Exergetic Assessment of a Hybrid Steam Biomass Gasification and SOFC System for Hydrogen, Power, and Heat Production

Abstract

In this chapter, an integrated process of steam biomass gasification and a solid oxide fuel cell (SOFC) for multi-generation purposes (hydrogen, power, and heat) is thermodynamically studied, and its performance is assessed through exergy efficiency. The scheme combines SOFC at 1,000 K and 1.2 bar and a gasifier which is used to gasify saw dust with a steam–biomass ratio of 0.8 kmol/kmol and a gasification temperature range of 1,023–1,423 K at an atmospheric pressure. A parametric study is performed to assess exergetic efficiency and investigate the effect of various parameters related to the different system components such as airflow rate and preheating temperature on the efficiency. The results show that SOFC is a major source of the system destruction exergy. For the gasification temperature range studied here, the system exergetic efficiency increases with hydrogen yield from about 22 to 32 % and the overall exergy efficiency, which considers electricity production, decreases from 57.5 to 51 %, respectively.

Nomenclature

C :

Carbon content in biomass (w %)

D _aeff :

Effective gaseous diffusivity through the anode (cm²/s)

D _ceff :

Effective gaseous diffusivity through the cathode (cm²/s)

E :

Ohmic symmetry factor

Ex :

Exergy (kJ/kg or kJ/kmol)

Ex _o :

Standard exergy (kJ/kmol)

F :

Faraday constant (96,485 coulombs/g mol)

H :

Hydrogen content in biomass (w %)

h :

Specific enthalpy (kJ/kg or kJ/kmol)

I :

Circuit current (A)

i :

Current density (mA/cm²)

i _o :

Apparent exchange current density (A/cm²)

LHV :

Lower heating value (kJ/kg)

$\dot{m}$ :

Mass flow rate (kg/s)

N :

Nitrogen content in biomass (w %)

$\dot{N}$ :

Molar flow rate (kmol/s)

${\dot{n}}_{H2}$ :

Hydrogen fed to SOFC (kmol/s)

O :

Oxygen content in biomass (w %)

P :

Pressure (pa or atm)

R :

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

S :

Total entropy (kJ/K)

s :

Specific entropy (kJ/kg K or kJ/kmol K)

T :

Temperature (K)

t :

Thickness (cm)

U _F :

Utilization factor (–)

V :

Circuit or overpotential volt (volts)

$\dot{W}$ :

Power (W or kW)

X :

Mole fraction (–)

a :

Anode

act :

Activation

biomass :

Biomass

c :

Cathode

ch :

Chemical

con :

Concentration

dc :

Power from DC

des :

Destroyed

e :

Exit

el :

Electrical

gen :

Generation

H ₂ :

Hydrogen

H ₂ O :

Water

i :

Inlet

o :

Reference or ambient

O ₂ :

Oxygen

ohm :

Ohmic

ph :

Physical

pol :

Polarization

res :

Resistance

SOFC :

Solid oxide fuel cell

t :

Turbine

tar :

Tar

Over dot :

Quantity per time

Over bar :

Quantity per kmol

SOFC :

Solid oxide fuel cell

β :

Quality coefficient (−)

ΔG :

Standard Gibbs function of reaction (kJ/kg)

η :

Efficiency (−)

ρ :

Resistivity (Ω-cm)