FLUIDIZED BED REHEAT PROCESSES FOR BIOMASS POWER PLANT APPLICATIONS

摘要

Several combustion technologies have been used for biomass. Stoker-fired units have a long history, but result in relatively high emissions, high unburned carbon loss, and provide limited capability to burn high moisture fuels. Circulating Fluidized Bed (CFB) boilers are another option but are associated with higher capital and operating costs compared to Bubbling Fluidized Bed (BFB) boilers. Modern BFB boilers are simple, reliable, and can even burn high moisture biomass fuels with low emissions. Power plants utilizing BFB boilers typically range from 10 to 100 MW_e of power generation. For non-reheat boilers, this translates to main steam flows from 100,000 lb/hr to 900,000 lb/hr (45 t/h .. 400 t/h). BFB based power plants have a long history in electricity and process steam generation, but are sometimes utilized for power generation only. In condensing power generation units, the plant efficiency is a very important parameter affecting the plant economics. A reheat cycle provides one way to improve the plant efficiency. Steam reheat is well proven in utility size boilers larger than 150 MW_e. Biomass boilers are typically smaller than 150 MW_e due to fuel supply limitations. Biomass fuel prices have remained low enough to not encourage the higher investment costs associated with reheat cycles. Increasing fuel costs, however, have resulted in increased interest in reheat boilers. Incorporating steam reheat into the power plant process improves the cycle efficiency by two percentage points or more. This increase in efficiency will reduce the fuel consumption and emissions by approximately five percent. With limited fuel supply, the higher efficiency results in higher power output. The paper addresses optimum reheat conditions and processes, including reheat steam pressure and temperature and boundary conditions for process optimization. Other aspects of incorporating a reheat system to the boiler, such as some methods for reheat steam temperature control and boiler layout, are also discussed. The limitations imposed by the steam turbine on the whole power plant process are also evaluated. The performed case study provides a techno-economic comparison between a 55 MWe (net) reheat and non-reheat power plant. The differences of reheat in condensing power plant applications and combined heat and power (CHP) plants are also reviewed.