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Repowering Steam Plant with Gas Turbines

A number of methods have been proposed and used to integrate gas turbines into existing conventional steam plants to increase plant efficiency, capacity, or both. RE-MASTER is our Application-Specific program for steam plant repowering. It simplifies and automates repowering calculations which inevitably require ability to simulate existing equipment, and design new equipment to work with existing units. However, THERMOFLEX alone, or in conjunction with STEAM MASTER can be also used for repowering, providing greater freedom and flexibility, albeit with less automation.

In this model, steam plant modifications are designed in a THERMOFLEX model, and the original steam plant is modeled in a linked STEAM MASTER file. This combination is extremely effective because the entire steam plant in all its detail is modeled in a sort of single icon, one with over 1500 inputs. This repowering scheme is known as “Direct Hot Windbox Repowering” because hot gas turbine exhaust is sent directly to the boiler in place of cooler ambient air. Depending on details, the repowered plant may produce more or less power, but will have higher efficiency, and will burn less of its original fuel.

The original single reheat Steam plant to be repowered is shown below. It is subcritical with nominal throttle steam conditions of 2690 psia (185 bar) 1040 F (560 C), and reheat steam conditions of 640 psia (44 bar) 1040 F (560 C). The plant burns pulverized coal and has a net nominal efficiency LHV of 40.5% at 334 MW net output.

An overview THERMOFLEX graphic of the repowered plant is displayed below. Note that the connections between THERMOFLEX and STEAM MASTER are indicated by icons labeled “STM”. To accomodate the hot gas turbine exhaust, the air–preheater in the original plant is removed and replaced by additional heat exchangers that heat feedwater in parallel to the original feedheaters. This is shown in the yellow box where two preheaters in series receive boiler flue gasses after they leave the economizer in the original plant. The plant’s primary and secondary air fans are replaced by ductwork connecting GT exhaust to boiler windbox and to the tempering air connection at the furnace exit. This is shown in the blue region. The green region surrounds the slurry preparation system that delivers fuel to the boiler in amounts that leave the excess oxygen at 10% or higher.

The repowered plant generates 325.5 MW electricity, a decrease of roughly 8 MW, but improves net LHV efficiency from 40.5 to 42.2%. Because the repowered plant burns partly natural gas and partly coal, CO2 emission rates are reduced from approximately 0.9 ton/MWhr to 0.75 ton/MWhr, a reduction of about 15% in emission rate.