A fuel whose exergy is times its lower heating value burns adiabatically to…

PHYSICS · Multi-part · Fermi University Physics · Thermal, Statistical & Complex Systems · Earth, Climate & Energy Physics · The laws of thermodynamics

Energy from Physics, Ch Heat engines on fuel: exergy, real cycles, biomass and the nuclear heat source

Problem

A fuel whose exergy is

1.04

times its lower heating value burns adiabatically to combustion products at

T_{\rm ad} = 2000\,\mathrm{K}

. Treat the products as a stream of constant heat capacity that can be cooled to the surroundings at

T_0 = 300\,\mathrm{K}

. ((a)) Find the share of the fuel's exergy destroyed by the combustion itself. ((b)) Find the entropy-weighted mean temperature

\bar T

of the products' heat, and the reversible efficiency

1 - T_0/\bar T

an engine could reach on it. ((c)) Now model the engine as endoreversible, taking heat from a reservoir at

\bar T

, rejecting it at

T_0

and running at maximum power. Find

\eta_{\rm CA}

and the further loss this model charges, as a share of the fuel's exergy. Which loss is larger, the flame's or the finite-rate one? ((d)) A real combined-cycle plant on such a fuel reaches about

0.60

of the heating value. What does that say about treating

\eta_{\rm CA}

as a limit?

Browse the problem library