Question

What do you understand by the cogeneration of power and process heat? Explain its thermodynamic advantage? Q2) Consider a regenerative gas-turbine power plant with two stages of compression and two stages of expansion. The overall pressure ratio of the cycle is 9. The air enters each stage of the compressor at 300 K and each stage of the turbine at 1200 K. Accounting for the variation of specific heats with temperature, determine the minimum mass flow rate of air needed to develop a net power output of 10 MW. 2

Answer 1

1. Cogeneration:- It is the simultaneous process of production of heat and power ina thermmodynamic process in the same power station It uses the heat which would normally be wasted in the process of power generation.It increases the overall plant efficiency.The initial high temperature heat drives the steam turbine and the low temperature heat rejected is used for heating space or water.

2. T₁=T₃ =300K, T₆=T₈=1200K, W_net=10MW, overall pressure ratio = 9,C_p = 1.005 kJ/kg.K , = 1.4

pressure ratio for each stage, r = (9)^0.5 = 3

T₂ = 300 x 3^(0.4/1.4) = 410.62K = T₄

T₇ = 876.72K = T₉

W_net = m C_p { (T₆ - T₇) + (T₈ - T₉) - (T₂ - T₁) - (T₄ - T₃) }

10x10^3 = m x 1.005 { 646.56 - 221.24}

m = 23.394 kg/s