Exergoeconomic Analysis of a Hybrid Steam Biomass Gasification-Based Tri-Generation System

Abstract

In this chapter, exergoeconomic analysis is performed for a hybrid system combining gasifier and solid oxide fuel cell (SOFC) as the core units. The pressurised SOFC considered is a planar type in geometry, operating at 1,000 K, and the gasifier gasifies biomass (sawdust) in a media of steam and operates near atmospheric pressure and at a range of operating temperature of 1,023–1,423 K. The analysis is conducted at steam–biomass ratio of 0.8 kmol-steam per kmol-biomass. The gasification system has a capacity of 8.1–8.6 kg h⁻¹ from the steam gasification-derived hydrogen, and the SOFC has an efficiency of 50.3 % and utilises the hydrogen produced from gasifier to generate power. Exergoeconomic analyses are performed to investigate and describe the exergetic and economic interaction between the system components through calculating the exergy costs of the streams for each component of this hybrid system. In the studied gasification temperature range and on the basis of electricity cost of 0.1046 $/kWh, it is found that both primary and secondary hydrogen costs decrease. The unit of exergy from primary hydrogen costs 0.103–0.045 $/kWh, while the unit of exergy from secondary hydrogen costs 0.064–0.039 $/kWh.

Nomenclature

A :

Area (m²)

C :

Carbon content in biomass (wt %)

c :

Cost per unit of exergy ($/kWh)

$\dot{C}$ :

Exergy cost rate ($/h)

${\dot{C}}_o$ :

Annualised cost of a component ($/y)

C ₀ :

Initial investment cost of a component ($)

CRF :

Capital recovery factor (–)

Cp :

Constant pressure-specific heat (kJ/kg K)

$\overline{C}p$ :

Constant pressure-specific heat (kJ/kmol K)

ER :

Exchange rate (CA$/US$)

Ex :

Specific exergy (kJ/kg or kJ/kmol)

$\dot{E}x$ :

Exergy rate (kW)

Ex _o :

Standard exergy (kJ/kmol)

h :

Specific enthalpy (kJ/kg or kJ/kmol)

h ^O _f :

Standard enthalpy of formation (kJ/kmol)

H :

Hydrogen content in biomass (wt %)

i :

Interest rate (%)

j :

Salvage ratio (%)

LHV :

Lower heating value (kJ/kg)

$\dot{m}$ :

Mass flow rate (kg/s)

N :

Nitrogen content in biomass (wt %)

n :

Number of moles (kmol) or lifetime of components (years)

$\dot{N}$ :

Molar flow rate (kmol/s)

O :

Oxygen content in biomass (wt %)

P ₀ :

Reference pressure (101.325 kPa)

Pr :

Biomass cost ($/GJ)

PW :

Present worth ($)

PWF :

Present worth factor (–)

R :

Universal gas constant (8.314 kJ kmol⁻¹ K⁻¹)

s :

Specific entropy (kJ/kg K)

$\overline{s}$ :

Specific entropy (kJ/kmol K)

S :

Sulphur content in biomass (wt %)

S _n :

Salvage value ($)

$\dot{S}$ :

Entropy generation (kW/K)

T :

Temperature (K)

T ₀ :

Reference temperature (298 K)

U _f :

Utilisation factor (–)

$\dot{W}$ :

Power (kW)

X :

Mole fraction (–)

$\dot{Z}$ :

Cost of owning and operating a component ($/h)

Ø :

Maintenance factor (–)

τ :

Annual component operation time at the nominal capacity (h)

β :

Quality coefficient (–)

air :

Air

biomass :

Biomass

ch :

Related to chemical exergy

CO ₂ :

Related to carbon dioxide

e :

Exit

f :

Fuel

F :

Filter

HE :

Heat exchanger

H ₂ :

Related to hydrogen

H ₂ O :

Related to water

i :

Inlet

o :

At reference or ambient or initial

O ₂ :

Related to oxygen

ph :

Related to physical exergy

des :

Destruction

Sep :

Separator

SOFC :

Solid oxide fuel cell

SRR :

Steam-reforming reactor

SSR :

Steam shift reactor

STACK :

Stack

t :

Turbine

tar :

Related to tar

W :

Power or electricity

Over dot :

Quantity per time

Over bar :

Quantity per kmol