Steam system cogeneration

Cog enera.tion in a. Steam System. The value of energy in a process stream can always be estimated from the theoretical work potential, ie, the deterrnination of how much power can be obtained by miming an ideal cycle between the actual temperature and the rejection temperature. However, in a steam system a more tangible approach is possible, because steam at high pressure can be let down through a turbine for power. The shaft work developed by the turbine is sometimes referred to as by-product power, and the process is referred to as cogeneration. 

As a precursor to discussing the fundamental waterside problems, two schematic drawings of steam cycles are provided. These examples (Figures 4.1 and 4.2) display the water-steam path and the key items of equipment in a FT boiler plant steam system cycle and a WT cogeneration steam system cycle. 

In setting the appropriate amount of heat recovery across the site through the steam system, various trade-offs need to be considered. Before this can be done, the cogeneration implications of the site heat recovery need to be quantified. 

Thus, large let-down flows are usually seen to be a missed cogeneration opportunity but might provide a degree of freedom to bypass bottlenecks in the steam system. Also, flow through let-down stations allows the level of superheat in the lower-pressure mains to be controlled independently of the flow through the steam turbines. 

Finally, on the facility level an economic comparison of three cogeneration steam systems for a wood pulping mill was carried out [20], finding that economic and environmental optimisation could not be achieved simultaneously. 

Prasad, S. B., Biomass-fired steam power cogeneration system - A theoretical-study. Energy Conversion Manage 1995, 36 (1), 65-77. 

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