South Africa-based Ardau USC Power has announced a significant efficiency increase in power generation using waste materials. A breakthrough to the so-called ultra-super critical (USC) phase has been made possible by the availability of new oxidation-resistant alloys capable of withstanding temperatures of 700°C and pressures of 30MPa or 300bar.
The technology offers a generating process, either mobile or fixed, for the conversion of mixed carbonaceous waste into electricity, water, fertiliser and gaseous end products, all of which are separate revenue streams.
The company has incorporated well-established science, known operating technologies and advanced simulated engineering to achieve this breakthrough. The inventor, Frans Bezuidenhout, has a record of 30 years of R&D experience in the field of waste management, power generation and patent-pending processes.
Biowaste carboniferous materials are ideal raw materials for the USC slurry-based process within a reactor. By inducing an underwater exothermic reaction, heat is released up to a temperature level where it attains USC characteristics. The process produces electricity at efficiencies that are at least four times higher than coal combustion power stations.
Super-critical (SC) fluid, which includes miscible gases such as carbon dioxide (CO2) and nitrogen, is produced in the reactor during the USC phase, which is then introduced into the turbo expander/genset for the generation of electricity. Under USC conditions, not only can CO2 be prevented from forming but nitrogen oxides (NOx) and sulphur oxides (SOx) cannot occur, and only useful derivative materials such as sulphur and nitrogen are produced.
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Metals and minerals in the slurry are reduced to their base metal and gradually accumulate in the reactor. Once discharged, metals will be stored separately until substantial quantities have been accumulated for final separation and sale.
The technology derives from Bezuidenhout’s research over 18 years, notably in the field of waste and discard coal. He established a research lab with the purpose of developing a methodology where electricity could be generated from waste coal and biomass without producing greenhouse gases.
Eight sub-critical pilot plants have been developed to incorporate research developments, drawing on the science and skills of other well-known steam boiler and nuclear technologies. A range of experimental work on various carboniferous waste materials has been carried out in which mixed materials, including coal/wood/paper waste, lignite, cereal husks, horticultural cuts, sewage, seaweed and everyday plastics, were introduced into the reactor as slurry.
The various phases of testing have included different types of simulation models, leading eventually to the USC phase. A suitable alloy, developed in the US, became available only in October 2017, which meant that the Ardau reactor could be built to meet all of the American Society for Testing and Materials standards and relevant code case.
In India, for example, there is a major problem in managing waste mountains of rice husks throughout the rice belt. Ardau could solve the problem with its methodology, fuelled by slurry consisting of husks and liquid such as river water, acidic/foul water, seawater and so on. In the off-seasons or drought periods where sufficient rice husks are not available, carboniferous materials such as waste coal and wood/ forest waste can be used.
The USC plant has a small footprint. A complete 5-10MWh USC plant can be accommodated in a mobile 40ft container or multiples thereof. Larger units up to 500MWh can be fabricated to retrofit into existing power stations, so saving infrastructure and jobs.
Anticipating the fabrication of the first set of 5MWh units, detailed designs exist, process simulations have been undertaken and specialist suppliers selected.
By moving away from power generation that uses boilers and tubes, the inventor has shown that significant environmental, economic and practical advantages are possible using the Ardau technology. Ennobling biowaste will render profits, create jobs and enhance the environment and social cohesion, to the benefit of everyone.
USC describes the very high pressures (300bar) and temperatures (700oC) that can be achieved in a reactor. Under these conditions, super-critical fluid is produced, which has all the characteristics of water but at only 50% of the density; does not have the steam phase and virtually all gases (including CO2) become miscible, through fusion not bonding.
Under USC conditions, NOx and SOx cannot be formed, other elements are reduced to their elemental parts and then percolate down to the base of the reactor where they can be drawn off during operational conditions.
Hugh Grace is chief executive of Ardau Community Waste Power
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