Mathematical modelling of a small biomass gasifier for synthesis gas production

The depletion of fossil fuels coupled with the growing demands of the world energy has ignited the interest for renewable energies including biomass for energy production. A reliable affordable and clean energy supply is of major importance to the environment and economy of the society. In this context, modern use of biomass is considered a promising clean energy alternative for the reduction of greenhouse gas emissions and energy dependency. The use of biomass as a renewable energy source for industrial application has increased over the last decade and is now considered as one of the most promising renewable sources. The direct combustion of biomass in small scales often results in incomplete and inconsistent burning process which could produce carbon monoxide, particulates and other pollutants. Therefore, biomass is required to be transformed into more easily handled fuel such as gases, liquids and charcoal using technologies such as pyrolysis, gasification, fermentation, digestion etc. Biomass gasification upon which this thesis focuses is one of the promising routes amongst the renewable energy options for future deployment. Gasification is a process of conversion of solid biomass into combustible gas, known as producer gas by partial oxidation. This research work is carried out to investigate various methods employed for modelling biomass gasifiers, it also studies the chemistry of gasification and reviews various gasification models. In this work, a mathematical model is developed to simulate the behaviour of downdraft gasifiers operating under steady state and determine the synthesis gas composition. The model distinctly analyses the processes in each of the three zones of the gasifier; pyrolysis, oxidation and reduction zones. Air is used as the gasifying agent and is introduced into the pyrolysis and oxidation zones of the gasifier for both single and double air operations. These zones have been modelled based on thermodynamic equilibrium and kinetic modelling; the model equations are solved in MATLAB. Given the biomass properties, consumption, air input, moisture content and gasifier specifications, the MATLAB model is able to accurately predict the temperature and distribution of the molar concentrations of the synthesis gas constituents. The downdraft gasifier is also represented in Aspen HYSYS based on the same models to study the effect of both single and double air gasification operation. For known biomass properties, consumption, air input, moisture content and gasifier operating conditions, the Aspen HYSYS model can accurately predict the distributions of the molar concentrations of the syngas constituents (CO, CO2, H2, CH4, and N2). The models were validated by comparing obtained theoretical results with experimental data published in the open literature. Parametric studies were carried out to study the effects of equivalence ratio, moisture content, temperature on the gas compositions and its energy content. The proposed equilibrium model displayed a variable ability for the prediction of various product yields with this being a function of the feedstock studied. It also demonstrated the ability to predict product gases from various biomasses using both single and double stage air input. In the case of gasification with double air stage supply, higher amounts of methane are obtained with specific tendencies of the gases reaching a peak at certain conditions. The kinetic model was partially successful in predicting results and comparable with experimentally published results for a range of conditions. There were discrepancies particularly with CH4 formation and the operating temperatures predictions which were usually consistently lower than those actually measured experimentally. The use of the PFR, however, did show a greater potential for the use in further modelling

Generation of synthesis gas for fuels and chemicals production

Many scientists believe that the oil production will peak in the near future, if the peak has not already occurred. Peak oil theories and uncertain future oil deliveries have stimulated interest in alternative sources of fuel and chemicals. This interest has been enhanced by concerns about energy security and about the climate change caused by emissions of carbon dioxide. The result has been increased interest in substituting fossil fuels with renewable energy sources such as wind, solar and biomass. However, this has proved particularly difficult in the transportation sector. The most likely source of renewable hydrocarbon fuels for transportation is biomass. It comes in many forms, none of which are suitable for direct use in internal combustion engines and gas turbines. Thus the biomass has to be refined to convert its energy into a more usable form. The most versatile conversion of biomass is thermochemical conversion via gasification and downstream synthesis, which allows the production of both fuels and chemicals. In the biomass gasification process, a gasifier converts the solid biomass into a gaseous product known as producer gas. The producer gas contains the desired components carbon monoxide and hydrogen, but it also contains water, carbon dioxide, lower hydrocarbons, tars and impurities that need to be removed from the gas. Reforming the tars and hydrocarbons in producer gas is difficult because of the amount of sulphur present. This thesis investigates the use of reverse-flow reactors to reform the tars and hydrocarbons in biomass generated producer gas.. Reverse-flow reactors operate by periodically reversing the direction of flow to enable high levels of heat recovery. The high heat recovery enables non-catalytic reformers to be operated at efficiencies near that of catalytic reformers. The operation of reverse-flow reactors is investigated experimentally in a tar-cracking reactor using dolomite as bed material and also theoretically using computer models. The investigations show that reverse-flow reactors have great potential, offering a chemically robust alternative to conventional reformers when operating on sulphur-containing biomass-generated producer gas. Furthermore, operation of reverse-flow tar crackers using dolomite as bed material is an efficient and viable solution for tar removal and syngas boosting. The producer gas also contains ammonia in varying amounts depending on the gasifier’s operating parameters and feedstock. Ammonia can be a poison for catalysts and, if the producer gas is burnt, will produce elevated levels of NOX in the flue gas. The selective catalytic oxidation of ammonia in synthesis gas was thus also investigated by experiments on a model synthesis gas. This thesis also covers mass and energy balance calculations to determine the efficiency and economics of synthetic fuels and chemicals plants. Several possible plant configurations were investigated, both stand-alone and integrated. The integration of a pulp and paper mill with a fuel synthesis plant is a very likely scenario as the biomass logistics are already located on-site. Another possible integration scenario involves steel plants, where large quantities of energy-rich gases are handled as off-gases in coke production. Utilisation of this off-gas coupled with biomass gasification was also investigated. In the stand-alone plants, the difference between reverse-flow reformers and conventional non-catalytic reformers was investigated as front-ends to well-head gas upgrading to produce crude oil via the Fischer-Tropsch synthesis. Furthermore a well-to-wheel comparison of synthetic natural gas, methanol, ethanol, dimethyl ether, Fischer-Tropsch diesel and synthetic gasoline was performed. The comparison used woody biomass as feedstock and computed mass and energy balances for complete plants from gasifier to fuel as well as for lignocellulosic ethanol production by fermentation. Efficiency in regard to feedstock to travel distance (Well-to-Wheel) and the cost of transportation was also investigated. Ammonia is one of the most valuable chemicals for modern agriculture. Current production is almost entirely based on fossil fuels. Thus small-scale production of ammonia from renewable feedstocks was also investigated

Study on feasibility of coir dust as feedstock for entrained flow gasification system

Experimental investigations of the influence of equivalence ratio on gas composition, adiabatic flame temperature, calorific value and rate of gas generation were performed using coir dust as feedstock in entrained flow gasification system. Experiments were carried out on a pilot scale (2.5 m high X 0.25 m i.d.) existing entrained flow biomass gasifier installed at IMMT, Bhubaneswar. Model calculations were made to find out the composition and other properties of the gas taking coir dust as feedstock. Results were realized through comparison of output from theoretical as well as experimental value obtained with varying equivalence ratio. The outputs obtained from Entrained flow gasifier are of high quality and the process can be industrialized

Simulation Studies of Biomass Gasification System

Biomass gasification plant with steady state and dynamics modelling exhibits promising prospects for the process modelling of the system. Extensive studies are required in understanding the biomass gasification system using extensive values of parameters. Previous works are only focusing on the experiment in isolated pilot plant while process modelling in steady state system and dynamics state system for hydrogen production using oil palm empty fruit kernel using Aspen Plus is another steps forward in using simulation study. Several variables are been identified that able to increase the production of hydrogen however because of limited time and knowledge the research was done focusing on one variable. Temperature has been manipulated to increase the production of hydrogen plus enhance carbon conversion efficiency. It plays an important role in the gasification reaction as it increase the conversion of hydrogen respect to the biomass used. While, increasing steam-to-biomass ratio increases hydrogen and carbon monoxide production hence decreases carbon dioxide and carbon conversion efficiency. Previous papers show the increment from 80.89% to 82.78% as result of increasing the biomass to steam ratio. This project focused on at least achieving the values above as validation of the simulation studies

Steam Gasification of Biomass Surrogates: Catalyst Development and Kinetic Modelling

This study reports a new fluidizable La2O3 promoted Ni/γ-Al2O3 catalyst. Prepared catalysts are characterized using BET specific surface area, XRD, TPR, TPO, H2-pulse chemisorptions, Pyridine FTIR, NH3-TPD and CO2-TPD. Catalytic steam gasification of biomass surrogates (glucose and 2-methoxy-4-methylphenol) are conducted in a CREC Riser Simulator under the expected conditions of a twin circulating fludized bed gasifier. Catalyst structure-property and structure-reactivity relationships are established using characterization and gasification results. Gasification performance of a catalyst is found to be well-correlated as a function of its Ni dispersion and basicity/acidity ratio. It is hypothesized that acid sites of γ-Al2O3 are responsible for coke deposition via hydrocarbon cracking, whereas basic sites facilitated coke reforming. The relative proportion of octahedral and tetrahedral sites in γ-Al2O3, which is a main determinant of metal-support interaction and acid-base properties, is assessed using H2 TPR and NH3-TPD. A 20% Ni/5% La2O3-γAl2O3 catalyst is developed, in this study, optimizing catalyst formulation and preparation conditions. This catalyst yields a 98.3% carbon conversion of glucose to permanent gases with no tar formation and negligible coke deposition. In the case of 2-methoxy-4-methylphenol gasification, a 89.8% carbon conversion with tar formation reduced to only 5.7% is achieved using this catalyst. The developed catalyst yields a high quality synthesis gas (H2/CO \u3e 2) performing very close of the equilibrium. A mechanistic based kinetic model with statistically significant intrinsic kinetic parameters is also developed and validated using an independent set of experimental results

Simulation Studies of Biomass Gasification System

As the fossil fuels is depleting in this world, renewable energy has become an important source of energy. Biomass gasification is one of the potential technologies that can convert biomass into environmental friendly energy. This technology is the possible method in reducing the emission of CO2 to environment. Palm kernel shell is used as its feedstock because of its ability of producing high production of hydrogen gas. This research paper is to develop a model of biomass gasification system which is located at Block P in University Teknologi PETRONAS. To fulfill this objective, information regarding the operating conditions of the equipment, and process flow diagram of the system need to be gathered. With using Aspen Plus software, a simulation model of biomass gasification is developed in this paper. In this research the temperature and steam to biomass ratio are manipulated to see the effect on gas production

Thermodynamic evaluation of the gasification of municipal solid waste

The dependency on energy use is unavoidable in modern civilization. The burning of fossil fuels for energy use is regarded as one of the human activities that has a harmful environmental impact. Waste to energy is slowly becoming an evident argument that energy can be obtained from waste at a level that is enough to meet energy demands. Waste is viewed as a renewable source of energy and can lower emissions from the greenhouse gas (GHG) and mitigate climate change. The exploitation of municipal solid waste (MSW) can be implemented using various routes, either through thermal or biological conversion. The thermal conversion can be achieved through combustion, gasification, or pyrolysis. This study aimed to evaluate the gasification of municipal solid waste. The investigation focused on the effects the selected operating parameters have on the syngas composition, H2/CO ratio, and calorific value. The selection of the modelling approach focused on the problem statement. It was necessary to use a model that did not have a lot of limitations or relied on the geometry of the gasifier. A mathematical model that could analyse the selected operating parameters of the gasification process was utilized. A step-by-step procedure of the thermodynamic equilibrium model was implemented using MATLAB. The model was validated by comparing the predicted results of this study and empirical data in published literature. The results showed that operating parameters affected the amount of syngas quality, calorific value, and H2/CO ratio. The amount of carbon monoxide and nitrogen reduced with an increase in moisture content, and the amount of carbon dioxide increased with increased moisture content. A small amount of methane was recorded, with increased moisture content. Enhanced temperature brought about increased hydrogen while the amount of nitrogen remained constant. With high temperature, carbon dioxide composition reduced, and just over 1% of methane was recorded. The increased (ER) from 0.2 to 0.6 showed that ER has a notable impact on nitrogen. A sharp increase in nitrogen was noted when the ER increased while the amount of hydrogen and carbon monoxide decreased. Results showed acceptable agreement between the modelled data from this investigation and the experimental values reported in the literature. The overall conclusion is that the thermodynamic model gives accurate prediction results of the gasification process. Additionally, when the investigated operating parameters were adjusted, syngas composition, H2/CO ratio and calorific value were all affected (they either increased or reduced). Furthermore, it is concluded that the ER ratio is the most influential parameter in the gasification process