Model-based evaluation of an integrated autothermal biomass gasification and solid oxide fuel cell combined heat and power system

• Published in: Proceedings of the Institution of Mechanical Engineers. Part C, Journal of mechanical engineering science Journal of mechanical engineering science, 2017-02, Vol.231 (4), p.672-694
• Main Authors: Borji, Mehdi, Atashkari, Kazem, Ghorbani, Saba, Nariman-Zadeh, Nader
• Format: Article
• Language: English
• Subjects: Biomass; Calorific value; Carbon monoxide; Chemical reactions; Cogeneration; Cold gas; Downdraft; Efficiency; Electric power systems; Equilibrium analysis; Equivalence ratio; Gaseous fuels; Gasification; Heat; Internal reforming; Methane; Numerical analysis; One dimensional models; Operating temperature; Parameter modification; Power efficiency; Power plants; Reaction kinetics; Solid oxide fuel cells; Temperature gradients; Thermodynamic equilibrium; Wildlife conservation
• ISSN: 0954-4062; 2041-2983
• DOI: 10.1177/0954406215621338

Numerical analysis of combined heat and power plant consisting of a solid oxide fuel cell and autothermal gasification system has been made for several cases of different composition of fuel relevant to air and steam blown biomass gasification process. Wet wood is fed to the fixed-bed downdraft gasifier and gaseous fuel is produced then after gas cleaning and conditioning can be used in solid oxide fuel cells. The integrated plant is investigated by thermodynamic modeling combining a one-dimensional model of direct internal intermediate planar type solid oxide fuel cell which allows monitoring the temperature gradients along the cell length in different operating conditions and a zero-dimensional autothermal gasifier. The solid oxide fuel cell mathematical model is developed based on gas species mass balances, energy balance, and an electrochemical model beside the kinetics describing internal reforming and water-gas shift reactions. Such a model can be integrated with adiabatic gasification modeling which includes atom balance conservation for assumed gas species and a modified thermodynamic equilibrium analysis. Both gasifier and solid oxide fuel cell models are verified against experimental and previous numerical data available in the literature. Two main parameters, namely modified equivalence ratio and air-to-steam ratio are investigated and the most important cycle parameters such as power, electric and combined heat and power efficiencies, temperature gradients along the cell length, and mole fractions of gaseous species of the produced fuel are analyzed. It has been revealed that any increase in air-to-steam ratio at fixed modified equivalence ratio leads to penalty on cold gas efficiency of the gasifier and both solid oxide fuel cell and combined heat and power plant electric efficiencies. Increased air-to-steam ratio at constant modified equivalence ratio produces a mixture with lower low heating value, higher steam-to-carbon ratio, rich in CO and lower in CH4 content. Under this condition the operating temperature of the cycle and solid oxide fuel cell increases and consequently improves the operating voltage of the cell and combined heat and power efficiency of the plant. On the other hand, results show that gasification with increased modified equivalence ratio at constant air-to-steam ratio produces mixtures richer in CH4 and CO, poorer in H2 with higher low heating value and cold gas efficiency, and lower steam-to-carbon ratio. Such conditio