Effect of gasification agent on co-gasification of rice production wastes mixtures

Fuel 180 (2016) 407–416

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Full Length Article

Effect of gasification agent on co-gasification of rice production wastes mixtures Filomena Pinto ⇑, Rui André, Miguel Miranda, Diogo Neves, Francisco Varela, João Santos LNEG, Estrada do Paço do Lumiar, 22, 1649-038 Lisboa, Portugal

h i g h l i g h t s
Co-gasification of biomass rice wastes blended with polyethylene (PE).
Use of mixtures of air, oxygen steam and CO2 with different compositions of two or more components as gasification agent.
Study of CO2-blown gasification.
Effect of gasification agent on the release of H2S and NH3.

a r t i c l e

i n f o

Article history: Received 5 November 2015 Received in revised form 10 April 2016 Accepted 11 April 2016 Available online 19 April 2016 Keywords: Co-gasification Rice biomass wastes Oxygen enriched air CO2 gasification

a b s t r a c t Rice production generates different types of wastes, rice husk and straw and plastics, mainly polyethylene (PE) used in bags for rice packaging and for seeds and fertilizers transport. Due to their contamination they are not suitable for physical recycling and end-up in landfills. Biomass rice wastes may have some utilisations, but each of them has drawbacks. This paper studies the possibility of using fluidised bed co-gasification of all these wastes for their energetic valorisation. Gasification gas composition and heating value is affected by the gasification and fluidisation agent. Mixtures of air, oxygen, steam and CO2 with different compositions of two or more components were tested. Besides the high cost of producing oxygen, the results obtained showed that the best technical option was the use of steam and oxygen, because the gas was not diluted in nitrogen and thus gas HHV (higher heating value) on dry basis increased around 42%. However, the use of enriched air with up to 40% (v/v) of oxygen may be an alternative, due to the lower cost of producing this gas and also because better results were obtained than those for air-blown gasification. The use of CO2 as gasification and fluidisation agent may be a good route, as the results obtained showed that CO2 reforming reactions were promoted. The increase of CO2 content in gasification agent led to a decrease of 45% in tar content, which was followed by a great increase in gas yield, around 70%. The main drawback of using steam and CO2 is the need to supply the energy for the endothermic reactions, hence a good option could be the use of mixtures of CO2, O2 and steam as gasification agent. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Worldwide rice production is about 700.7 million tonnes per year, which produce around 100 million tonnes of rice husk annually [1]. Rice straw and rice husk are produced in Portugal in significant amounts, but currently, there is no energy recovery of these wastes. In Portugal the production of rice straw was around 37.7 kt year1, while rice husk was about 23.7 kt year1 in 2013. The estimated amount of plastics produced due to rice production was around 300 t year1 [2], the main plastic was polyethylene ⇑ Corresponding author. E-mail address: [email protected] (F. Pinto). http://dx.doi.org/10.1016/j.fuel.2016.04.048 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

(PE), used to transport seeds, fertilisers and pesticides which are contaminated with chemical products that prevents their physical recycling. Biomass rice wastes are sometimes incinerated at openair or deposit in landfields, which are not environmentally desirable. Other options are the incorporation in animal food, which is not a good option, due to the high contents of silica, or the use as animal bedding, but again the final disposal is incineration or landfilling. The high energetic content of plastics and biomass rice wastes makes attractive their valorisation by thermochemical processes, namely co-gasification for the production of gaseous biofuel. There is already some knowledge about gasification of rice husk [3–6], nevertheless the presence of plastics brings some challenges.

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Calvo et al. [3] gasified rice straw in an atmospheric fluidized bed at a temperature up to 850 °C and found that it was possible to produce high quality syngas, some bed agglomeration could be avoided by using a mixture of alumina silicate sand and MgO as gasification bed. Murakami et al. [4] used rice straw in a twostage process, hydrothermal treatment was done in the first one and in the second stage steam gasification in a fixed-bed reactor using nickel as catalyst was performed. Yoon et al. [5] gasified rice husk in a bench-scale downdraft fixed-bed gasifier and observed that the heating value of the gas and the cold gas efficiency were higher when rice husk was used as pellets. Fluidised bed gasification is usually a good option to deal with heterogeneous feedstocks like those obtained during rice production. Gasification gas heating value and composition depends on gasification conditions and mainly on the gasification and fluidisation agent, air, oxygen steam and CO2 may be used either alone or in mixtures of two or more components. As one of the most promising technologies available for energy conversion, air–steam gasification leads to a gas with low heating value, which decreases its potential value, due to the dilution effect of nitrogen [7]. Air could be replaced by pure oxygen, but the cost of oxygen production for the gasification installation would reduce its economic viability. A compromise option would be the use of enriched air, with oxygen amounts higher than that found in air, as nitrogen dilution effect would be reduced. One of the preferred compositions would be enriched air with 40% (v/v) of oxygen, as this concentration can be produced at a lower cost using membrane technology [7]. Campoy et al. [7] reported that the use of enriched air with 40% of oxygen and a steam/biomass ratio around 0.3, increased CO and H2 yields, heating value and carbon conversion. A maximum efficiency of 70% was obtained [7]. Cheng et al. [8] also observed that an increase of the oxygen concentration from 21% to 31.4%, increased gas yield, carbon conversion efficiency and gasification efficiency. Under the best operating conditions, the maximum LHV (lower heating value) of the produced gas reached 6200 kJ/ Nm3 [8]. Huynh et al. [9,10] also reported that increasing oxygen amounts from 21% to 40% (v/v) during gasification of three kinds of wood caused a great increase in H2 production and a smaller one in CO. Thus, an increase in gas LHV of more than 43% was observed [9]. However, significantly higher amounts of steam were required to control the reactivity of the system at high oxygen levels [10]. Thanapal et al. [11] also observed an increase in CO, H2 and CH4 concentrations, while CO2 decreased when enriched air with 28% (v/v) of oxygen was used for the gasification of dairy biomass wastes. Wang et al. [12] reported that biomass gasification using enriched air up to 99.5% (v/v) of oxygen, but without steam, led to increases in H2 + CO from about 30% (v/v) to more than 70% (v/v), but H2/CO ratio decreased for the highest oxygen concentration. Enriched air with oxygen generated a syngas with higher LHV and higher carbon conversion efficiency, but with a lower yield, corresponding to lower amount of nitrogen on the obtained gas [12]. Due to the need of obtaining a viable low CO2 emission energy source, several studies have proposed the use of CO2 as a gasification agent, as it could act as gasification promoter, through gas– solid Boudouard reaction, acting in the solid carbon from the feedstock and through CO2 reforming reactions of the hydrocarbons initially formed. Butterman et al. [13] studied biomass gasification with different amounts of CO2 mixed with steam. The use of CO2 significantly enhanced CO evolution and reduced H2 and CH4 concentrations. The same authors also reported the increase in carbon conversion, probably due to the ability for CO2 to enhance the pore structure of the residual carbon skeleton after devolatilisation, providing access for CO2 to efficiently gasify the solid [14].

Cheng et al. [15] observed that by increasing CO2/biomass ratio, the mole fraction of CO in the gasification gases increased, while H2 and CO2 decreased. When CO2 mass percentage in the gasifying agent reached 60%, the fractions of CO and CH4 attained the maximum, as well as gas LHV and cold gas efficiency. Hanaoka et al. [16] studied the gasification of aquatic biomass using O2/CO2 mixtures with different proportions. The increase in CO2 content led to the increase in the conversion to gas and enhanced CO release, while H2 content decreased. Several researchers [11,12,16–18] stated that CO2 has a positive effect on char gasification, due to the promotion of the gas–solid Boudouard reaction. These conclusions are corroborated by Huo et al. [17] who compared the reactivities of different origin chars during gasification with CO2. Biomass chars with higher BET surfaces than coal or coke char, presented the highest reactivity. Kirtania et al. [19] compared gasification of chars, obtained from wood and algae, with steam or CO2. A significant difference in gasification reactivity, which was attributed to char structure, was reported. These results agree with those presented by Nilsson et al. [20], who reported that the char gasification rates measured in a mixture containing both CO2 and H2O, were well approximated to the sum of the individual rates measured with only CO2 or H2O. Guizani et al. [21] studied the diffusion–reaction competition for steam and CO2 in the gasification of biomass chars and reported that H2O had an almost twice higher reactivity and diffusivity than CO2. The combined reactivity obtained with both gasification agents was a linear combination of the two individual reactivities. Nevertheless, Roberts et al. [22] disagreed partially with the former results, indicating that the rate of reaction in a mixture with CO2 and H2O was not the sum of the two pure-gas reaction rates. A complex combination of the two reaction rates that appeared to be dependent on the blocking of reaction sites by the relatively slow Boudouard reaction was stated. Farzaneh et al. [23] also studied the gasification of chars using either CO2 or steam. The experiments led to almost similar results for activation energies of CO2 gasification and steam gasification. The information about co-gasification of rice straw and husks and plastic wastes blends is still scarce and some problems still need to be solved, due to the heterogeneity and particularities of such wastes. The work presented main objective was to study the viability of using co-gasification of biomass rice wastes and plastic to produce a gas suitable to be used as a biofuel. As the type and composition of gasification agent is an important parameter in the composition of the gas produced by gasification, the effect of using air, O2, CO2 and steam in different compositions was studied. As mentioned before, several authors have studied the effect of gasification agent on gasification performance, however, similar results and trends were not always obtained, which justifies the work presented. The results reported refer to the first stage of the work that has been developed about co-gasification of rice husks and plastic wastes. Other important aspects like controlling gasification bed agglomeration will be addressed in a future work.

2. Experimental part 2.1. Co-gasification bench-scale installation Co-gasification bench-scale installation is shown in Fig. 1. The reactor was a bubbling fluidised bed gasifier made of a refractory steel pipe. The reactor was circular in cross-section with an inside diameter of 80 mm and with a height of 1500 mm. The feeding system was water cooled to avoid some clogging, that might be due to pyrolysis of the feedstock, prior to the entry into the gasifier, especially when PE was used in the feedstock. A nitrogen flow was also

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1 – Hopper 2 – Motor 3 – Screw feeder 4 – Drop pipe 5 – Gasifier 6 – Thermocouple 7 – Flow meters 8 – Pressure probe 9 – Cyclone 10 – Condensation system 11 – Liquid collector 12 – By-pass 13 – Glass wool filters 14 – Filter

15 – Valve 16 – Gas sampling 17 – Pump 18 – CO/CO2 online analysers 19 – Gas meter 20 – Refrigeration system 21 – Steam Generator 22 – Data acquisition system 23 – Gasifier controller 24 – Ice bath 25 – Water reservoir 26 – Pressure indicator

Fig. 1. Schematic diagram of bench-scale fluidised bed gasification installation.

used to help waste feeding, to avoid the formation of a compact plug, due to some pyrolysis of the feeding, and to prevent gas back flow. The amount of N2 quantified in gasification gas obtained when only O2 or CO2 were used instead of air was only around 5% (maximum), while when air was fed into the gasifier it was in the range of 40–50% depending on the gasification conditions used. The gasification gas formed passed through a cyclone to remove particulates. Tar and condensable liquids were removed in a quenching system. Next, the gas was filtered, before it was injected into CO and CO2 on-line analysers. Each experiment lasted between 90 and 120 min, depending on the time necessary to collect all the samples at stabilised conditions. Replicates were done for each experimental test. At least two sets of runs were repeated at the same experimental conditions, when deviations higher than 5% were observed, mostly due to oscillations in feeding of solid material, more tests were performed to assure the reproducibility of experimental results to be below 5%.

2.2. Experimental conditions and syngas analysis Previous results have shown that temperature is one of the most important parameters on gasification performance and gasification gas (also referred as syngas) composition [24]. According to previous results co-gasification temperature should be around 850 °C, as the content of heavier gaseous hydrocarbons and tar was minimised. Gasifier temperature was kept at this value for most gasification tests by external electrical heating supplied by the oven where the gasifier was placed. Though it was possible to work the gasifier at auto-thermal conditions, it was decided to use external heating to be sure that temperature would be kept at the selected value when studying the effect of changing other operational conditions. Feedstocks flow rate was adjusted to 5 g daf/min (daf means dry and ash free). Feedstock flow rate was adjusted to daf basis to account for the effect of moisture and ash contents in different feedstocks and allow results comparison between those obtained in this work and others obtained with different feedstocks. No significant changes in ash and moisture contents were observed in the feedstocks used in this work, as ash and moisture contents were kept constant in the feedstocks co-gasified.

Mixtures of steam, air, oxygen and CO2 were tested as gasification and fluidisation agent, which was introduced through a gas distributor at the base of the reactor. The content of oxidation gas also varied, being the equivalent ratios tested between 0 and 0.3. The equivalent ratio (ER) is defined as the ratio between the amount of oxygen added and the stoichiometric oxygen needed for complete combustion of the feedstock. The effect of varying steam flow rate was previously analysed, by testing values between 0 and 1.4 for steam/biomass ratio. Previous results also led to the selection of steam/feedstock ratio of around 1. However, some results obtained without steam are also reported. The ratio between the velocity and the minimum fluidisation velocity (U/Umf) was calculated for each gasification test and the values obtained usually changed from 2.5 to 4, depending on gasification conditions used. The lowest values were achieved when steam was not used. The objective was to maintain ER values, thus oxidising flow rates were adjusted accordingly. Though some changes in U/Umf were needed, all the values used ensure that the fluidisation conditions were always achieved. Blends of rice husk and straw and PE wastes were used in cogasification studies. In Table 1 are presented the ultimate and proximate analysis of PE and rice biomass wastes. In previous studies the amount of PE in rice biomass blends was studied [24]. The results obtained showed that high contents of PE led to the formation of tar and black particulates in higher amounts, which compli-

Table 1 Ultimate and proximate analysis of rice husk, rice straw and PE wastes. Rice husk

Rice straw

PE

52.3 7.3 1.3 0.1

85.7 14.3 – – –

Proximate analysis (% w/w) (as received) Volatile matter 67.6 Ash 16.6 Moisture 9.5

59.3 15.1 10.4

99.8 0.1 –

HHV (MJ/kg daf)

19.7

46.1

Ultimate analysis (% w/w) (daf) Carbon 49.2 Hydrogen 2.2 Nitrogen 0.44 Sulphur 0.06 Chlorine 0.08

daf – dry and ash free.

19.8

F. Pinto et al. / Fuel 180 (2016) 407–416

DH ¼ 242 kJ=mol

ð2Þ

CO þ 1=2O2 ¡ CO2

DH ¼ 283 kJ=mol

ð3Þ

On the other hand, the use of only steam seems to have favoured steam reforming reactions (4)–(6) and thus the formation of H2, which agrees with the decrease in the contents of both CH4 and CnHm. In fact, the use of only steam allowed decreasing the total release of hydrocarbons of around 10%. These results showed that the presence of steam and oxidation agent was beneficial for the gasification process and hence in all the other experiments presented in Fig. 2, steam was also used, being the feedstock/steam ration around 1 as mentioned before. The results obtained with air and steam mixtures showed that steam promoted the enrichment of the gas in hydrogen, while air increased the amount of both CO and CO2. Thus, H2/CO and H2/(CO2 + CO) molar ratios increased in relation to the use of only air and decreased in relation to the use of only steam. These results generally agree with those found in literature [7,25–29].

DH ¼ 206 kJ=mol

CH4 þ H2 O ¡ CO þ 3H2

ð4Þ

DH ¼ 165 kJ=mol

CH4 þ 2H2 O ¡ CO2 þ 4H2

ð5Þ

DH ¼ 210 kJ=mol

Cn Hm þ nH2 O ¡ nCO þ ðn þ m=2ÞH2

ð6Þ

As expected, the rise of ER during air-blown gasification further promoted the partial oxidation reactions and thus, the release of both CO and CO2 was observed to increase. With the rise of ER, the conversion of C-char into the total amount of carbon oxides was around 14%. The release of H2 and hydrocarbons decreased,

CO

CO2

H2

CH4

CnHm

40

20

3 St

ea m

d an

an ir A

O 2

d

St

St ea d an ir A

ea m ,E

R ,E

R m ,E

y nl

R

=0 .

.2 =0

m St ea

ir A

=0 .2

0

O

To analyse the effect of gasification/fluidisation medium, the temperature of 850 °C was selected, as stated before. Air, or oxygen and mixtures of both gases with steam were used as gasification/ fluidisation medium. The effect of varying ER, using air or pure oxygen, was also studied. The use of oxygen instead of air lowered U/Umf ratio to values around 3, while for air and steam mixtures

ð1Þ

H2 þ 1=2O2 ¡ H2 O

O

3.1. Effect of gasification medium on co-gasification of rice husk blended with PE wastes

DH ¼ 111 kJ=mol

C þ 1=2O2 ¡ CO

y

3. Results and discussion

values around 4 were achieved. When no steam was used U/Umf ratio was lower, however, fluidisation conditions were ensured. Gas compositions on dry and inert free basis are presented in Fig. 2 to easier the comparison of gas composition obtained in presence of different gasification agents with and without nitrogen. Nevertheless, when air was used instead of oxygen, for instance keeping all the other parameters constant, the gas produced was diluted in nitrogen. Fig. 2 shows that on dry and inert free basis the use of only air as gasification/fluidisation agent led to higher release of both CO and CO2, than when only steam was used, as the partial oxidation reactions (1)–(3) were favoured, being the increase in the total release of CO and CO2 around 25%.

nl

cated gasifier control. The presence of PE in rice husk blends with contents higher than 20% (w/w) should be avoided to easier the control of the gasifier and to ensure operation at smooth conditions. However, the presence of plastics was beneficial to increase the energetic content of gasification gas. Hence, the blend with 80% of rice biomass mixed with 20% (w/w) of PE was selected for the present study. Usually the amounts of plastics generated by rice production activities in Portugal are not enough to treat all the biomass rice wastes available, considering waste mixtures with 20% (w/w) of PE. However, additional plastics from other agriculture sectors or even from municipal solid wastes could be used, as the presence of PE is beneficial to increase gas HHV [24]. Gasification gas was sampled and collected in bags to be analysed by gas chromatography (GC) to determine the contents of CO, CO2, H2, CH4, N2, O2 and other heavier gaseous hydrocarbons, which were presented as CnHm. Gas compositions are presented on a dry inert free basis, to avoid the dilution effect of nitrogen and moisture and to allow the comparison of gas compositions obtained with different gasification agents. Gasification gas was also sampled to determine tar content using CEN/TS 15439:2006 Standard. The solvent isopropanol (2propanol) was used for tar collection. The homogeneous liquid sampled was evaporated under well-defined conditions and the evaporation residue was weighed to calculate the amount of tar in g/Nm3 (CEN/TS 15439:2006). Gasification gas was also sampled to determine H2S and NH3 contents. Besides H2S, some other sulphur compounds may be found in syngas, however, as around 95% of the gaseous sulphur appeared as H2S, only this compound was analysed in gasification gas. Several nitrogen gaseous compounds may be formed during gasification. The formation of HCN may occur, but in much lower contents than NH3. NOx is not usually released, due to the reduction conditions used during gasification. Therefore, only the content of NH3 was measured in gasification gas. H2S was analysed by method 11 of EPA (Environmental Protection Agency). Sulphide was retained in an absorbing solution of CdSO4 and then analysed by iodometry. Sulphur held in the condensation system was also analysed, as SO2 4 , using Capillary Ion Electrophoresis. NH3 was analysed according to the method CTM-027 of EPA. Ammonia was retained in an acidic absorbing solution of H2SO4 0.1 N and then analysed potentiometrically with a specific ion electrode. After each experiment, solid bed residue, containing silica sand, ashes and unconverted carbon, was collected and analysed. Silica sand was separated from the remaining solids and char content was determined. Gas composition was presented on dry basis. Gas yield was calculated based on the production of inert-free gas per weight of dryash-free feedstock, excluding water from gas composition. Gas HHV is defined as the gross calorific value of the inert-dry-free gas on a volumetric basis. Energy conversion was determined as the ratio between the energy present in the produced gas and the energy contained in the gasified feedstock.

Concentration (% v/v)

410

Fig. 2. Effect of co-gasification conditions on syngas composition obtained during co-gasification of rice husk blended with 20% (w/w) of PE. Gas composition is presented on dry inert basis. When not mentioned otherwise experimental conditions were: Temperature – 850 °C, ER – 0.2, steam/feedstock ratio – 1.0.

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because of their oxidation due to the presence of higher amounts of oxygen. The use of oxygen instead of air did not led to significant changes in gas composition on dry inert free basis. Nevertheless, in presence of air, the gasification gas contained around 45% of nitrogen and thus all the other gaseous components decreased accordingly. For oxygen-blown gasification, keeping ER constant, lower gas flow rate was used, due to the absence of air nitrogen, thus higher residence times were observed, which could lead to some destruction of hydrocarbons, as observed for CnHm. Nevertheless, as tar and char conversion would also be promoted, more hydrocarbons could be formed. This together with the high PE tendency to produce hydrocarbons could explain the smaller variation in CH4 contents. Hydrocarbons destruction by steam reforming reactions would lead to the formation of CO and H2. In Fig. 2 may also be observed a great increase in H2 contents, some decrease in CO and a great increase in CO2, which is the result of feedstock devolatilisation, carbon and CO oxidation and water gas shift reaction. CO initially formed may be converted by secondary gas reactions like water gas shift reaction (7). These results agree with those found in literature [26].

DH ¼ 41 kJ=mol

CO þ H2 O ¡ CO2 þ H2

ð7Þ

Besides gas composition, other parameters, such as gas yield and gas HHV should also be considered for the selection of gasification conditions. In Fig. 3 tar content and syngas yield are presented on dry basis, while gas HHV is presented on dry inert basis (excluding nitrogen). As shown in Fig. 3, the lowest gas production was obtained when only steam was used as gasification agent, while the highest production of gas was obtained when mixtures of air and steam were introduced into the reactor, especially for the highest ER values tested, as more nitrogen was added. Exchanging air by oxygen decreased the total gas yield, as in presence of air nitrogen was also included. The use of oxygen-blown gasification increased gas HHV up to 37% on dry inert free basis, as the diluting effect of nitrogen was absent. Though the use of air led to higher gas yields, the gas produced had lower HHV, mainly due to the diluting effect of nitrogen. This is even more important for the ER value of 0.3, as a decrease of around 16% was observed, consequently the lowest cold gas efficiency was obtained. The use of only steam also led to a great increase in gas HHV, thus despite the low gas yield the second highest cold gas efficiency was obtained. Tar is one of the main concerns during the gasification process, as it may originate clogging of the installations and higher maintenance and downtime related costs. The use of both steam and air

allowed decreasing the release of tar, as steam promoted the destruction of tar by steam reforming reactions and air favoured the partial oxidation of tar compounds. The lower tar concentration was obtained while using steam and air, for the ER value of 0.3, which led to tar reduction of about 41%, in relation to the content obtained for the ER value of 0.2. On the other hand, oxygenblown gasification originated higher tar concentrations, as in this case there was no nitrogen diluting effect. However on dry and inert free basis tar content in presence of pure oxygen was lower, as the higher residence time of gases promoted tar destruction by reforming reactions and also by cracking ones, reaction (8).

ð8Þ The analysis of the overall results showed that the use of steam is a good option for the gasification of rice husks mixed with up to 20% (w/w) PE. The presence of air and steam led to the highest gas yields, acceptable tar amounts and low gas HHV. Nevertheless, the use of oxygen instead of air is recommended when the gas is intended to be used as fuel or as raw material for chemical synthesis, not only because gas HHV increased, but mainly because the presence of nitrogen is eliminated. In Fig. 4 is presented the effect of gasification/fluidisation conditions on the release of H2S and NH3 into the gasification gas obtained by a mixture containing 80% of rice husk blended with 20% (w/w). H2S and NH3 contents were relatively low when compared with other feedstocks, which agrees with the low sulphur and nitrogen contents in the initial feedstocks. As shown in Fig. 4, the presence of steam increased the formation of both H2S and NH3, as steam seems to have promoted the release of char-S and char-N. As expected the rise of ER during air-blown gasification led to a decrease in both H2S and NH3 contents, the presence of more oxygen should have favoured the formation of sulphur and nitrogen oxides, which might explain the decreases observed in H2S and NH3 contents of around 20% and 40%, respectively. On the other hand, the use of oxygen instead of air, keeping ER constant, meant a lower gas flow rate and thus a higher residence time, which might have favoured the release of char-S and char-N and consequently the formation of H2S and NH3. The use of oxygen and steam mixtures as gasification agents led to a large increase in both NH3 and H2S also due to the disappearance of the diluting effect of nitrogen from air.

H2S

Tar

Gas Yield

DH > 0

pCn Hm ¡ qCx Hy þ Cz Hu þ rH2 ðx; z < n and y; u < mÞ

NH3

HHV/10

1.0

10

0.5

0

0.0 ly

On

Air ly

On

eam

St

Air

R

,E

am

te dS

an

.2 =0

Air

.3 =0

R

,E

am

an

te dS

O2

.2 =0

R

,E

am

te dS

an

Fig. 3. Effect of co-gasification conditions on tar content, gas HHV and syngas yield obtained during co-gasification of rice husk blended with 20% (w/w) of PE. Experimental conditions as mentioned in Fig. 2. Tar and syngas yield are presented on dry basis and HHV on dry inert basis.

200

200

100

100

0

NH 3 (ppmv)

20

(ppmv)

Tar (g/m3)

1.5

H2S

2.0 30

Gas Yield(Ndm3/g daf), HHV/10 (kJ/Ndm3)

40

0 ir

A nly

O

ly

On r

Ai

0 R=

r

Ai

am

te dS

an

0 R=

,E

,E

am

te dS

an

.3

.2

m

a Ste

O2

m,

.2

0 R=

E

tea

S nd

a

Fig. 4. Effect of gasification conditions on the contents of H2S and NH3 obtained during co-gasification of rice husk blended with 20% (w/w) of PE at 850 °C. Gas composition is presented on dry basis. Other experimental conditions as mentioned in Fig. 2.

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F. Pinto et al. / Fuel 180 (2016) 407–416

The results obtained generally agree with others found in literature [27], though H2S and NH3 contents shown in Fig. 4 were much smaller in accordance to the lower sulphur and nitrogen contents in the feedstocks. For coal and wastes blends oxygen-blown gasification led to the decrease of NH3 in gasification gas, the lower nitrogen contents found in coal char seem to indicate that more nitrogen was released, mainly as NH3, which was converted afterwards, due to the higher syngas residence time. The changes obtained with different feedstocks shows the importance of feedstock type and composition [27]. The quality of the gasification gas was improved by the presence of low cost minerals (limestone, dolomite or olivine) in the gasification process, as gaseous hydrocarbons, NH3 and tar destruction were promoted, together with sulphur retention inside the bed, leading to lower releases of H2S. Though, the tendency for bed agglomeration was not a problem during co-gasification of biomass rice wastes blended with PE, it is predictable that during longer time experiments bed agglomeration may become a problem, due to rice biomass wastes composition. These minerals would also decrease the tendency for bed agglomeration. This subject will be further reported elsewhere. 3.2. Effect of using air enriched in O2 co-gasification of rice production wastes Results reported previously showed that the use of steam and oxygen is the best option for gasification of rice biomass wastes blended with PE. The main drawback of using oxygen is the cost associated to its production. Another option is to use enriched air, containing up to 40% (v/v) of oxygen, as this content can be reached using commercial air separators based on membrane technology. This technology has a lower cost than the production of pure oxygen. Enriched air with 60% (v/v) of oxygen was also tested to try to understand the phenomena involved in co-gasification of rice biomass wastes blended with PE. In general U/Umf ratio varied between 3 and 4, the lowest value was used when pure oxygen and steam were used as gasification and fluidisation agents, while the highest value was attained when air was used instead of oxygen. In Fig. 5 gas compositions are shown on dry inert basis, to discount nitrogen diluting effect. Blends with different biomass rice wastes were tested, but in all of them the amount of PE was kept

CO

CO2

constant, 20% (w/w). Rice husk, rice straw and a mixture of equal contents of husk and straw were used. The effect of using pure oxygen against the use of air (with 21% of oxygen) or enriched air with 40% and 60% (v/v) of oxygen was analysed. The results obtained showed that the same tendencies were obtained for all the feedstocks mixtures, though some differences in gas composition were observed, due to the use of different biomass wastes. The rise of oxygen content in the gasification agent led to an increase in CO2 and H2 contents in the produced gas, some decrease in hydrocarbons, mainly in CnHm was also observed. ER values were kept constant in all experiments. As the amount of oxygen in the gasification agent increased, there was a reduction in the gas introduced into the gasifier, as the amount of nitrogen decreased, thus the residence time was observed to increase. Therefore, gasification reactions had more time to occur, which may explain the reduction observed in hydrocarbons contents of about 30%, as cracking (8) and steam reforming reactions (4)–(6) were promoted. Consequently, CO and H2 concentrations were expected to increase, as reported by Campoy et al. [7] who used enriched air with up to 40% (v/v) of oxygen. Nevertheless, in Fig. 5 is observed that CO contents decreased when more oxygen was added into the gasifier. This might have occurred because CO was converted into CO2 and H2 by water gas shift reaction (7), which would explain the rise of CO2 and the great increase of H2. The increase of oxygen partial pressure in the emulsion phase when more oxygen was used could also have favoured the full oxidation of carbon to CO2 [30]. Cheng et al. [8] also reported a sharp increase in CO2 concentration when oxygen increased from 21% to 31.4%. On the other hand, Campoy et al. [7] did not report the increase in CO2 and the consequent decrease in CO, probably because the gasification conditions used did not fully promote water gas shift reaction. Tar and gas yield presented in Fig. 6 refer to the total dried gas produced, including nitrogen. Therefore, a decrease in gas yield with the rise of oxygen content was observed, as it corresponded to a reduction in nitrogen and thus to a lower gasification agent flow rate. However, gas yield on dry and inert free basis was observed to increase from 20% to 38% depending on feedstock composition. This agrees with the higher residence time when pure oxygen was introduced and thus with the greater extent of overall gasification reactions. Cheng et al. [8] also observed an increase in gas yield from 72% to 81% when oxygen concentration increased from 21% to 31.4%.

H2

CH4

CnHm

40

Concentration (% v/v)

80%Husk, 20%PE

80%Straw, 20%PE

40%Husk, 40%Straw, 20%PE

20

En ri 10 ch 0% ed En ai A r, ir ri ch 40 ed % ai O r, 2 60 % O 2 10 0% En O ri 2 10 ch 0 ed % En ai A r, ir ri ch 40 ed % ai O r, 2 60 % O 2 10 0% En O ri 2 10 ch 0 ed % En ai A r, ir ri ch 40 ed % ai O r, 2 60 % O 2 10 0% O 2

0

Fig. 5. Effect of using air enriched with O2 on gas composition obtained by co-gasification of rice wastes mixed with 20% (w/w) of PE. Gas composition is on dry inert basis. Other conditions: Temperature – 850 °C, ER – 0.2, steam/feedstock ratio – 1.0.

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Gas Yield

80%Husk, 20%PE

80%Straw, 20%PE

HHV/10 40%Husk, 40%Straw, 20%PE 2.0 1.5

20 1.0 10

0.5

0

0.0

En ri ch 10 En ed a 0% ir ri A , ch i ed 40% r ai O r, 60 2 % O 2 10 0% En ri O ch 10 2 En ed a 0% i ri ch r, 4 Air 0% ed ai O r, 60 2 % O 2 10 0% En ri O ch 1 2 En ed a 00% ir ri A , ch ed 40% ir ai O r, 60 2 % O 2 10 0% O 2

Tar (g/m3)

30

Gas Yield (Ndm3/g daf), HHV/10 (kJ/Ndm 3)

Tar 40

Fig. 6. Effect of using air enriched with O2 on tar content, gas HHV and gas yield obtained by co-gasification of rice wastes mixed with 20% (w/w) of PE. Experimental conditions as mentioned in Fig. 5. Tar and syngas yield are presented on dry basis and HHV on dry inert basis.

It is also due to the lower diluting effect of nitrogen that tar contents obtained with 100% of oxygen were higher than the values obtained with lower oxygen contents. The initial rise of oxygen content in gasification agent led to a decrease in tar, because cracking and oxidation reactions that promoted tar destruction were favoured. However, the formation of tar on dry and inert free basis was observed to increase. Ruoppolo et al. [30] also reported that the use of oxygen and steam mixtures during biomass gasification led to higher total tar content than that resulting from air and steam mixtures. However, with air and steam, tars were more refractory [26,30], thus the use of pure oxygen and steam was beneficial for tar composition and further destruction. The use of pure oxygen and steam was also beneficial to decrease the amount of char, as carbon conversion was favoured, not only because the residence time was higher, but also because an higher partial pressure of oxygen was achieved in the emulsion phase of the bed, as stated by Ruoppolo et al. [30]. The results obtained quite agree with those reported by these authors. Gas HHV on dry and inert basis was observed to decrease with the rise of oxygen, Fig. 6, reductions around 20% were obtained depending on feedstock composition. These reductions were due to the decrease of CO and hydrocarbons contribution to HHV, as the rise of H2 content was not enough to compensate the decreases of CO and hydrocarbons. However, gas HHV on dry basis increased around 42% when pure oxygen was used, because of decreasing the diluting effect of nitrogen. Thanapal et al. [11] also stated that the HHV of the obtained syngas increased with enriched air. The rise of oxygen content in the gasification agent allowed increasing cold gas efficiency on dry and inert basis, which reached values around 67% for the highest oxygen content. Campoy et al. [7] also reported an increase in cold gas efficiency with the rise of oxygen content up to 40% (v/v), though there was not a clear increase in gas yield. Results presented in Fig. 7 generally agree with those of Fig. 4, as the rise of oxygen content in gasification agent led to an increase in H2S and NH3 for all the feedstocks tested. As mentioned before, the increase in oxygen content led to a reduction in gasification agent flow rate, due to the decrease in nitrogen, thus residence may increase around 30–45%, depending on feedstock composition. Consequently, gasification reactions had more time to occur, which increased the release of char-S and char-N and the formation of H2S and NH3. Besides this, with the decrease of nitrogen

dilution effect in gasification gas, there was an increase in both H2S and NH3. Even with air and air enriched with oxygen up to 40% (v/v), H2S and NH3 contents were not acceptable for much gas utilisations. To decrease H2S and NH3 contents low cost minerals like dolomite or olivine were used in the gasification bed. Calcium, magnesium and iron oxides present in dolomite or olivine reacted with H2S to form metal sulphides, which were retained inside the gasifier. However, for utilisations requiring low contents of both these compounds it is advisable to treat gasification gas in a hot gas conditioning system to further decrease the contents of these compounds and also to favour the conversion of heavier gaseous hydrocarbons and tar into H2 and CO. Huynh et al. [10] stated that with the rise of oxygen content more free oxygen exists to form NOx. There might be some competition between this reaction and that leading to NH3. Once formed NH3, it may react with oxygen to form NO, if there is enough free oxygen available. However, it is very unlikely that NO could be stable under the gasification conditions used. In fact the formation and destruction of nitrogen and sulphur compounds are quite complex and depend mainly on the gasification medium and on nitrogen content in the feedstock. The results obtained showed that the best option is the use of pure oxygen and steam, as a better gas composition with a higher HHV and a higher energy conversion were obtained. The highest char conversion, the release of tar with a composition easier to destroy and mainly the absence of nitrogen in syngas are also important advantages. The use of air enriched with oxygen increased the H2/CO ratio due to the rise of H2 and to the decrease of CO content when more oxygen was added to the gasifier. For the feedstock with 80% of husk blended with 20% (w/w) of PE the use of air enriched with 60% of oxygen led to an H2/CO of around 2.4. This means that the gas produced had an H2/CO ratio suitable for its conversion into liquid fuels by Fischer–Tropsch synthesis, though the gas still needed to be upgraded to fulfil other requirements for chemical synthesis, namely the contents of tar, H2S, NH3, alkali metals and total chlorine. For mixtures with straw, only the use of pure oxygen and steam allowed to reach values around 2 for the H2/CO ratio. On the other hand, the syngas produced was not suitable for methanol synthesis, because H2/(2CO + 3CO2) ratio ranged from 0.5 to 0.8, instead of being 1.05 as required [29]. As, the rise of oxygen

F. Pinto et al. / Fuel 180 (2016) 407–416

H2S 80%Husk, 20%PE

NH3 80%Straw, 20%PE

40%Husk, 40%Straw,

300

400

NH 3 (ppmv)

414

H2S (ppmv)

300

200 200

100

100

0

0

2 O2 Air 2 2 O2 2 O2 Air 2 Air 2 % 0% O 0% O 0% 00% 0% O 0% O 0% 00% 0% O 0% O 0% 0 0 0 0 0 1 r, 4 r, 6 1 r, 4 r, 6 1 r, 4 r, 6 1 1 1 i i i i i i d a hed a d a hed a d a hed a e e e h h h ric ric ric ric ric ric En En En En En En Fig. 7. Effect of using air enriched with O2 on the contents of H2S and NH3 obtained during co-gasification of rice husk blended with 20% (v/v) of PE at 850 °C. Gas composition is presented on dry basis. Other experimental conditions as mentioned in Fig. 5.

3.3. Effect of CO2 content on gasification medium There is an increasing interest in using as gasification and fluidisation agent CO2 that could come from capture installations. The use of CO2 would decrease this gas overall emissions and at the same would promote gasification process by favouring char conversion by reaction (9) and tar and gaseous hydrocarbons destruction by CO2 reforming reactions (10)–(12).

Two CO2/O2 ratios were tested, 0.1 and 1. The results obtained were compared with those obtained with only oxygen and only CO2. In all experiments similar flow rates of gasification agent and steam/feedstock ratio of around 1 were used. In general U/ Umf ratios were around 3 for all the tests done with and without oxygen. The use of only CO2 brings an extra problem, the supply of the heat necessary for the gasification endothermic reactions. The results obtained are shown in Fig. 8. The rise of CO2 content in gasification agent led to an increase in CO, probably because Boudouard reaction (9) was promoted, which would also explain the reduction in CO2 content in gasification gas. This decrease may be also explained by CO2 reforming reactions (10)–(12), which would favour further formation of CO. The decrease observed in CH4 and especially in other gaseous hydrocarbons concentrations might have been due to their destruction by reforming reactions, which would lead to the increase of H2 content. The conversion of CH4 is more difficult and thus milder variations were observed. Nevertheless, in Fig. 8 no great variations in H2 concentrations were observed. Butterman et al. [13] and Cheng et al. [15] reported that in presence of high CO2 contents the reverse of water gas shift reaction (7) may occur, which would lead to further formation of

CO

Concentration (% v/v)

content increased this ratio, gas cleaning and conditioning processes, to attain the required H2/(2CO + 3CO2) ratio would be easier when pure oxygen was used instead of air. The main drawback of using pure oxygen, or air enriched with more than 40% of oxygen, is the cost production of such gases. Membrane technology is usually limited to the production of enriched air with oxygen contents in the range 25–50%, it tolerates air contaminated with H2O and CO2 and may produce around 20 tons/day of enriched air [31]. Lin et al. [32] reported a new and energy efficient membrane process to produce oxygen enriched air at a cost of 1.4–1.7 MMBtu/ton EPO2 (equivalent pure oxygen), which is competitive to conventional technologies like: cryogenics and vacuum pressure swing adsorption, mainly for small-scale oxygen production. According to these authors it was possible to obtain 30% oxygen-enriched air at $35/ton EPO2, which is lower than the values achieved by commercial membranes for nitrogen generation from air. Even if oxygen-blown gasification is not nowadays economic attractive, its viability in the future depends on gasification gas end-use and on the development of new technologies that may decrease the cost of more demanding processes. Nevertheless, depending on gasification gas utilisation the use of air enriched with 30–40% (v/v) of oxygen may be also a good alternative, with the great advantage of lowering operation costs.

CO2

H2

CH4

CnHm

40

20

DH ¼ 292 kJ=mol

ð11Þ

Cn Hm þ n=4CO2 ¡ n=2CO þ ðm=2  3n=2ÞH2 þ ð3n=4ÞCH4

DH ¼ 45 kJ=mol

ð12Þ

O

2

0%

C

10

2, O

C

O 50

0% 2, 9 O C

50 %

Cn Hm þ nCO2 ¡ 2nCO þ m=2H2

ð10Þ

10 %

DH ¼ 247 kJ=mol

10

CH4 þ CO2 ¡ 2CO þ 2H2

%

O

O

2

2

ð9Þ 0%

DH ¼ 172 kJ=mol

2

0

C þ CO2 ¡ 2CO

Fig. 8. Effect of CO2 content on gasification agent on gasification gas composition obtained by co-gasification of rice wastes mixed with 20% (w/w) PE. Gas composition is shown on dry inert basis. Other conditions: Temperature – 850 °C, ER – 0.2, steam/feedstock ratio – 1.0.

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1.5 1.0

10

0.5

200

200

100

100

2, O

C 0%

0

0

C

10

% 50

% 90 2, O

2 %O

100

50

%

C % 10

O

2 O

2 O

O 0% 10

2

0.0

2

0

NH3

NH 3 (ppmv)

2.0

(ppmv)

Tar (g/m 3)

H2S

HHV/10

H2S

Gas Yield

Gas Yield (Ndm3/g daf), HHV/10 (kJ/Ndm3)

Tar 20

10% Fig. 9. Effect of CO2 content on gasification agent on tar content, gas HHV and gas yield obtained by co-gasification rice wastes mixed with 20% (w/w) PE. Experimental conditions as mentioned in Fig. 8. Tar and syngas yield are presented on dry basis and HHV on dry inert basis.

CO, this would help to explain the great increase of around 76% observed in Fig. 8. Thanapal et al. [11] also observed that CO increased with addition of CO2 into the gasification medium, due to the promotion of Boudouard reaction. Pfeifer et al. [33] also stated that the internal recycling of CO2 led to higher CO contents and to lower H2 in the gas produced. In Fig. 9 is observed that the decrease in tar content was around 45% with the rise of CO2 content in gasification agent, which was followed by a great increase in gas yield, around 70%. Tar destruction by CO2 reforming reactions was favoured, leading to higher gas formation. Besides this, the presence of CO2 might have promoted char gasification, by reaction (9), thus also leading to further gas formation. However, reaction (9) is slower than the direct oxidation of feedstock carbon to form CO2, thus the use of CO2 is expected to have enhanced char accumulation, as stated by Ruoppolo et al. [30]. This agrees with the lower formation of char observed with the rise of O2 in gasification agent. Hanaoka et al. [16] also observed that the presence of CO2 led to the decrease of tar by CO2 reforming reactions, thus leading to the rise of gas yield of around 15%, when CO2 in gasification agent varied from 0% to 79%. Ruoppolo et al. [30] also stated that char accumulation in presence of more CO2 could favour tar adsorption and it further conversion. Cheng et al. [15] also reported an increase in gas yield when the amount of CO2 introduced into the gasifier increased, as a higher gas HHV was obtained, cold gas efficiency also increased. On the other hand, no great changes were observed in gas HHV, Fig. 9, as the decrease in hydrocarbons with the rise of CO2 was compensated by the increase in CO. Due to the rise in gas yield in presence of CO2, an increase in cold gas efficiency was obtained. The highest value around 80% was obtained when only CO2 and steam were used as gasification agent. On the other hand, Thanapal et al. [11] observed that in presence of CO2, gas HHV increased, due to the rise in CO content. In Fig. 10 may be observed that the increase of oxygen content favoured the formation of both H2S and NH3, probably because oxygen is a stronger reactant than CO2, promoting in a harder way the release of char-S and char-N and thus leading to the formation of H2S and NH3, due to co-gasification reduction medium. On the other, Guizani et al. [21] suggested that CO2 could increase additional micro-porosity and thus enhance the access of H2O to the active sites. Hence, the gasification process would be easier and thus the release of char-S and char-N. Nevertheless, the same amount of steam was used in all Fig. 10 experiments, being the only difference the amount of CO2 in the reaction medium.

0%

O2

2, 9

CO

50%

0%

2, 5

CO

2

O2 %

100

CO

Fig. 10. Effect of CO2 content on gasification agent on the release of H2S and NH3 obtained by co-gasification of rice husk blended with 20% (v/v) of PE. Other experimental conditions as mentioned in Fig. 8. Gas composition is presented on dry basis.

Fig. 10 results do not agree with those reported by Hanaoka et al. [16] who gasified gulfweed with high nitrogen and sulphur contents and reported that by increasing CO2 content, the release of H2S and COS enhanced. The biomass studied by Hanaoka et al. [16] had a higher mineral content, which might have played an important role in H2S and COS formation and destruction, including sulphur retention inside the bed by the formation of metal sulphides. This may explain the different results obtained. The huge number of reactions that occur simultaneously, being the products of some of them the reactants of other reactions makes difficult the clear understanding of the gasification mechanism. On the other hand, the type of feedstock and composition, including mineral matter may have a clear participation in gasification reactions by acting as catalysts or by retaining inside the bed some of the compounds initially formed or elements released. Gasification in presence of several gasification agents is quite complex, thus further investigations are still needed to have more insights on biomass co-gasification with CO2, using captured CO2 or the recirculation of some of the gasification gas into the gasifier. The use of CO2 improves gasification process, promotes CO2 reforming reactions and at the same time reduces CO2 emissions, but it requires external heating. The main challenge is achieving conditions for auto-thermal operation, thus the presence of oxygen is fundamental, especially when using CO2 and steam as gasification agents. It is difficult to select the O2 and CO2 mixture to ensure auto-thermal operation. Generally it is found that auto-thermal operation requires the use of a higher ER, thus at least an ER value of 0.3 would be advisable. In relation to the O2 and CO2 mixtures tested, that with 10% of CO2 and 90% of O2 would possibly be the best option. Nevertheless, further tests are needed to select the suitable composition for the gasification agent containing O2, steam and CO2 mixtures. 4. Conclusions Co-gasification of rice biomass wastes blended with PE was favoured by using as gasification and fluidisation agent an oxidant reactant (air or oxygen) and steam. Steam promoted steam reforming reactions, thus leading to a gas enriched in H2 and with lower tar and hydrocarbons contents. The decrease in tar and hydrocarbons was also achieved by increasing ER, which increased gas yields, but it also decreased gas HHV, due to the reduction of H2 and hydrocarbons.

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The results obtained showed that the use of pure oxygen and steam was a good option, as due to the lack of nitrogen diluting effect, the gas produced may have a wider range of possible applications, being gas HHV around 42% higher than the value obtained when air was used instead of oxygen. The main disadvantage is the cost of producing oxygen. Thus air enriched with O2 was also tested. The best option was the use of pure oxygen and steam, as it led to the highest energy conversion and the gas obtained had the best gas composition and the highest HHV. The use of air enriched with 40% (v/v) of oxygen may be also an alternative, as gasification performance and gas quality were better than those obtained for air-blown gasification and the cost production of such enriched air is lower than the production of pure O2. For co-gasification of rice biomass wastes blended with PE, the use of CO2 as gasification agent may be a good option, either by recirculating part of the gasification gas or by using gas captured from combustion processes. CO2 promoted tar and hydrocarbons destruction by CO2 reforming reactions, thus leading to CO release and decreasing CO2 emissions. The use of CO2 and steam as gasification agent led to tar reduction of about 45%, while gas yield increased around 70%. However, the addition of CO2 needs to be limited, because of the need to supply energy for the gasification process. The recirculation of part of the gasification hot gas or the use of hot combustion gases by a nearby installation would help to solve this problem. The use of steam, CO2 and O2 mixtures are a good option, as the partial oxidation reactions would supply the energy needed for the endothermic reactions. CO2-blown gasification is quite complex and more investigation is still needed to find the best composition of CO2, oxygen and steam mixtures. Acknowledgement This research was financed by FEDER through the Operational Program for Competitive Factors of COMPETE and by National Funds through FCT – Foundation for Science and Technology by supporting the project PTDC/AAG – REC/3477/2012 – RICEVALOR – Energetic valorisation of wastes obtained during rice production in Portugal, FCOMP-01-0124-FEDER-027827, a project sponsored by FCT/MTCES, QREN, COMPETE and FEDER. References [1] FAO Rice Market Monitor, vol. XVI1I (2). Food and Agriculture Organization of the United Nations; 2015. [2] INE Statistical Yearbook of Portugal 2012; 2013. ISBN 978-989-25-0219-9. [3] Calvo LF, Gil MV, Otero M, Morán A, García AI. Gasification of rice straw in a fluidized-bed gasifier for syngas application in close-coupled boiler-gasifier systems. Bioresour Technol 2012;109:206–14. [4] Murakami K, Kasai K, Kato T, Sugawara K. Conversion of rice straw into valuable products by hydrothermal treatment and steam gasification. Fuel 2012;93:37–43. [5] Yoon SJ, Son Y-I, Kim Y-K, Lee J-G. Gasification and power generation characteristics of rice husk and rice husk pellet using a downdraft fixed-bed gasifier. Renew Energy 2012;42:163–7. [6] Zhang K, Chang J, Guan Y, Chen H, Yang Y, Jiang J. Lignocellulosic biomass gasification technology in China. Renew Energy 2013;49:175–84. [7] Campoy M, Gómez-Barea A, Vidal FB, Ollero P. Air–steam gasification of biomass in a fluidised bed: process optimisation by enriched air. Fuel Process Technol 2009;90:677–85.

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