• Energy Research

For a chemical process to be feasible, two levels of energy must be met: the heat and work requirements of the process. Whereas, for most processes, the heat requirement can easily be satisfied, supplying the amount of work needed is a major challenge and is usually the determinant of the process complexity. In some cases, heat, by virtue of its temperature, can satisfy the work requirement for a process; it is the simplest method for supplying work but could result in major irreversibility when applied inappropriately to a process. This article discusses different techniques that can be used to supply work to a process. A graphical approach, namely, the gh diagram, is used to analyze the heat and work requirements of chemical processes and to determine which method of supplying work is suitable for the process to be feasible and reversible. An ammonia process is analyzed as a case study, in which different methods of supplying work are compared and an attempt is made to elucidate the consequences of oper...

The energy needs of the world continue to grow, as does the resulting environmental impact. Policy makers continue to call for alternative energies to replace today's petroleum‐based liquid fuels. However, liquid fuels have significant advantages, and it is probably unwise to abandon the existing infrastructure without appropriately exploring alternatives to lessen the environmental burden of producing liquid fuels. Biomass and coal are often proposed as alternatives to petroleum‐based carbon sources, but those processes lose a significant amount of their potential product to unwanted carbon dioxide emissions. However, combining biomass and coal with cleaner natural gas yields processes with less environmental impact to produce liquid fuels with small, zero, or even negative carbon dioxide emissions. Our process synthesis approach is applied to commonly encountered liquid fuel production methods to identify promising routes and to establish feasibility limits on those less promising alternatives. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2062–2078, 2013

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.

Given the damage that the natural environment suffers from human activities, it is relevant to provide ecological literacy to all Chemical Engineering students. Sometimes, this information is offered through elective courses and/or seminars and consequently it might not reach the whole class. Some courses have more obvious connections to environmental issues, while others do not appear to. In this paper, we aim to show through some solved examples how to introduce an environmental topic in the subject Mathematical Optimization. The problems goal is to decide on the best logistics for the transport and management of human waste that will be used in the production of sustainable energy. The context is that of improving the sanitation and hygiene in areas of the developing world, while simultaneously creating job opportunities within the communities. The research that we have conducted for finding the proper way to address the environmental analysis in class, led us first to the Sustainable Development Goals (SDGs), but later on, other theories such as the Cradle-to-Cradle (C2C) have proven to be more comprehensive and therefore, better. We believe that this multidisciplinary paper shows how to integrate environmental concerns and understanding in the Chemical Engineering curricula.

Abstract Waste tyre generation in South Africa is an issue for which no sustainable solution has been found. South Africa generates over 177 385 tons of waste tyres per year, and only around 25 % is recycled, the remaining 75 % accumulates in storage depots, and landfills across the country. In this work, a slurry fed IGCC system is analysed. This system does not require the use of oxygen during gasification and is designed to be self-sustaining and produces electrical power. A sensitivity analysis shows that considerable gains in thermal efficiency are made by using a turbine pressure ratio of between 20 and 30 bar. This data was then used to develop a system that processes 518 ton/day of waste tyres and operates at a gas turbine pressure ratio of 30 bar and a 1600 °C combustion temperature. The net power production from the system is 89 MW, with a thermal efficiency of 45.65 % and work efficiency of 44.97 %. However, the results from Aspen Plus were significantly less than the predicted with an overall net efficiency of 32 %. Despite the discrepancy waste tyre IGCC net-work output was found to be 10.5 GJ/ton of tyre much higher than that of conventional coal IGCC at 9.6 GJ/ton of coal.

Abstract Energy in the form of plasma was used to thermally decompose wood in the presence of O2. Heat required to sustain the gasification reactions was provided indirectly by: (i) the electricity fed to the nitrogen N2 plasma torch; (ii) the chemical potential of the oxygen fed to the reactor. Two sets of experiments were carried out at 700 °C and 900 °C in a plasma reactor to investigate syngas composition variation. The results show that increasing the O2 flow rate reduced the lost work potential caused by the plasma electrical energy being degraded to heat, but increased the lost work during the reaction, as well as across the process, thus increasing the irreversibility of the overall process. Furthermore, a plasma torch that requires cooling is not the best way to add high temperature heat in form of electricity to the gasifier, as this results in enormous heat and work loss.

Abstract The sustainability of biomass use as a primary energy source depends on the efficiency of its conversion processes. The key contributing factors are well understood, owing to extensive experimental and theoretical modeling efforts in literature. In this manuscript, we present a systematic study of the thermochemical conversion route that allows us to target desirable outcomes when converting biomass to other fuels and products. Using process synthesis techniques that include material, energy and work balances, we identify the best targets to consider for highly efficient processes given specific constraints. Our analysis shows that by supplying the right amount of oxygen, a 100% carbon conversion efficiency can be achieved for certain applications that require gas as product. If the objective is to obtain a cleaner fuel from biomass, converting it to char is most efficient in terms of carbon and energy conversion. According to our analysis, an energy neutral biomass gasification process is theoretically possible over a wide range of H2 and CO production rates. We demonstrate its feasibility by simulating the process on Aspen Plus®. The simulation reveals that with heat integration, we can achieve the energy neutral target at a hydrogen production rate of 0.9 mol/mol biomass. We further show that even at zero energy requirement, biomass gasification processes can have excess chemical potential, which can be recovered as useful work or conserved by producing more H2. Adding low temperature heat in the form of steam at 102 °C gives an 8% gain in chemical potential conservation and increases the hydrogen production rate by 60%. The insights revealed in this work allow for better decision making in early stages of process design, and consequently, more efficient biomass gasification processes.

Abstract This paper proposes that the approach of social acceptance of renewable energy technology needs to include the concept of naturalness to understand the social rejection of biogas technology. Because naturalness concerns are not only strongly associated with the physical emotions of disgust and fear but also with disgust as a moral emotion, which is experienced as an indignity to the community, they have the potential to prevent energy projects from succeeding. Results from a survey and a case study conducted in South Africa demonstrate that relative to other renewable energy technologies, biogas technology elicited stronger naturalness concerns and the emotions of disgust and fear (Study 1: N = 452) and that indignity experiences of community members of an informal settlement were sufficient to reject a small scale biogas technology project (Study 2: N = 155). The implications of our findings are discussed and solutions are provided to address the naturalness concerns about biogas technology.

Abstract Waste tyres are a particularly problematic pollutant; persistent, highly toxic, flammable, and difficult to process or store. However, waste tyres need not be viewed solely as a waste material, as they also offer promising properties as an energy material. Waste tyres have a higher energy density than coal, as well as lower ash content and favourable quantities of carbon and hydrogen. Extensive experimental research has demonstrated that thermochemical valorisation pathways including pyrolysis and gasification are viable for producing valuable chemical products from waste tyre. Despite this, there is as yet no established technology for waste tyre conversion. In this paper, fundamental thermodynamic and economic analysis is used to evaluate a range of process pathways to determine their economic favourability and environmental impact. The process performance targets derived in this way can serve as a basis for preliminary process design and provide estimates for the commodity value of waste tyre, informing long-range planning in both corporate and legislative settings. A range of pyrolysis and gasification pathways have been evaluated in terms of the fundamental thermodynamic metrics of carbon efficiency, atom economy, e-factor and chemical potential efficiency, and also their market-related revenue potential. It was found that pyrolysis pathways perform better in terms of thermodynamic efficiency and carbon footprint than gasification processes, which lose about 45% of the carbon feed to carbon dioxide. However, the gasification routes offer higher potential revenue, yielding as much as $625 per ton of waste tyre as compared to $205 from the pyrolysis route.