Page 45 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

24 Biomass Gasification, Pyrolysis, and Torrefaction

Example 1.1
Compare the thermodynamic efficiency of electricity generation from biomass
through the following two routes:
1. Biomass is combusted in a boiler with 95% thermal efficiency (on lower
heating value (LHV) basis) to generate steam, which expands in a steam
turbine from 600 C to 100 C driving an electrical generator.

2. Biomass is gasified with a cold gas efficiency of 85% and the product gas
is burnt to produce hot flue gas at 1200 C, which expands in a gas turbine to

600 C. Waste gas from the gas turbine enters a heat recovery steam generator

that produces steam at 400 C. This steam expands to 100 C in a steam turbine.

Both turbines are connected to electricity generators. Neglect losses in the
generators.
Solution
For the steam power plant:
Given:
Boiler thermal efficiency on LHV: η b 5 0.95

Inlet steam temperature: T 1 5 600 C 5 873 K
Exhaust steam temperature: T 2 5 100 C 5 373 K

We assume the turbine to be an ideal heat engine, operating on a Carnot cycle.
So, ideal steam turbine efficiency, η st 5 1 2 ((T 2 /T 1 ))5 1 2 (373/873)5 0.573.
The overall efficiency of the first route, η sc is the combination of the boiler
and the turbine.
η 5 η 3 η 5 0:95 3 0:573 5 0:544% or 54:4%
sc
b
st
For the IGCC plant:
Given:
Gasification efficiency: η g 5 0.85
Inlet gas temperature: T g1 5 1200 C 5 1473 K

Exhaust gas temperature: T g2 5 600 C 5 873 K

Ideal gas turbine efficiency, η gt 5 1 2 ((T g2 /T g1 )) 5 1 2 (873/1473) 5 0.407.
Considering the cold gas efficiency of the gasifier efficiency
Gas turbine plant efficiency, η gtp 5 η g 3 η gt 5 0.85 3 0.407 5 0.346.
The downstream steam turbine is also a Carnot heat engine operating
between 400 C or 673 K and 100 C or 373 K. So, the ideal cycle efficiency of

the steam turbine in a IGCC plant is written as:

T 2 373
η 5 1 2 5 1 2 5 0:446
st
T 1 673
Both the steam and gas turbines have been assumed to be ideal, so the ideal
efficiency of the combined cycle can be calculated using the expression for
combined cycle efficiency given in basic thermodynamics:
η igcc 5η gtp 1η 2η 3η gtp 50:34610:446 ð0:44630:346Þ50:638% or 63:8%
st
st
Thus, the gasification-based IGCC plant has an overall efficiency of 63.8%
compared to 54.4% for combustion-based Rankin cycle steam power plant.