Application of response surface methodology to assess the combined effect of operating variables on high-pressure coal gasification for H2-rich gas production

Coal gasification was performed by means of a high-pressure fixed bed gasifier fitted with a solids feeding system in continuous mode, using oxygen and steam as gasifying agents. The main aim of the paper was to assess the combined effects of the operating variables (temperature, oxygen and steam concentrations) on high-pressure coal gasification. To this end a face centered central composite design (FCCCD) based on response surface methodology (RSM) was used. The response variables studied were: H₂, CO and syngas production, H₂/CO ratio, cold gas efficiency (η), and carbon conversion (X). The study was carried out at temperatures of 900, 950 and 1000 °C, using oxygen concentrations of 5, 10 and 15 vol.%, and steam concentrations of 25, 40 and 55 vol.%. The gasification temperature was found to be the most influential variable, with high temperatures leading to an increase in all the response variables studied. An increase in the oxygen content of the gasifying agent led to a decrease in H₂ and CO production, and cold gas efficiency, whilst carbon conversion was favoured. An increase in steam concentration, on the other hand, favoured the production of H₂ and syngas production, whereas CO production underwent a reduction; cold gas efficiency and carbon conversion were observed to increase. Response surface methodology (RSM) revealed the effects of interaction between the operating variables, which would not have been identified by the traditional “one-factor-at-a-time” method. The models developed successfully fitted the experimental results for all the response variables studied.     

Steam and air fed biomass gasification: Comparisons based on energy and exergy

Results are reported of thermodynamic analyses of a biomass gasification unit in which sawdust is the biomass feed and the gasifying medium is either air or steam. Energy and exergy analyses are performed for the system and each of its components. A parametric study reveals the effect of design and operating parameters on the system's performance and energy and exergy efficiencies. The results show that the adiabatic temperature of biomass gasification significantly changes with the type of the gasifying medium. In addition, the exergy and energy efficiencies are observed to be higher when air is the gasifying medium rather than steam, while the system performance and exergy efficiencies are dependent on the moisture content of the feed biomass. The results are significant because they quantify the strong dependence of biomass gasification, which can be used for syngas or hydrogen production, on moisture content, and gasifying medium.     

Sustainable hydrogen production options and the role of IAHE

In this paper, some potential sustainable hydrogen production options are identified and discussed. There are natural resources from which hydrogen can be extracted such as water, fossil hydrocarbons, biomass and hydrogen sulphide. In addition, hydrogen can be extracted from a large palette of anthropogenic wastes starting with biomass residuals, municipal wastes, plastics, sewage waters etc. In order to extract hydrogen from these resources one needs to use sustainable energy sources like renewables and nuclear. A total of 24 options for sustainable hydrogen production are then identified. Sustainable water splitting is the most important method of hydrogen production. Five sustainable options are discussed to split water, which include electrolysis, high temperature electrolysis, pure and hybrid thermochemical cycles, and photochemical/radiochemical methods. Other 19 methods refer to extraction of hydrogen from other materials than water or in conjunction with water (e.g., coal gasification with CO₂ capture and sequestration). For each case the achievable energy and exergy efficiency of the method were estimated based on state of the art literature screening for each involved process. In addition, a range of hydrogen production capacity is determined for each of the option. For a transition period to hydrogen economy nuclear or solar assisted coal gasification and fossil fuel reforming technologies – with efficiencies of 10–55% including CO₂ sequestration – should be considered as a viable option. Other “ready to be implemented” technology is hydro-power coupled to alkaline electrolysers which shows the highest hydrogen generation efficiency amongst all electrical driven options with 60–65%. Next generation nuclear reactors as to be coupled with thermochemical cycles have the potential to generate hydrogen with 40–43% energy efficiency (based on LHV of hydrogen) and 35–37% exergy efficiency (based on chemical exergy of hydrogen). Furthermore, recycling anthropogenic waste, including waste heat, waste plastic materials, waste biomass and sewage waters, shows also good potential as a sustainable option for hydrogen production. Biomass conversion to hydrogen is found as potentially the most efficient amongst all studied options in this paper with up to 70% energy efficiency and 65% exergy efficiency.     

Energy and exergy analyses and optimization study of an integrated solar heliostat field system for hydrogen production

In this paper, we propose an integrated system, consisting of a heliostat field, a steam cycle, an organic Rankine cycle (ORC) and an electrolyzer for hydrogen production. Some parameters, such as the heliostat field area and the solar flux are varied to investigate their effect on the power output, the rate of hydrogen produced, and energy and exergy efficiencies of the individual systems and the overall system. An optimization study using direct search method is also carried out to obtain the highest energy and exergy efficiencies and rate of hydrogen produced by choosing several independent variables. The results show that the power and rate of hydrogen produced increase with increase in the heliostat field area and the solar flux. The rate of hydrogen produced increases from 0.006 kg/s to 0.063 kg/s with increase in the heliostat field area from 8000 m² to 50,000 m². Moreover, when the solar flux is increased from 400 W/m² to 1200 W/m², the rate of hydrogen produced increases from 0.005 kg/s to 0.018 kg/s. The optimization study yields maximum energy and exergy efficiencies and the rate of hydrogen produced of 18.74%, 39.55% and 1571 L/s, respectively.     

Decentralized generation of electricity from biomass with proton exchange membrane fuel cell

Biomass can be applied as the primary source for the production of hydrogen in the future. The biomass is converted in an atmospheric fluidized bed gasification process using steam as the gasifying agent. The producer gas needs further cleaning and processing before the hydrogen can be converted in a fuel cell; it is assumed that the gas cleaning processes are able to meet the requirements for a PEM-FC. The compressed hydrogen is supplied to a hydrogen grid and can be used in small-scale decentralized CHP units. In this study it is assumed that the CHP units are based on low temperature PEM fuel cells. For the evaluation of alternative technologies the whole chain of centralized hydrogen production from biomass up to and including decentralized electricity production in PEM fuel cells is considered.Two models for the production of hydrogen from biomass and three models for the combined production of electricity and heat with PEM fuel cells are built using the computer program Cycle-Tempo. Two different levels of hydrogen purity are considered in this evaluation: 60% and 99.99% pure hydrogen. The purity of the hydrogen affects both the efficiencies of the hydrogen production as well as the PEM-FC systems. The electrical exergy efficiency of the PEM-FC system without additional heat production is calculated to be 27.66% in the case of 60% hydrogen and 29.06% in the case of 99.99% pure hydrogen. The electrical exergy efficiencies of the whole conversion chain appear to be 21.68% and 18.74%, respectively. The high losses during purification of the hydrogen gas result in a higher efficiency for the case with low purity hydrogen. The removal of the last impurities strongly increases the overall exergy losses of the conversion chain.     

Exergy analysis of sugarcane bagasse gasification

The gasification technology has been object of study of many researchers, especially those involved in promoting large-scale electricity generation in sugarcane mills. This paper presents a simplified model for the gasification process based on chemical equilibrium considerations. The model consists in the minimization of the Gibbs free energy of the produced gas, constrained by mass and energy balances for the system. Despite the simplicity of the model, its results are reliable in identifying the tendencies of the working parameters of the system. A parametric study has been carried aiming the verification of the influence of many variables inherent to the model, such as: gasification temperature, moisture content, and air temperature, among others. The results were compared with those found in literature and real systems. Following this parametric study, an exergy analysis has been performed in order to evaluate irreversibilities associated to the process, and the influence of temperature, moisture, charcoal production, and thermal losses on them. Finally, a first attempt to integrate a gasifier into a sugarcane mill was performed, which showed the potential benefits regarding the use of such technology.     

A multi-criteria assessment of six energy conversion processes for H2 production

The study of Very Large Complex systems (“VLCS”), of which modern energy conversion systems are an important subset, requires a holistic approach to analyze the system itself and all of its “external” and “internal” interactions. The view taken in this paper is that the VLCS should be considered as an “extended” (in a sense specified below) thermodynamic system. The evaluation of the flows of matter and energy sustaining a VLCS and the knowledge of the transformations therein can be used to describe the rate of exploitation of the available natural resources, to assess the efficiency of the conversion process, and to provide a quantitative estimate of the impact of the system on the environment. This kind of information is an important part of the essential database of any Decision Support System for both the internal and global policy planning and for resources management. Several assessment methods are in use at present, and each one of them provides a different insight in the “performance” of the conversion chain under examination.     

Decentralized generation of electricity with solid oxide fuel cells from centrally converted biomass

A thermodynamic evaluation of different energy conversion chains based on centralized biomass gasification and decentralized heat and power production by a solid oxide fuel cell (SOFC) has been performed. Three different chains have been evaluated, the main difference between the chains is the secondary fuel produced via biomass gasification. The secondary fuels considered are hydrogen, synthetic natural gas (SNG) and syngas. These fuels are assumed to be distributed through a transport and distribution grid to the micro-combined heat and power (μ-CHP) systems based on a SOFC and a heat pump.     

Enhancement of hydrogen production in a modified moving bed downdraft gasifier – A thermodynamic study by including tar

A new approach on thermodynamic simulation of the gasification process is conducted by considering the formation of tar using Aspen Plus. The present model shows higher accuracy as compared to the conventional model in term of the composition of producer gas. The tar from pyrolysis process is successfully reduced with high reaction temperature in the combustion zone. A parametric study is performed by varying the split ratio of gasifying agents (steam/oxygen) through three different zones: (i) combustion zone, (ii) counter-current reduction zone, and/or (iii) co-current reduction zone. Introduction of the gasifying agents through the counter-current reduction zone has positive effects on the gasification performances in term of hydrogen concentration, cold gas efficiency, and gasification system efficiency. The effects of O₂ equivalence ratio and steam to carbon ratio (S/C) on the performance of gasification are also investigated. The gasification with oxygen provided the highest cold gas efficiency. A remarkable hydrogen production is achieved from gasification with both oxygen and steam.     

Optimization of conditions for hydrogen production from complex dairy wastewater by anaerobic sludge using desirability function approach

Present study deals with the multiple-response optimization for biohydrogen production using anaerobic sludge and outstanding approach to overcome the drawbacks of conventional response surface methodology (RSM). Dairy wastewater was used as source in batch fermentation was followed for this study. Response surface methodology (RSM), based on a three level, four variable Box–Behnken design, was employed to obtain the best possible combination of substrate concentration, pH, COD/N ratio and COD/P ratio for maximum H₂ yield (HY) and specific hydrogen production rate (SHPR). Experimental data were evaluated by applying RSM integrating a desirability function approach. The optimum H₂ yield and SHPR conditions were: substrate concentration 15.3 g COD/L, pH 5.5, COD/N ratio 100.5 and COD/P ratio 120 with maximum overall desirability D of 0.94. The confirmation experiment under these optimal condition showed a HY and SHPR of 13.54 mmol H₂/g COD and 29.91 mmol H₂/g-VSS.d, respectively. This was only 0.22% and 0.20%, respectively, different from the predicted values, suggesting that the desirability function approach with RSM was a useful technique to get the maximum H₂ yield and SHPR simultaneously.     

Exergy analysis of hydrogen production from biomass gasification

For a given set of operating conditions, the hydrogen production from biomass gasification can be improved through optimization of the operating parameters and efficiencies. The present approach can predict hydrogen production via biomass gasification in a range of 10–32 kg/s from biomass (sawdust wood). The biomass is introduced to a gasifier at an operating temperature range of 1000–1500 K. Also, 4.5 kg/s of steam at 500 K is used as gasification medium. Results indicate that improvement in hydrogen production from biomass steam gasification depending on the amount of steam and quantity of biomass feeding to the gasifier as well the operating temperature. Over the range of feeding biomass, the hydrogen yield reaches 80–130 g H₂/kg biomass while in the operating temperature examined, the hydrogen yield reaches 80 g H₂/kg biomass. On mole basis it is found that, in the first range of H₂ varies from 51 to 63% in the studied range of feeding biomass in existing 4.5 kg/s from steam while H₂ gets to 51–53% in existing of 6.3 kg/s from steam.