Combined heat and power (CHP) retrofit for supplemental on-site power generation at Engen oil refinery.
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This thesis is a critical evaluation of an opportunity project for on-site generation of electricity at the ENGEN refinery in Durban, South Africa. The key equipment discussed is a 2.5MW special purpose backpressure turbine which (prior to July of 2014), operated in continuous service as a compressor prime mover. The availability of the turbine since the plant decommissioning, has drawn business interest in a retrofit service application as a turbo-generator capable of electrical power production if re-engineered with a an optimal gearbox and electrical generator configuration. The assessment method employed for the data extraction and calculations in this thesis is the “Plant performance triangle”. Historical and current process data are filtered for meaningful calculations and engineering analysis. Data segmentation methods are used to analyse the refinery operation at varying boiler loads where High Pressure (HP) steam at 40 barg is routed to the turbine and let down to 10 barg Medium Pressure (MP) header. The thesis evaluates the profitability of the devaluation of this steam by the isentropic steam expansion from thermal to mechanical to finally electrical energy, as opposed to isenthalpic (adiabatic) steam “let-down” (throttling) or pressure relief. The design basis for the turbine operation is 42 tons/hr high-pressure (HP) steam to the turbine casing inlet. Calculations show that between 2.0 MW to 2.5 MW of electrical energy generation is possible with minimal additional consumption of HP steam from the refinery HP header. This is due to the steam load balancing of five onsite boilers between the high and medium pressure steam header mains. In essence, additional MP steam for power generation is “let-down” into the MP header resulting in the back-up of HP to MP “let-down” from parallel boilers into the MP header. By this, the refinery demand for steam at varying pressure headers is adjusted by automated boiler advanced control. The resultant economic value of electricity cost savings is approximately (conservatively – based on 2016 electricity prices) R9.9m per year. Two key parameters in the techno-economic assessment are fuel gas (combustible energy) and treated feed water cost. The cost of boiler feed water is assumed a fixed cost to the operation, however since the refinery steam headers require a mere 2.37 additional tons of HP steam to support the new turbine operation, added water costs do not pose a significant operating expense. Sensitivities are performed on varying water costs (R/kL), as this is a factor of the project profitability given the scarce water availability challenges in South Africa.