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.     

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.     

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.     

Characteristics of cyclone gasification of rice husk

The gasification characteristics of the rice husk were studied in a cyclone gasifier using air as the gasifying medium to generate the fuel gas with available heating value and less tar content. The influence of equivalence ratio on temperature profiles, composition and low heating value of the produced gas, tar content, carbon conversion and cold gas efficiency was investigated. The equivalence ratios considered in this study were 0.20–0.32. The results show that the optimal equivalence ratio is 0.29 and the maximum temperature of gasification should be lower than 1000 °C. In order to optimize the performance of the cyclone gasifier, the main body of the gasifier was lengthened and air staged gasification was carried out. The low heating value of the produced gas, carbon conversion, cold gas efficiency and tar content are 4.72 MJ/Nm³, 57.5%, 37.3% and 1.85 g/Nm³, respectively.     

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.     

Rice husk combustion evolved gas analysis experiments and modelling

Rice husk is a major agricultural waste which could be a major source of fuel for boilers and furnaces if its calorific value could be realized efficiently. The oxidation kinetics of rice husks combustion were investigated using an evolved gas analysis technique. Rice husk samples were heated from 100 °C to 500 °C at a constant rate inside a small pressurised reactor. An oxygen-containing gas was passed through the reactor at a controlled flow rate and the evolved gas was continually analysed for its oxygen, carbon monoxide and carbon dioxide contents after moisture had been removed. A model for the oxidation of the rice husks samples is proposed that considers that the many simultaneous and competing oxidation reactions may be adequately represented by grouping them into three overlapping and competing reaction regimes in which CO₂, CO and H₂O are the only reaction products. The activation energies, and peak oxygen consumption temperatures were all found to be linear functions of the oxygen partial pressure in the reactor. Increasing the oxygen partial pressure decreased the temperatures at which peak oxygen consumption occurred. The total system pressure had no effect on the combustion behaviour other than through the oxygen partial pressure. At a heating rate of 80 K h⁻¹ and a system pressure of 500 kPa values for E/R for the low temperature, medium temperature and high temperature oxidation reactions are 14.7, 19.2 and 17.4 respectively.     

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.     

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.     

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.     

稻壳和褐煤热解特性的初步研究

利用TG-DSC联用分析稻壳与褐煤热解过程中热失重规律及吸放热情况,结果发现,稻壳的热失重率较大,共热解失重过程相当于二者单独热失重过程的叠加。通过DSC曲线分析稻壳与褐煤热解过程的吸放热量显示,与二者单独热解过程不同的是共热解在高温热分解阶段须吸收大量的热量。利用气相色谱分析不同温度下稻壳与褐煤热解气体产物各组分比例,并与热失重过程相对应分析气体产物变化规律,结果发现,H2和CH4气体组分变化规律相同;与褐煤热解相比,稻壳热解气体产物中CO气体组分较多。总体而言,共热解产物是二者单独热解产物的简单加和,但共热解过程吸放热量变化却显示二者存在热量交换和相互影响。     

Application of Nagchampa biodiesel and rice husk gas as fuel

This paper elaborates the application of Nagchampa biodiesel and rice husk producer gas in dual mode using rice husk producer gas. Nagchampa biodiesel was examined under both single and dual fuel mode having a persistent gas flow rate of 21.69 kg/h at discrete loads. The test results disclosed that the engine shows improved performance and emission characteristics at optimum load of 8 kW. There is a decrease in NO_x, smoke opacity, and performance, while HC, CO, and CO₂ increase moderately with respect to gas flow rate. It can finally be concluded that biodiesel exhibits lower performance and improved emission than that of neat diesel.     

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.     

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.     

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%.     

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.     

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.     

Gaseous mercury removal using biogenic porous silica modified with potassium bromide

Rice husk, as an abundant agricultural waste and renewable biomass, is adopted to produce porous silica which is used as a potential carbon-free adsorbent for mercury emission control of coal-fired power plants. The rice husk derived porous silica (RHS) exhibits a good Hg⁰ adsorption capability with Hg⁰ removal efficiency above 80% in the temperature range of 60–140 °C, which is plausibly due to the big specific surface area and mesoporous structure. The mercury removal performance of RHS can be evidently reinforced by KBr modification. The optimal KBr loading value and reaction temperature are 2 wt% and 140 °C, respectively. Acidic gas components, such as NO and SO₂, both show slight inhibitive effects on Hg⁰ adsorption process probably owing to the competitive adsorption or the elimination of adsorption sites.     

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.     

Effect of combining Al,Mg, Ce or La oxides to extracted rice husk nanosilica on the catalytic performance of NiO during COx-free hydrogen production via methane decomposition

Amorphous nanosilica powder was extracted from rice husk and used as a catalyst support as well as a starting material for the preparation of different binary oxides, i.e., SiO₂Al₂O₃, SiO₂MgO, SiO₂CeO₂ and SiO₂La₂O₃. A series of supported nickel catalysts with the metal loading of 50 wt % were prepared by wet impregnation method and evaluated in methane decomposition to “CO_x-free” hydrogen production. The fresh and spent catalysts were extensively characterized by different techniques. Among the evaluated catalysts, both Ni/SiO₂Al₂O₃ and Ni/SiO₂La₂O₃ catalysts were the most active with an over-all H₂ yield of ca. 80% at the initial period of the reaction. This distinguishable higher catalytic activity is mainly referred to the presence of free mobile surface NiO and/or that NiO fraction weakly interacted with the support easily reducible at low temperatures. The Ni/SiO₂CeO₂ catalyst has proven a great potential for application in the hydrogen production in terms of its catalytic stability. The formation of Mg_xNi_(1?x)O solid solution caused the Ni/SiO₂MgO catalyst to lose its activity and stability at a long reaction time. Various types of carbon materials were formed on the catalyst surface depending on the type of support used. TEM images of as-deposited carbon showed that multi-walled carbon nanotubes (MWCNTs) and graphene platelets were formed on Ni/SiO₂, while only MWCNTs were deposited on all binary oxide supported Ni catalysts.