A downdraft high temperature steam-only solar gasifier of biomass char: A modelling study

A numerical model of a solar downdraft gasifier of biomass char (biochar) with steam based on the systems kinetics is developed. The model calculates the dynamic and steady state profiles, predicting the temperature and concentration profiles of gas and solid phases, based on the mass and heat balances. The Rosseland equation is used to calculate the radiative transfer within the bed. The char reactivity factor (CFR) is taken into account with an exponential variation. The bed heating dynamics as well as the steam velocity effects are tested. The model results are compared with different experimental results from a solar packed bed gasifier, and the temperature profile is compared to an experimental downdraft gasifier. Hydrogen is the principal product followed by carbon monoxide, the carbon dioxide production is small and the methane production is negligible, indicating a high quality syngas production. By applying the temperature gradient theory in the steam-only gasification process for a solar gasifier design, a solar downdraft gasifier improves the energy conversion efficiency by over 20% when compared to a solar packed bed gasifier. The model predictions are in good agreement with the experimental results found in the literature.     

A steady state model of gas-char reactions in a downdraft biomass gasifier

A phenomenological model of downdraft gasification under steady state operation is developed based on previously published values for the reaction kinetics in the reduction zone. The model predicts a product gas with a composition similar to that found experimentally, although the model over-predicts the methane concentration. The accuracy of the model is limited by the availability of data on the initial conditions at the top of the reduction zone.     

Experimental study on non-woody biomass gasification in a downdraft gasifier

The current paper concerns the process of non-woody biomass gasification, particularly about releasing processes of detrimental elements. The gasification of corn straw was carried out in a downdraft fixed bed gasifier under atmospheric pressure, using air as an oxidizer. The effects of the operating conditions on gasification performance in terms of the temperature profiles of the gasifier, the composition distribution of the producer gas and the release of sulphur and chlorine compounds during gasification of corn straw were investigated. Besides, the gasification characteristics were evaluated in terms of low heating value (LHV), gas yield, gasification efficiency and tar concentration in the raw gas.     

Gasification and power generation characteristics of woody biomass utilizing a downdraft gasifier

Energy utilization from biomass resources has started to attract public attention as a method to reduce CO₂ emissions. In this study, the characteristics of syngas production from biomass gasification were investigated in a downdraft gasifier that was combined with a small gas engine system for power generation. Syngas temperatures from the gasifier were maintained at a level of 700-1000 °C. When the air ratio for gasification was 0.3-0.35, the low heating value of syngas was 1100-1200 kcal Nm⁻³ and the cold gas efficiency 69-72%. Tar concentration in raw syngas was around 3.9-4.4 g Nm⁻³. Syngas combustion in the gas engine after purification showed that HC concentration was below 200 ppm, and NOx concentration was below 40 ppm in the exhaust gas.     

Assessment of cow dung as a supplementary fuel in a downdraft biomass gasifier

A model of downdraft gasifier has been described considering thermodynamic equilibrium of species in the pyro-oxidation zone and kinetically controlled reduction reactions in the reduction zone. It is found that the sole use of cow dung as the gasifier fuel is not technically feasible. This is due to very low heating value of the producer gas with much carbon leaving the gasifier as char. However, cow dung can be used as a supplementary fuel blended with a conventional woody biomass, like sawdust. The increased fraction of cow dung in the fuel blend renders the gasification process less efficient, when the gasifier is operated at a particular equivalence ratio. Both the producer gas production rate and its heating value reduce with the increase in the cow dung content in the biomass fuel blend, leading to an overall reduction in the gasifier conversion efficiency. It is observed that an increase in the cow dung content from 0 to 90% in the blended fuel reduces the heating value by 46.8% and the conversion efficiency by 45%. The use of cow dung in between 40 and 50% by mass in the fuel mix would result in an overall fuel economy.     

Results with a bench scale downdraft biomass gasifier for agricultural and forestry residues

A small scale fixed bed downdraft gasifier system to be fed with agricultural and forestry residues has been designed and constructed. The downdraft gasifier has four consecutive reaction zones from the top to the bottom, namely drying, pyrolysis, oxidation and reduction zones. Both the biomass fuel and the gases move in the same direction. A throat has been incorporated into the design to achieve gasification with lower tar production. The experimental system consists of the downdraft gasifier and the gas cleaning unit made up by a cyclone, a scrubber and a filter box. A pilot burner is utilized for initial ignition of the biomass fuel. The product gases are combusted in the flare built up as part of the gasification system. The gasification medium is air. The air to fuel ratio is adjusted to produce a gas with acceptably high heating value and low pollutants. Within this frame, different types of biomass, namely wood chips, barks, olive pomace and hazelnut shells are to be processed. The developed downdraft gasifier appears to handle the investigated biomass sources in a technically and environmentally feasible manner. This paper summarizes selected design related issues along with the results obtained with wood chips and hazelnut shells.     

First and second law thermodynamic analysis of a single and dual-stage ignition biomass downdraft gasifier

The dual-stage ignition biomass downdraft gasifier is an enormous tar reduction technology as against a single-stage ignition biomass gasification. Exergetic analysis of the system guides toward a possible performance enhancement. In dual-stage gasification, around 67.76% of input exergy is destructed in the several components, while 9.16% is obtained as a useful exergy output and 24.34% is found to be as a useful energy output there. The entire unit was assessed with a progressively rising electric load from 15.24 kW to 38.86 kW. The enhanced producer gas quality comes from 57% combustible gas with a higher heating value of 6.524 MJ/Nm³ and tar content of 7 mg/Nm³ after the paper filter, whereas the biomass consumption rate is 58 kg/h at the greatest load with the grate temperature of 1310–1370 °C. The samples of exhaust gas emissions are obtained environmentally favorable. The results even described that the dual-stage ignition biomass downdraft gasifier has significantly greater energetic and exergetic efficiency as compared to the single-stage gasifier.     

Hydrogen-rich gas production from biomass air and oxygen/steam gasification in a downdraft gasifier

Biomass gasification is an important method to obtain renewable hydrogen. However, this technology still stagnates in a laboratory scale because of its high-energy consumption. In order to get maximum hydrogen yield and decrease energy consumption, this study applies a self-heated downdraft gasifier as the reactor and uses char as the catalyst to study the characteristics of hydrogen production from biomass gasification. Air and oxygen/steam are utilized as the gasifying agents. The experimental results indicate that compared to biomass air gasification, biomass oxygen/steam gasification improves hydrogen yield depending on the volume of downdraft gasifier, and also nearly doubles the heating value of fuel gas. The maximum lower heating value of fuel gas reaches 11.11 MJ/N m³ for biomass oxygen/steam gasification. Over the ranges of operating conditions examined, the maximum hydrogen yield reaches 45.16 g H₂/kg biomass. For biomass oxygen/steam gasification, the content of H₂ and CO reaches 63.27–72.56%, while the content of H₂ and CO gets to 52.19–63.31% for biomass air gasification. The ratio of H₂/CO for biomass oxygen/steam gasification reaches 0.70–0.90, which is lower than that of biomass air gasification, 1.06–1.27. The experimental and comparison results prove that biomass oxygen/steam gasification in a downdraft gasifier is an effective, relatively low energy consumption technology for hydrogen-rich gas production.     

2-D Modeling of thermo-kinetics coupled with heat and mass transfer in the reduction zone of a fixed bed downdraft biomass gasifier

A two dimensional modeling is developed in the reduction zone of a fixed bed downdraft biomass gasifier based on mass, energy and momentum conservation equations written for the solid and fluid phases and coupled with chemical kinetics. Kinetics parameters are derived from previous works and an effectiveness factor was used in the reaction rate correlation to quantify the mass transfer resistance in the bed. The obtained numerical results are compared with experimental and numerical data from literature and a reasonable agreement is observed. Fields of temperature, gaseous concentrations are investigated for the two-dimensional domain. Results show that the solid and fluid inlet temperatures to the reduction zone and the reactivity of the bio-char including the effectiveness factor are the main variables affecting the conversion of char to syngas in the gasification zone of the fixed bed reactor.     

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.     

两级下吸式生物质气化炉气化性能的研究

在自行设计的两级下吸式生物质气化炉中,研究了空气当量比(ER)对气体组成、气体热值、气化效率以及焦油含量的影响。试验结果表明,该新型两级气化炉能够产生焦油含量较低的燃气;在空气预热的条件下,焦油含量更低,可达238 mg/m3。该新型两级气化炉的最佳ER为0.33~0.35,当ER=0.34时,气化气低位热值(LHV)最高为4 409 kJ/m3,气化效率为63.7%,焦油含量低于300 mg/m3。     

Thermodynamic equilibrium model and second law analysis of a downdraft waste gasifier

The management of municipal solid waste (MSW) and the current status of world energy resources crisis are important problems. Gasification is a kind of waste-to- energy conversion scheme that offers the most attractive solution to both waste disposal and energy problems. In this study, the thermodynamic equilibrium model based on equilibrium constant for predicting the composition of producer gas in a downdraft waste gasifier was developed. To enhance the performance of the model, further modification was made by multiplying the equilibrium constants with coefficients. The modified model was validated with the data reported by different researchers. MSW in Thailand was then used to simulate and to study the effects of moisture content (MC) of the waste on the gasifier's performance. The results showed that the mole fraction of H₂ gradually increases; CO decreases; CH₄, which has a very low percentage in the producer gas increases; N₂ slightly decreases; and CO₂ increases with increasing MC. The reaction temperature, the calorific value, and the second law efficiency, decrease when MC increases.     

Gasification of biomass: comparison of fixed bed and fluidized bed gasifier

Gasification as a thermochemical process is defined and limited to combustion and pyrolysis. A systematic overview of reactor designs categorizes fixed bed and fluidized bed reactors. Criteria for a comparison of these reactors are worked out, i.e. technology, use of material, use of energy, environment and economy. A utility analysis for thermochemical processes is suggested. It shows that the advantages of one of the reactor types are marginal. An advantage mainly depends on the physical consistency of the input. As a result there is no significant advantage for the fixed bed or the fluidized bed reactor.     

A numerical model of a fluidized bed biomass gasifier

A one-dimensional, steady state, numerical model was developed for a fluidized bed biomass gasifier. The gasifier model consists of a fuel pyrolysis model, an oxidation model, a gasification model and a freeboard model. Given the bed temperature, ambient air flow rate and humidity ratio, fuel moisture content and reactor parameters, the model predicts the fuel feed rate for steady state operation, composition of the producer gas and fuel energy conversion. The gasifier model was validated with experimental results. The effects of major mechanisms (fuel pyrolysis and the chemical and the physical rate processes) were assessed in a sensitivity study of the gasification model. A parametric study was also conducted for the gasifier model. It is concluded that the model can be used for gasifier performance analysis.     

Kinetic modeling and optimization of parameters for biomass pyrolysis: A comparison of different lignocellulosic biomass

A primitive element for the development of sustainable pyrolysis processes is the study of thermal degradation kinetics of lignocellulosic waste materials for optimal energy conversion. The study presented here was conducted to predict and compare the optimal kinetic parameters for pyrolysis of various lignocellulosic biomass such as wood sawdust, bagasse, rice husk, etc., under both isothermal and non-isothermal conditions. The pyrolysis was simulated over the temperature range of 500–2400 K for isothermal process and for heating rate range of 25–165 K/s under non-isothermal conditions to assess the maximum pyrolysis rate of virgin biomass in both cases. Results revealed that by increasing the temperature, the pyrolysis rate was enhanced. However, after a certain higher temperature, the pyrolysis rate was diminished which could be due to the destruction of the active sites of char. Conversely, a decrease in the optimum pyrolysis rate was noted with increasing reaction order of the virgin biomass. Although each lignocellulosic material attained its maximum pyrolysis rate at the optimum conditions of 1071 K and 31 K/s for isothermal and non-isothermal conditions, respectively, but under these conditions, only wood sawdust exhibited complete thermal utilization and achieved final concentrations of 0.000154 and 0.001238 under non-isothermal and isothermal conditions, respectively.     

Development of a novel updraft multifuel biomass gasifier

The paper reports the development of a novel multifuel biomass reactor for direct heating applications such as water heating, air heating or steam generation. Studies were conducted on the reactor to maximize its heat release rate by evaluation of the sensitivities of the fuel and charging parameters to firing rate. Maximum firing rates measured for the different fuels were in the range of 8–43 kg/h for the natural draft mode and 45–130 kg/h for the forced draft mode. It is observed that the heat release rate is enhanced by up to 360% by preheating of the charge, increase of the surface-area/volume ratio of the fuel charge, decrease of the charge per feed, and adoption of forced draft mode. For wood, a fuel preheat to 80°C, a surface-area/volume ratio of 600 m^?1 and a charge per feed of 0.5–1.0 kg give a higher heat release and minimize gas temperature fluctuations.     

Experimental study on biomass gasification in a double air stage downdraft reactor

This work presents an experimental study of the gasification of a wood biomass in a moving bed downdraft reactor with two-air supply stages. This configuration is considered as primary method to improve the quality of the producer gas, regarding its tar reduction. By varying the air flow fed to the gasifier and the distribution of gasification air between stages (AR), being the controllable and measurable variables for this type of gasifiers, measuring the CO, CH₄ and H₂ gas concentrations and through a mass and energy balance, the gas yield and its power, the cold efficiency of the process and the equivalence ratio (ER), as well as other performance variables were calculated. The gasifier produces a combustible gas with a CO, CH₄ and H₂ concentrations of 19.04, 0.89 and 16.78% v respectively, at a total flow of air of 20 Nm³ h⁻¹ and an AR of 80%. For these conditions, the low heating value of the gas was 4539 kJ Nm⁻³. Results from the calculation model show a useful gas power and cold efficiency around 40 kW and 68%, respectively. The resulting ER under the referred operation condition is around 0.40. The results suggested a considerable effect of the secondary stage over the reduction of the CH₄ concentration which is associated with the decreases of the tar content in the produced gas. Under these conditions the biomass devolatilization in the pyrolysis zone gives much lighter compounds which are more easily cracked when the gas stream passes through the combustion zone.     

Combined-gasification of biomass and municipal solid waste in a fluidized bed gasifier

Due to the environmental problems associated with burning of fossil fuels and population growth, more attention has been paid to develop renewable energies in recent years. Among all options for renewable energy utilization, biomass gasification is more popular because of environmental benefits and economic issues. In the present study, a series of experiments were carried out to study the influence of blending ratio, reaction temperature, equivalence ratio (ER) on co-gasification characteristics of pine sawdust (SD) and municipal solid waste (MSW). By increasing the blending ratio from 100% SD to 100% MSW, CO and CH₄ respectively increased from 16.7 to 18.8 vol% and from 4.1 to 5.1 vol%, while an opposite trend was found for H₂ and CO₂. Over the ranges of the experimental conditions used, the tar content and gas yield varied from 5.4 to 10.1 g/Nm³ and 1.34 to 1.15 Nm³/kg, respectively.     

Numerical modeling of a steam-assisted tubular coal gasifier

Understanding the heat and mass transfer phenomena in a coal gasifier is very useful for the assessment of gasifier performance and optimization of the design and operating parameters. In this paper, performance of an entrained flow air blown laboratory scale gasifier is numerically simulated with Fluent software. In the model, the continuous phase conservation equations are solved in an Eulerian frame, while those of particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The dispersion of the particles due to turbulence is predicted using a stochastic tracking model, in conjunction with the k–ε equations for the gas phase. The coal gasification model adopted includes devolatilization, combustion of volatiles, char combustion and gasification. The gasification performance inside the gasifier has been predicted for different air ratios as well as for different air and steam inlet temperatures. The overall temperature inside the gasifier is found to increase when the degree of air/steam pre-heating is increased, resulting in acceleration of the different reaction steps in the gasifier. The overall gasification performance indices such as carbon conversion, heating value of the exit gas and cold gas efficiency have been predicted. The predicted results show good agreement with available experimental data in literature.