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.     

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.     

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.     

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.     

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.     

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.     

Comparisons of the hydrogen-rich syngas compositions from wet rice husk slurry steam reforming reactions using different catalysts

Rice husk slurry is pumped into a packed reactor and the products from the steam reforming reactions using different catalysts are studied. The steam/biomass weight ratio of such a system is between 3.47 and 5.25. The solids, liquid and gaseous products are a mass fraction of 2.8-4.1%, a mass fraction of 92.4-93.0% and a mass fraction of 3.5-4.7%, respectively. The hydrogen concentration in the gaseous product is approximate a volume fraction of 41% using the Al₂O₃ catalyst of a CuO mass fraction of 13%, a volume fraction of 38% using the Al₂O₃ catalyst of a Ni mass fraction of 13%, a volume fraction of 31% using the Al₂O₃ catalyst of a ZnO mass fraction of 13%, and a volume fraction of 20% using the Al₂O₃ catalyst at the reactor temperature of 800 °C. In the reactor temperature range studied (350-800 °C), the hydrogen concentration in the product stream increases monotonically with the increasing of the reactor temperature and the steam/carbon molar ratio. The value of dry gas LHV is between 9.4 MJ m⁻³ and 12 MJ m⁻³ at the reaction temperature of 600-800 °C. Considering the simple catalyst used in current study, the syngas of a hydrogen volume fraction of approximate 40% is obtained by pumping the biomass slurry to carry out the catalytic steam reforming reaction.     

Influence of impregnated copper and zinc on the pyrolysis of rice husk in a micro-fluidized bed reactor: Characterization and kinetics

Catalytic performance of Cu and Zn catalysts was investigated during rice husk (RH) high-temperature pyrolysis under isothermal conditions in a micro-fluidized bed reactor. The results showed that the presence of Cu and Zn evidently influenced the release characteristics and conversion of the gas components. The impregnated Cu promoted the conversion of H₂, CH₄, CO and CO₂, while Zn showed positive catalytic effect on the conversion of H₂, CH₄ and CO₂ and negative effect on the conversion of CO. The X-ray diffraction patterns of the residue chars revealed that metallic copper nanoparticles (Cu⁰) were formed during Cu impregnated biomass pyrolysis. Textural characterization and SEM images showed that the impregnation of Cu and Zn, particularly Zn, promoted the generation of micropores and mesopores, with the pore sizes predominantly at around 1.3 nm and 3.9 nm. Reaction kinetics for generating these gases was studied based on model fitting method, and the most probable reaction mechanism was obtained based on the relative error between experimental and calculated conversion data. The resulting apparent activation energies were 85.08, 12.56, 49.72 and 38.37 kJ/mol for the formation of H₂, CO, CH₄ and CO₂ from pure RH pyrolysis. The presence of Cu decreased the forming activation energies of the four gases, and Zn decrease the forming activation energies of H₂, CH₄ and CO₂ while increased the value for the formation of CO.     

稻壳在循环流化床中燃烧现象的分析

为了研究稻壳在循环流化床中的燃烧特性，在生物质循环流化床试验台上对稻壳进行了燃烧试验。通过对试验过程中各测点温度及压力变化的分析，探讨了二次风和循环回料对稻壳在循环流化床燃烧炉内燃烧过程的影响。试验结果表明：二次风可以促进挥发分在稀相区的燃烧，对提升稀相区的温度作用明显；正常循环回料使得温度沿炉膛高度均匀分布。所得结论对生物质循环流化床的试验研究及实际运行有一定的参考意义。     

A study on the consecutive preparation of silica powders and active carbon from rice husk ash

Rice husk ash (RHA) is an abundant agricultural by-product. The present research work deals with the production of silica powders and active carbon from RHA with a consecutive method. The RHA is firstly treated with acid leaching to remove mineral composition, and then is boiled with base to leach silica. The filtrate is used to synthesize silica powders with CO₂ precipitator and solid residue is used to prepare active carbon. The optimum conditions of preparing silica powders are as follows: the concentration of Na₂CO₃ is 25 wt.%, the base-leached time is 4 h, and the impregnation ratio of Na₂CO₃ solution to RHA is 6:1. The yield of silica leached from RHA is 84.57 wt.%. The synthesized silica powders are hydrated with amorphous structure, moreover, with a relative smooth surface and high purity. The residue is activated with potassium hydroxide (KOH) after base-leached. The activated carbons are found to be a mixture of micropore and mesopore pore structures. The maximum pore volume, BET surface area and iodine adsorption capacity of as-prepared active carbon can reach 1.22 cm³/g, 1936.62 m²/g and 1259.06 mg/g, respectively. Field emission scanning electron microscopy (SEM) is used to characterize the morphological features of the ash after step by step treatment.     

Feasibility study of Jatropha seed husk as an open core gasifier feedstock

This paper presents the results of investigation carried out in studying the fuel properties of Jatropha seed husk and its gasification feasibility for open core down draft gasifier. Jatropha seed husk was converted to producer gas in an open core down draft gasifier whose performance was evaluated in terms of fuel consumption rate, calorific value of producer gas and gasification efficiency at different gas flow rates. It was found that producer gas calorific value and concentration of CO, along with gasification efficiency, in general, increased with the increase in gas flow rate. The maximum gasification efficiency was found to be 68.31% at a gas flow rate of 5.5 m³ h⁻¹ and specific gasification rate of 270 kg h⁻¹ m⁻². Studies revealed that Jatropha seed husk could successfully be used as feedstock for open core down draft gasifier.     

Converting rice husk activated carbon into active material for capacitor using three-dimensional porous current collector

We have successfully applied rice husk activated carbon (RHAC) as an active material for the electric double layer capacitor using a three-dimensional (3D) porous current collector. The capacity and cycle stability were evaluated in a 1.0 mol dm⁻³ tetraethylammonium tetrafluoroborate/propylene carbonate solution in the range of 0-2.5 V. The specific capacity of the RHAC was about 14 mAh g⁻¹ at the 50 mA g⁻¹ discharge rate, corresponding to 19 F g⁻¹ under the present conditions. The RHAC cell using the 3D porous current collector possessed a lower internal resistance and better high-rate discharge properties than the RHAC cell using a conventional aluminum (Al) foil collector. After 5000 cycles of charging and discharging, the RHAC cell with the 3D current collector maintained 95% of its initial capacity, while the capacity of the one with the Al foil collector dropped to only 30%.     

Removal of Cu(II) from aqueous solution using the rice husk carbons prepared by the physical activation process

The adsorption of Cu(II) from aqueous solution by carbons prepared from rice husk through pyrolysis and steam activation was studied. The rice husk carbon was characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), and its pore structure was also examined. After comparing different characteristics of the carbons prepared under different conditions and their adsorption abilities of Cu(II), the optimum temperature for pyrolysis and steam activation was chosen as 700 and 750 °C, respectively, using 3% (V/V) steam as the best activation gas. It was found that the Cu(II) adsorption on the rice husk derived carbons was pH and temperature dependent with an optimum pH value of 5.0, and an equilibrium time of 24 h. The adsorption kinetics and isotherms of Cu(II) by the rice husk derived carbons were also investigated under four different temperatures, and good correlation coefficients were obtained for the pseudo-second-order kinetic models, and the Langmuir isotherm model fitted very well with the experimental data. The mean free energy E (kJ mol⁻¹) obtained in the Dubinin-Radushkevitch (D-R) adsorption isortherm equation indicated a chemical ion-exchange mechanism. Several thermodynamic parameters were also caculated to predict the nature of adsorption process.