Kinetics of rice husk char gasification

The gasification of rice husk char in carbon dioxide and steam was investigated for determining the kinetic parameters. Experiments were conducted with rice husk char in its original grain form in a silica tube reactor with steam at temperatures of 750°C, 800°C, 850°C and 900°C and experiments were conducted with rice husk char powder in a thermogravimetric balance in a carbon dioxide medium at temperatures of 750°C, 800°C, 850°C and 900°C. The data was analysed based on the volume reaction and shrinking core models. The activation energies obtained for the rice husk grain sample were 200 kJ/mol and for the rice husk powder, about 180 kJ/mol respectively. The results obtained are in good agreement with literature values of different char gasification reactions.     

Thermodynamic analysis of hydrogen rich synthetic gas generation from fluidized bed gasification of rice husk

In the present work, the generation of hydrogen rich synthetic gas from fluidized bed steam gasification of rice husk has been studied. An equilibrium model based on equilibrium constant and material balance has been developed to predict the gas compositions. The equilibrium gas compositions are compared with the experimental data of the present group as well as of available literature. The energy and exergy analysis of the process have been carried out by varying steam to biomass ratio (ψ) within the range between 0.1-1.5 and gasification temperature from 600 °C to 900 °C. It is observed that both the energy and exergy efficiencies are maximum at the CBP (carbon boundary point) though the hydrogen production increases beyond the CBP. The HHV (higher heating value) and the external energy input both continuously increase with ψ. However, the hydrogen production initially increases with increase in temperature up to 800 °C and then becomes nearly asymptotic. The HHV decreases rapidly with increase in temperature and energy input increases. Therefore, gasification in lower temperature region is observed to be economical in terms of a trade off between external energy input and HHV of the product gas.     

Modelling and simulation of a downdraft rice husk gasifier

The behaviour of a downdraft rice husk gasifier of diameter 200 mm and a height 940 mm has been studied. The gasification rate was varied in the range 1.8–4.3 × 10^?2 kg/m²s. The air velocity was varied in the range 0.032–0.099 m/s. The producer gas obtained from the gasifier has a calorific value in the range 3240–4382 kJ/m³. A set of theoretical kinetic equations on the assumption of nonequilibrium conditions has been developed and solved numerically. The simulated temperature profile and outlet gas composition have been compared with those obtained from experimental runs. The model developed from a mechanistic approach is found to explain the behaviour of the present system appreciably within the range of variables studied.     

Syngas production by chemical looping gasification of rice husk using Fe-based oxygen carrier

The chemical looping gasification (CLG) of rice husk was conducted in a fixed bed reactor to analyze the effects of the ratio of oxygen carrier to rice husk (O/C), temperature, residence time and preparation methods of Fe-based oxygen carriers. The yield of gas, H₂/CO, lower heating value of syngas (LHV), conversion efficiency and performance parameters were analyzed to obtain CLG reaction characterization and optimal reaction conditions. Results showed that when O/C increased from 0.5 to 3.0, the gas production, H₂/CO, CO₂ yield and carbon conversion efficiency gradually increased, while the yield of H₂, CO and CH₄ and LHV gradually decreased. At the same time, a highest gasification efficiency was obtained when O/C was 1.5. As increasing temperature, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, while the yield of H₂, CH₄ and CO₂, H₂/CO and LHV gradually decreased. Sintering and agglomeration was obvious when the temperature was higher than 850 °C. When the reaction time increased from 10 min to 60 min, the gas production, CO yield, carbon conversion efficiency and gasification efficiency gradually increased, but the yield of H₂, H₂/CO and LHV decreased, among which 30 min was the best reaction residence time. In addition, coprecipitation was the best preparation method among several preparation methods of oxygen carrier. Finally, O/C of 1.5, 800 °C, 30 min and coprecipitation preparation method of oxygen carrier were the optimal parameters to obtain a gasification efficiency of 26.88%, H₂ content of 35.64%, syngas content of 56.40%, H₂/CO ratio of 1.72 and LHV of 12.25 MJ/Nm³.     

The effect of biomass feeding location on rice husk gasification for hydrogen production

The objective of this study is to investigate the impact of biomass feeding location on rice husk gasification for hydrogen production. By comparing the results between top-feed and bottom-feed of the feedstock of the fluidized bed biomass gasification at the reaction temperature between 600～1000 °C and ER = 0.2, 0.27, and 0.33 without steam, the optimum low heating value was increase by 2.35 kJ/g-rice husk by the top-feed to gasifier. Although the yield of hydrogen was decreased by 42% for the rice husk gasification by the top-feed operation, the yield of CO, CO2, and CH4 were highly increased, which enhancing the heating value of the effluent gas. The study results suggested the potential route of the biomass gasification at the different feeding location.     

Overview of combustion and gasification of rice husk in fluidized bed reactors

Rice is cultivated in more than 75 countries in the world. The rice husk is the outer cover of the rice and on average it accounts for 20% of the paddy produced, on weight basis. The worldwide annual husk output is about 80 million tonnes with an annual energy potential of 1.2 × 10⁹ GJ corresponding to a heating value of 15 MJ/kg. India alone generates about 22 million tonnes of rice husk per year. If an efficient method is available, the husk can be converted to a useful form of energy to meet the thermal and mechanical energy requirements of the rice mills themselves. This paper provides an overview of previous works on combustion and gasification of rice husk in atmospheric bubbling fluidized bed reactors and summarizes the state of the art knowledge. As the high ash content, low bulk density, poor flow characteristics and low ash melting point makes the other types of reactors like grate furnaces and downdraft gasifiers either inefficient or unsuitable for rice husk conversion to energy, the fluidized bed reactor seems to be the promising choice. The overview shows that the reported results are from only small bench or lab scale units. Although a combustion efficiency of about 80% can normally be attained; the reported values in the literature, which are more than 95%, seem to be in higher order. Combustion intensity of about 530 kg/h/m² is reported. It is also technically feasible to gasify rice husk in a fluidized bed reactor to yield combustible producer gas, even with sufficient heating value for application in internal combustion engines. A combustible gas with heating value of 4-6 MJ/Nm³ at a rate of 2.8-4.6 MW_th/m² seems to be possible. Only very little information is available on the pollutant emissions in combustion and tar emissions from gasification. The major conclusion is that the results reported in the literature are limited and vary widely, emphasizing the need for further research to establish suitable and optimum operating conditions for commercial implementations.     

Chemical looping gasification of rice husk to produce hydrogen-rich syngas under different oxygen carrier preparation methods

The chemical looping gasification (CLG) of rice husk was conducted in a small fixed-bed reactor to investigate the effects of oxygen carrier preparation methods, active components, Fe loadings, water inflow and successive redox cycles. The gas content, the relative gas concentration, lower heating value of syngas (LHV), carbon conversion efficiency, gasification efficiency, H₂-TPR, XRD, BET and SEM were analyzed to obtain CLG reaction characterization and comprehensive performance of oxygen carrier. The results showed that compared with several methods and active components, the coprecipitation method with a Fe:Al ratio of 2 was the best preparation method for oxygen carrier. Furthermore, the H₂ concentration was 57.29%, while the gasification efficiency and carbon conversion efficiency were 95.79% and 51.56%. Meanwhile, the oxygen carrier prepared by coprecipitation had a greater performance of oxygen release and reduction. There was an optimal water inflow to obtain the highest gas content, carbon conversion efficiency and gasification efficiency. Thereinto, the H₂ concentration was stable at 54.07–57.65%. And the highest gas content of 78.33% and carbon conversion efficiency of 54.42% were obtained at a water inflow of 14.0 mL/h, while the highest gasification efficiency of 95.79% was obtained at 16.8 mL/h. In addition, after ten successive redox cycles, the gasification performance, crystal size, specific surface area and pore structure were relatively stable. Moreover, the material composition remained basically unchanged.     

Conversion of rice husk into amorphous silica and combustible gas

Results of the studies on thermogravimetric analysis (TGA) of raw and 1(N) HCl acid treated rice husk (in still air) reveal that its thermal degradation takes place in three main stages of mass loss, namely (i) drying, (ii) devolatilization and (iii) slow oxidation of fixed carbon. Hydrochloric acid leaching of husk at 75° for 1 h prior to combustion is necessary for production of amorphous silica of complete white colour. For production of low calorie-combustible gas along with amorphous silica from the rice husks containing 5.5–7% (wb) moisture, a furnace set temperature of 450°C appears to be optimal.     

Energy and chemicals from rice husk

Rice husk is the only agro residue having maximum siliceous ash content and available in dry form. The present work on rice husk describes processes for producing controlled carbon-silica mixtures in amorphous form, electronic-grade potassium silicate and activated carbon.

Rice husk is first pyrolised in a reactor at less than 973 K to obtain the char which is subsequently heated and activated with steam at 1073–1173 K for about 1 h in another reactor. The char is leached with HCl. Analysis of the leached char indicates that it is amorphous in nature and contains mainly 40% carbon and 56% silica with small quantities of volatile matter. The utility of the material as filler for reinforcement of rubbers is tested. Test results show that a tenfold increase in tensile strength and modulus of elasticity can be achieved with 100 phr.

In another development, the leached char is digested with 10–15% KOH solution in the temperature range of 303–373 K for about 1–10 h. It is found that electronic-grade potassium silicate could be easily produced compared to the conventional method of fusion of sand with alkali at 1273–1473 K. The residue from this process is a good-quality activated carbon. A process flowsheet is presented for producing these chemicals with energy recovery aspects from rice husk.     

Assessment of the rice husk lean-combustion in a bubbling fluidized bed for the production of amorphous silica-rich ash

Rice husk lean-combustion in a bubbling and atmospheric fluidized bed reactor (FBR) of 0.3 m diameter with expansion to 0.4 m in the freeboard zone and 3 m height was investigated. Experiment design - response surface methodology (RSM) - is used to evaluate both excess air and normal fluidizing velocity influence (independent and controllable variables), in the combustion efficiency (carbon transformation), bed and freeboard temperature and silica content in the ashes. Hot gases emissions (CO₂, CO and NO_x), crystallographic structure and morphology of the ash are also shown. A cold fluidization study is also presented. The values implemented in the equipment operation, excess air in the range of 40-125% and normal fluidization velocities (0.13-0.15 Nm/s) show that the values near the lower limit, encourage bed temperatures around 750 °C with higher carbon transformation efficiencies around 98%. However, this condition deteriorated the amorphous potential of silica present in the ash. An opposite behavior was evidenced at the upper limit of the excess air. This thermochemical process in this type of reactor shows the technical feasibility to valorize RH producing hot gases and an amorphous siliceous raw material.     

Gasification of rice husk in two-stage gasifier to produce syngas,silica and activated carbon

Rice husk was utilized in the production of syngas, silica and activated carbon. Experiments were performed in two-stage gasifier for the production of syngas. The syngas is generated with minimum tar yields due cracking of tar at high temperature. Rice husk char obtained from the pyrolysis stage of the reactor was used in the silica extraction process to obtain silica and activated carbon. Using nitrogen as pyrolysis agent high purity (88.46%) silica was obtained with a good quality of syngas as compared to air as pyrolysis agent. Highest surface area of 276.91 m²/gm of silica was found at 500^?C.     

Fractional characterization of a bio-oil derived from rice husk

Bio-oils usually contain many types of compounds with various chemical properties. A bio-oil sample derived from rice husk through rapid pyrolysis was fractioned using solvent- or solid-extraction techniques based on their various properties. Ultraviolet-visible spectroscopy, three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy and Fourier transform infrared spectroscopy were used to characterize their various spectral properties for further understanding the characteristics of the bio-oil. Bio-oil mostly contains many aromatic ring components, acidic polar fractions, few weak- and non-polar components. The results all show that the main compounds and functional groups in the various bio-oil fractions were different and depended on the fractionation methods. The compositions of the bio-oil fractions were also analyzed with a gas chromatography/mass spectrometry (GC/MS) method. The consistency of the results obtained from the spectrometric methods with the GC/MS method indicates that the spectrometric methods have a good potential for rapid and effective characterization of bio-oils.     

Effects of rice husk particle size on biohydrogen production under solid state fermentation

As a renewable energy source bio-hydrogen production from lignocellulosic wastes is a promising approach which can produce clean fuel with no CO₂ emissions. Utilization of agro-industrial residues in solid state fermentation (SSF) is offering a solution to solid wastes disposal and providing an economical process of value-added products such as hydrogen.In this study three different particle size of rice husk (<2000 μm, <300 μm, <74 μm) was subjected to batch SSF with a Clostridium termitidis: Clostridium intestinale ratio of 5:1. C. termitidis is a cellulolytic microorganism that has the ability to hydrolyze cellulosic substances and C. intestinale is able to grow on glucose having a potential of enhancing hydrogen production when used in the co-culture. 5 g dw rice husk with 75% humidity was used as substrate in SSF under mesophilic conditions. The highest HF Volume (29.26 mL) and the highest yield (5.9 mL H₂ g⁻¹ substrate) were obtained with the smallest particle size (<74 μm). The main metabolites obtained from the fermentation media were acetic, butyric, propionic and lactic acids. The second best production yield (3.99 mL H₂ g⁻¹ substrate) was obtained with the middle particle size (<300 μm) rice husk with a HF of 19.71 mL.     

Investigation of biomass gasification hydrogen and electricity co-production with carbon dioxide capture and storage

Biomass is one of the renewable energy resources which can be used instead of fossil fuels to diminish environment pollution and emission of greenhouse gases. Hydrogen as a biomass is considered as an alternative fuel which can be derived from a variety of domestically available primary sources. In this paper, a hydrogen and electricity co-generation plant with rice husk is proposed. Rice husk with water vapor and oxygen produces syngas in gasifier. In this design, electricity is generated by using two Rankine cycles. The Results show that the net electric efficiency and hydrogen production efficiency are 1.5% and 40.0%, respectively. Hydrogen production is 1.316 kg/s in case which carbon dioxide is gathered and stored. The electricity generation is 5.923 MW_e. The main propose of implementing Rankine cycle is to eliminate hydrogen combustion for generating electricity and to reduce NO_x production. Furthermore, three kinds of membranes are studied in this paper.     

Physical properties of rice husk and bran briquettes under low pressure densification for rural applications

Agriculture generates large amount of by-products that could be used to produce energy and reduce the amount of fuelwood required to meet the daily cooking needs, especially in developing countries. Rice is a major crop grown in West Africa and rice husk is a by-product of the milling process. The goal of this study was to develop a low cost system to produce biomass briquettes from rice husks in the context of a rural village. A manual press generating a pressure of 4.2 MPa was developed and used. The influence of the briquette formulation (type of binder, binder content, water addition, and bran content) was studied. The binders investigated were cassava wastewater, rice dust, and okra stem gum. The physical properties (density, moisture content, calorific value, durability, and compressive strength) were tested to identify the briquettes with the highest quality, i.e. greatest physical integrity. The briquettes made with rice dust had the highest durability (91.9%) and compressive strength (2.54 kN), while the briquettes made with cassava starch wastewater had the greatest density (441.18 kg m⁻³). Water added to the rice husk before densification positively influenced the briquette quality while bran seemed to mostly increase the density, but not necessarily the briquette quality. The briquette formulation did not significantly influence the calorific value. With a higher heating value of 16.08 MJ kg⁻¹ dry basis, rice husk briquettes represent an interesting alternative to fuelwood.     

Combustion behavior in a dual-staging vortex rice husk combustor with snail entry

The combustion characteristics of rice husk fuel in a dual-staging vortex-combustor (DSVC) are experimentally investigated. In the present work, the vortex flow is created by using a snail entrance mounted at the bottom of the combustor. The temperature distributions at selected locations inside the combustor, the flue gas emissions (CO, CO₂, O₂, NO_x), and the combustion/thermal efficiency are monitored. Measurements are made at a constant rice husk feed rate of 0.25 kg/min with various excess airs (37%, 56%, 74% and 92%) and different secondary air injection fractions (λ = 0.0, 0.15 and 0.2), respectively. The combustion chamber is 1800 mm high and 300 mm in diameter (D) with a centered exhausted pipe while the middle chamber of the combustor is set to 0.5D. The smaller section at the middle chamber is introduced to split the chamber to be dual-staging chamber where a large central toroidal recirculation zone induced by swirl flow through the small section is generated in the top chamber. The experimental results reveal that the highest temperature inside the combustor is about 1000 °C whereas both the thermal and the combustion efficiency are 41.6% and 99.8% for 74% excess air without the secondary air injection (λ = 0.0). In addition, the emissions are CO₂ = 8.1%, O₂ = 9.3%, CO = 352 ppm, NO_x = 294 ppm and small amount of fly ash. Therefore, the DSVC shows an excellent performance, low emissions, high stabilization and ease of operation in firing the rice husk.     

Kinetics of rice husk char gasification in an H2O or a CO2 atmosphere

Rice husk gasification has been attracting increasing attention in rice-producing countries, but the technology has not yet achieved optimal efficiency. Only a few studies have reported on the gasification kinetics of rice husk char, and the influence of some important parameters has not yet been investigated. This paper provides an experimental database and kinetic models of gasification of a rice husk char particle in an H₂O or a CO₂ atmosphere. A complete parametric study of rice husk char gasification was performed in a wide range of operating conditions, relevant to those that exist in industrial gasifiers. Two kinetic models were developed to predict the conversion of a particle, taking into account changes in the reactive surface. Results of this study could help researchers and engineers in the design, modeling or optimization of a new efficient rice husk gasifier.     

The effect of oyster shell powder and rice husk ash on the pyrolysis of rice husk for bio-oil

The aim of this study was to investigate the effect of oyster shell powder (OSP) and rice husk ash (RHA) on the pyrolysis of rice husk (RH) for bio-oil. The present study focuses on the effect of catalysts on pyrolysis of RH for bio-oil and the quantity of bio-oil produced. The results showed that both OSP and RHA could improve the yield and quality of bio-oil, and the catalytic effect of OSP was better than that of RHA. With the content of the two catalysts increased, the net increase range of bio-oil yield decreased gradually. With 3 wt.% of OSP or 2 wt.% of RHA, the yield of bio-oil achieved to 57.06% and 56.07% respectively, which increased by 6.03% and 4.20% compared to that of single pyrolysis of rice husk. Both OSP and RHA can increase the bio-oil heating value and decrease the acid value. With the presence of 1–5 wt.% of OSP or RHA in the RH pyrolysis process, the heating value of the bio-oil can be increased by 5.04–10.25% and 4.32–5.78%, the acid value of the bio-oil can be decreased by 5.30–13.54% and 9.81–33.01%, respectively. OSP was better than RHA on the heating value improvement, while RHA was superior to OSP in decreasing the acid value. The gas chromatography/mass spectrometry (GC-MS) analysis of bio-oil composition indicated that the formation of phenols, acids and ketones compounds were inhibited and alcohols and furan compounds were promoted with the addition of OSP and RHA catalysts. The study made the catalytic pyrolysis process more favorable for the production of high heating value fuel.     

Urea-assisted synthesis towards a renewable rice husk silica-derived Ni-phyllosilicate catalyst for CO2 methanation

In this work, nickel phyllosilicate was prepared through the hydrothermal reaction of rice husk-derived silica (SR) and nickel nitrate. Owing to the loss of surface silanol group on SR during the calcination process, a small amount of Ni-phyllosilicate with Ni content of 10.2 wt% was obtained on N₂₂₀/SR, which was prepared under a very severe hydrothermal condition of 220 °C for 48 h. After the addition of urea, the dense flower-like nanosheets attributing to Ni-phyllosilicate were observed on the surface of N₁₈₀/SR-U-24 with high Ni content of 22.6 wt%, which was prepared under a much milder hydrothermal temperature of 180 °C and a shorter reaction time of 24 h. This was because urea could facilitate the formation of an important intermediate (Ni(OH)₂) and leaching of SiO₂, resulting in the quick formation of Ni-phyllosilicate. N₁₈₀/SR-U-24 exhibited both good catalytic activity and high long-term stability for CO₂ methanation due to the relatively high Ni content, fine Ni particles and the strong metal-support interaction derived from Ni-phyllosilicate.     

Experimental investigation of combustion characteristics in a multi-staging vortex combustor firing rice husk

The paper described the combustion characteristics in a multi-staging vortex combustor by using rice husk as fuel. Effects of the operating conditions namely: equivalence ratio (Φ = 0.8, 1.0 and 1.2) and secondary air ratio (λ = 0.0, 0.15 and 0.25) on combustion characteristics (temperature distribution, fly ash and gas emission) were experimentally studied. In the experiments, the conventional vortex combustor consisted of two straight concentric cylindrical pipes, combustion chamber (outer chamber) and exhaust pipe (inner chamber). The variable size of middle section of the combustor was designed to be adjustable from 1.0D (conventional vortex combustor), to 0.75D and 0.5D as desired. The changes of the middle chamber size lead to multi-staging vortex inside the combustor. In the experiments, the rice husk was fed into the combustor at constant mass flow rate of 0.3 kg/min. Test results revealed that the mean temperature distribution for the multi-staging vortex combustor with middle chamber size of 0.5D was higher than those of 0.75D and 1.0D. The experimental results showed the maximum temperature of about 1176 °C in the vortex chamber with the middle chamber of 0.5D at equivalence ratio, Φ = 0.8 and no secondary air injection, λ = 0.0. Measurements of gas emissions from cyclone collector consisted of O₂ = 2.5%, CO₂ = 17.3%, and CO = 270 ppm, respectively.