• Energy Research

Abstract This work shows work flows supported by experimental work to analyse the efficiency of a plasma system in biomass conversion processes. The most common set of problems encountered when using biomass-to-energy (BTE) processes relate to tar formation and product gas composition. However, using plasma technology to convert biomass provides a solution because it unlocks more energy than can be achieved by other BTE systems by using a heat supply derived from electricity. The research presented in this paper focuses on the conversion of biomass to chemical energy (in gaseous form) with the aid of the electrical energy supplied by a water-cooled nitrogen plasma torch. The authors conducted a series of experiments in a continuous pyrolysis set up in which wood pellets were converted to syngas in a small-scale laboratory nitrogen plasma torch reactor with a maximum power supply of 15 kW. The efficiency of the process was measured in terms of the carbon conversion to all product gases which changed from 43 to 77%, at temperatures ranging from 400 °C to 1000 °C respectively. The combined carbon monoxide and hydrogen mole concentration in the product gas (without nitrogen) was 86% at 1:1 ratio for all temperatures studied. Syngas yield increased with increase in temperature. The overall biomass conversion obtained increased from 46% to 82% for the temperatures 400 °C to 1000 °C respectively, with the balance comprising carbon-rich solid residue and liquid. The work flow shows that a plasma system can get to high temperatures but work is also degraded in the overall process. Exergy analysis shows that the work lost by the overall process decreases with increase in process temperature.

Utilization of mesoporous materials to enhance structure–performance relationship of Fischer–Tropsch catalysts.

Abstract Due to its complexity, coal gasification is perhaps one industry’s least understood processes. This is despite the fact that this process is critical to countries such as South Africa, as it is responsible for producing a large portion of the country’s fuel needs through the Fischer–Tropsch process. Worldwide, this process has also become critical for applications such as IGCC, for the production of electricity. It is because of this importance that it is necessary to better understand this process. Another motivating factor is that gasifiers are very expensive and are big energy consumers as well as being large carbon dioxide producers. Much experimental work has been done in the area of gasification, but this can be very expensive and is time consuming. It is with this in mind, that we have developed a quick, relatively simple and yet very powerful graphical tool to assess and better understand gasification and to use this tool to look for opportunities to improve efficiencies of process and to reduce carbon dioxide emissions. The approach used here is to make a few reasonable assumptions to set up mass balances; the energy balance and reaction equilibria around a coal gasifier. This paper deals with how these balances can be set up; it also looks at what effect the feed composition and choice of reaction conditions (temperature and pressure), may have on the possible gasifier product. The result of this approach shows that we can work in a stoichiometric subspace defined by the energy and mass balance. Furthermore we can show that gasification is energy and not work limited which has implications for the design and operation of these units.

AbstractSouth Africa obtains over 80% of its energy needs from fossil fuels. Latest research indicate that South Africa emits over 500 million tons of carbon dioxide (CO2) per year and is therefore ranked number eight as the world's CO2 emitter. Unabated or controlled emission of CO2 and other green house gases has largely contributed to detrimental global warming and adverse climatic change. Microalgae are capable of converting hazardous CO2 into valuable biomass. A high CO2 tolerating microalgae was collected from Johannesburg Zoo Lake and identified as Desmodemus sp. Batch cultures were grown in 1000 mL. Erlenmeyer flasks. CO2 was bubbled into the microalgae culture media. The media contained optimal nutrients, an optimal photoperiod and light intensity and controlled pH. CO2 concentrations of 100%, 50%, 25%, 10%, and 5% and total gas flow rates of 20, 50 and 100 mL/min were used. The aim of this study was to determine the effective flow rate and CO2 concentration that gave optimal microalgae growth. Microalgae dry mass contain 50% carbon and carbon is known to be a limiting factor when all other nutrients and environmental conditions are presents. When atmospheric air was supplied at 50 mL/min the growth rate was 0.1151 per day and it increased drastically by almost 5 folds when 100% CO2 was supplied at 50 mL/min. Using a gas flow rate of 20 mL/min and 10% CO2, growth rate increased to 1.42 per day with a dry biomass yield of 809.96 mg/L after 12 days of growth. At a gas flow rate of 50 mL/min, 5% CO2 and the highest growth rate was 1.993 per day and an overall biomass yield of 1200 mg/L and the average pH was 6.36. At 100% CO2 the growth rate was 1.265 per day and dry biomass yield obtained was 469.81 mg/L while the average pH was 5.12. When gas flow rate was increased to 100 mL/min using 5% CO2 the growth rate slightly reduced to 1.30 per day with a dry biomass yield of 1000 mg/L. A growth rate was 0.34 and dry biomass yield of 279.47 mg/L at an average pH of 5.09 were achieved at 100% CO2. Higher flow rates and higher concentrations resulted in slightly reduced growth due to low pH. These investigations indicate that the species under study is CO2 tolerant and it presents a viable CO2 abatement strategy. © 2011 American Institute of Chemical Engineers Environ Prog, 2012

Abstract Background Engineers face increasing pressure to manage and utilize waste (whether of animal, human or municipal origin) in a sustainable way. We suggest that a solution to the problem of organic waste in rural communities lies in their being able to convert it to biogas technology. This would offer smallholders and farmers a long-term, cheap and sustainable energy source that is independent of the national electricity grid. However, although the technology involved in making biogas from waste has already been fully developed, there are obstacles impeding its adoption. First, there is a general ignorance about this source of energy among the very people who can most benefit from using it. Second, at present, South Africa has no regulatory framework to support the installation of biodigesters. Methods The research focused on the current gap between knowledge and need. The two objectives were raising general awareness of the many and varied benefits that biodigestion can offer, especially to rural communities, and demonstrating how it works. Using science events as a platform, the team introduced the concept of biodigestion, its functioning and uses, to their audiences, and then invited informal responses, which were recorded. The second stage, the case study, entailed the setting up of a small-scale (10 m3) household biodigester in the Muldersdrift community in Gauteng, South Africa. It was put into operation, using fresh cow dung as the feed. Members of the community were invited to watch every step of the process and afterwards were asked to participate in a more formal survey, which sought their opinions on whether biodigestion offers a power source the individual farmer could (and would) use. Results The results presented in this paper were derived from a comparison of the ‘before-and-after-installation’ responses of the persons interviewed. We found that the members of the Muldersdrift community who had been involved in both phases of the case study (explanation followed by experience of a hands-on educational example) had become more willing to adopt the technology. Conclusions The results justified our contention that, to ensure a greater adoption of biogas technology in South Africa, it is necessary to provide targeted communities with educational programmes and exposure to pilot plants.

Abstract UG2 is a low grade PGM ore, with value concentration in the range of 1–2 ppm. In this paper, we aim to determine whether the PGM values in the UG2 ore are associated with a soft or hard mineral phase. The anticipated benefits of determining which mineral phase the PGMs are associated with is that such knowledge may be very useful in the efficient processing of the ore. We developed and successfully applied a simple model in order to achieve our objective. The development of our model is from fundamental first principles, in which we mathematically manipulate the component mass fraction ratio of a hard component in the ore to develop an expression that we used to predict the mass fraction of the hard component in UG2 ore remaining in the largest size class material after a specified milling duration. Our results show that as the mass fraction of the tracer element retained in the largest size class increases with grinding time, so does the predicted mass fraction of the hard component. This similarity in grinding behaviour and the linear relationship that exists between the tracer element and the hard component theoretically supports that the PGMs in the UG2 ore are associated with the hard mineral phase.

The research shows theoretical calculations on the thermodynamics of digestion/gasification processes where glucose is used as a surrogate for biomass. The change in Enthalpy (∆H) and Gibbs Free Energy (∆G) is used to obtain the Attainable Region (AR) that shows the overall thermodynamic limits for digestion/gasification from 1 mol of glucose. Gibbs Free Energy and Enthalpy (G–H) plots were calculated for the temperature range 25–1500 °C. The results show the effect of temperature on the AR for the processes when water is in both liquid and gas states using 25 °C, 1 bar as the reference state. The AR results show that the production of CO, H2, CH4 and CO2 are feasible at all temperatures studied. The minimum Gibbs Free Energy becomes more negative from −418.68 kJ mol−1 at 25 °C to −3024.34 kJ mol−1 at 1500 °C while the process shifts from exothermic (−141.90 kJ mol−1) to endothermic (1161.80 kJ mol−1) for the respective temperatures. Methane and carbon dioxide are favoured products (minimum Gibbs Free Energy) for temperatures up to about 600 °C, and this therefore includes Anaerobic Digestion. The process is exothermic below 500 °C, and thus Anaerobic Digestion requires heat removal. As the temperature continues to increase, hydrogen production becomes more favourable than methane production. The production of gas is endothermic above 500 °C, and it needs a supply of heat that could be done, either by combustion or by electricity (plasma gasification). The calculations show that glucose conversion at temperatures around 700 °C favours the production of carbon dioxide and hydrogen at minimum G. Generally, the results show that the gas from high-temperature gasification (>~800 °C) typically carries the energy mainly in syngas components CO and H2, whereas at low-temperature gasification (<500 °C) the energy is carried in CH4. The overall analysis for the temperature range (25–1500 °C) also suggests a close relationship between biogas production/digestion and gasification as biogas production can be referred to as a form of low-temperature gasification.

Biomass integrated gasification combined cycle (IGCC) is attracting increased interest because it can achieve high system energy efficiency (>50%), which is predicted to increase with the increase in the solar share in biomass IGCC. This study evaluated the potential of crop residues numerically for the co-production of power and bio-fertilizer using ASPEN Plus® simulation software. The results showed that the gas yield increases with increasing temperature and decreasing pressure while the yield of bio-fertilizer is dependent on the biomass composition. The biomass with a low ash content produces high bio-fertilizer at the designated gasification temperature. The IGCC configuration conserves more energy than a directly-fired biomass power plant. In addition, the solar-assisted IGCC attains a higher net electricity output per unit of crop residue feed and achieves net thermal efficiencies of around 53%. The use of such hybrid systems offer the potential to produce 0.55 MW of electricity per unit of solar-thermal energy at a relatively low cost. The ASPEN Plus model predicted that the solar biomass-based IGCC set up is more efficient in increasing the power generation capacity than any other conversion system. The results showed that a solar to electricity efficiency of approximately 55% is achievable with potential improvements. This work will contribute for the sustainable bioenergy production as the relationship between energy production and biomass supplies very important to ensure the food security and environmental sustainability.

The gasification of coal is a process that has been commonly used to produce a mixture of gases containing primarily carbon dioxide and hydrogen, called syngas. This syngas is used as an intermediate in the production of many chemicals such as ammonia, synthetic hydrocarbons, and methanol (to name a few). Coal gasification has a reputation for being “dirty” in terms of its emissions in comparison with other syngas creation technologies, such as methane reforming. However, there is remarkably little information on what the “best case” for coal gasification could actually be and how existing process perform relative to that “best case.” The goal of this article is to formulate a preliminary and conceptual flow sheet for the gasification process; this flow sheet is not intended to be a finalized design or a definitive solution. It is intended to illustrate the method of setting and achieving design objectives and provide a basis of comparison for either new or existing processes. Thermodynamics can be used to describe any process, or system of processes. Of particular interest are the properties of enthalpy and Gibbs free energy. Using these two thermodynamic properties together as vectors on a diagram of free energy (ΔG) against enthalpy (ΔH), it becomes possible to develop better process flow sheets that combine the thermodynamics of chemical reactions and the dynamics of physical operations on a single diagram. This article will discuss the selection of the independent mass balances that best describe the process as a whole, then the choosing of design objectives, how these objectives might be achieved, and their implications for the process as a whole. Using these ideas one is able to show how to improve the carbon and operating efficiency of a gasification process, making the process more reversible. It was found that there will always be a price to pay for using coal as the feedstock for creating synthesis gas but there is room for improvement, most notably in how combustion is carried out and how energy is used internally within a process. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3258–3266, 2014