ºC)

(C) Energy Yield

80

95

60

75

40 175

Energy Yield (%)

Mass Yield

Mass Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

55 200 225 250 275 300 Torrefaction Temperature (ºC)

325

Figure 3. Effect of temperature during 30 min torrefaction of rice husks on volatile matter, moisture, ash (A), HHV (B) and mass and energy yields (C). ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Main compounds derived from carbohydrates obtained by pyrolysis of raw and torrefied rice husk. 1: furfural; 2: 3-hydroxypropanal; 3: 2-hydroxymethyl-5-hydroxy-2,3dihydro-(4H)-pyran-4-one; 4: acetic acid; 5: 4-Hydroxy-5,6-dihydro-(2H)-pyran-2-one; 6: 2-oxopropanal; 7: hydroxyacetaldehyde; 8: levoglucosan.

Figure 5. Main compounds derived from lignin obtained by pyrolysis of raw and torrefied rice husk. 1: trans-coniferyl alcohol; 2: 4-methylguaiacol; 3: guaiacol; 4: trans-coniferaldehyde; 5: vanillin; 6: 4-vinylguaiacol.


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(A)

CO

CO2

H2

CH4

CnHm

60 Raw Rice Husk

(%)

40

20

0 725

750

775

800

825

850

875

900

Gasification Temperature (ºC) (B)

Raw

Torrefied 250ºC

Torrefied 300ºC

Gasification at 850ºC

Concentration (% v/v)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40

20

0

CO

CO2

H2

CH4

CnHm

Figure 6. Profiles of syngas composition of raw rice husks obtained by gasification with oxygen at different temperatures (A) and comparison between raw and torrefied rice husks at 250 ºC and 300 ºC gasified at 850 ºC (B). Gas composition is shown on dry basis.


Energy & Fuels

(A) 15 Raw Rice Husk

Tar (g/m3)

10

5

0 750ºC

800ºC

850ºC

890ºC

(B)

15 Gasification at 850ºC

10 Tar (g/m3)

5

ie d To rr ef

To rr ef

ie d

25 0º C

30 0º C

0 Ra w

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 7. Effect of gasification temperature of raw rice husks (A) on tar content in syngas obtained by gasification with oxygen. Comparison between raw and torrefied rice husks (B). Gas composition is shown on dry basis.


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(A) HHV/10

CGE (%) 100.0 Raw Rice Husk 80.0

1.5

60.0 1 40.0

CGE (%)

Gas Yield (Nl/g daf), HHV/10 (kJ/NL)

Gas Yield

0.5 20.0

0

0.0 750ºC

800ºC

850ºC

890ºC

(B) Gas Yield

HHV/10

CGE (%) 100.0

Gasification at 850ºC 80.0

1.5

60.0 1 40.0

CGE (%)

Gas Yield (Nl/g daf), HHV/10 (kJ/NL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.5 20.0

0

0.0 Raw

Torrefied 250ºC

Torrefied 300ºC

Figure 8. Effect of gasification temperature of raw rice husks on CGE, gas yield and on HHV (dry basis) (A). Comparison between raw and torrefied rice husks (B).


Energy & Fuels

(A) H2S

NH3

200

Raw Rice Husk

150 1000 100

NH 3 (ppmv)

H2S

(ppmv)

1500

500

50

0

0 750ºC

800ºC

850ºC

890ºC

(B) H2S 200

NH3 Gasification at 850ºC

150 1000 100

500

50

NH 3 (ppmv)

(ppmv)

1500

H2S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

0 Raw

Torref. 250ºC

Torref. 300ºC

Figure 9. Effect of gasification temperature of raw rice husks on the release of H2S and NH3 during gasification with oxygen (A). Comparison between raw and torrefied rice husks (B). Gas composition is shown on dry basis.


Effect of Rice Husk Torrefaction on Syngas ... - ACS Publications

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