(324a) Improving a process site sustainability through waste heat recovery
In spite of depleting reserves of fossil fuels and increasing prices, energy is still being wasted. In the process industries, at least 40 %1 of the energy content of fuel is wasted. Using energy more efficiently can reduce demand for fuel, thereby conserving resources, reducing CO2 emissions and reducing operating costs. Apart from these benefits, when the useful energy recovered (such as heat and power) from waste heat is exported to buildings outside the site, there could be costs savings and CO2 emission reduction from fossil fuel that would have been used to satisfy demand for energy. To this ends, waste heat recovery could result in economic growth (measured using cost savings), environmental protection (measured using the percentage change in CO2 emissions) and social progress since this will create more jobs thereby improving the sustainability of a process site. Diverse mature and commercialized waste heat recovery technologies2 exist to recover energy in the form of work, cooling and heat from waste heat.
In this work, a definition of waste heat is presented and a methodology is developed to identify the potential for recovery of waste heat in a process site, considering the temperature, quality and quantity of sources of waste heat from site processes and the site utility system. To support the methodology, the concept of the energy efficiency of a site is introduced – the fraction of the energy inputs that is converted into useful energy (heat or power or cooling). The methodology takes into account energy flows in fired heaters and in the cogeneration, cooling and refrigeration systems. Mathematical models of waste heat recovery technologies, such as organic Rankine cycles (using both pure and mixed organics as working fluids), absorption chillers and absorption heat pumps, are provided and applied to assess the potential for recovery of waste heat.
To assess the potential for waste heat recovery, the concept of pinch analysis is extended; the waste heat source profile provides guidance on placement of waste heat recovery technologies in a process site. The methodology can be used to generate and evaluate alternative configurations of waste heat recovery technologies, propose improvements to existing technologies and determine the waste heat recovery temperatures for a site.
The evaluation of the different alternatives to generate work, chilled water and heat for space heating or hot water circulation was done using the energy performance of technologies, exergy degradation associated with the technologies and an enviro-economic criterion developed to measure the economic potential associated with reduced CO2 emissions resulting from waste heat recovery.
This methodology is illustrated using the case study of a petroleum refinery. In this case, from the available waste heat 14.3 MW electricity for export, 3.75 MW of chilling for site use, 2.7 MW heat to satisfy the domestic demand of 1500 people living outside the site and 31.3 MW of heat for preheating boiler feed water on-site can be generated. This resulted in 19% reduction in the site CO2 emissions and 40% reduction in the site operating costs i.e. an overall win-win situation for both the site and the neighbourhood of the site.
The methodology can be applied to the process industries and other facilities producing low-grade heat.
- Ammar, Y., Joyce, S., Norman, R., Wang, Y., Roskilly, A.P., 2012. Low grade thermal energy sources and uses from process industry in the UK, Applied Energy 89 (1), 3-20.
- Little, A.B, Garimella, S., 2011. Comparative assessment of alternative cycles for waste heat recovery and upgrade, Energy 36 (7), 4492-4504.
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