Modelling of a Combined Biomass Clc Combustion and Renewable-Energy-Based Methane Production System for CO2 Utilization

In this work, an innovative process layout to promote the integration among the chemical looping combustion of biomass, solar hydrogen, and carbon methanation is proposed and numerically investigated.

The core of the layout consists of a multiple interconnected fluidized bed system (MFB) equipped with a two-stage fuel reactor (t-FR), a riser used as Air Reactor (AR), a cyclone, a L-valve return leg, and a loop-seal. The two-stage FR is based on the concept presented in a previous work [1], where two bubbling beds were placed in series with respect to both gaseous and solids streams in order to overcome the limitations of a single-stage fuel reactor (poor char conversion, slip of unburnt volatiles, extensive elutriation of char fines). In the first stage, where fuel and fresh oxygen carrier (OC) from the AR enter, the solid carbon combustion took place. Volatile combustion occurs mainly in the second stage, where the partly reduced OC is present. An internal riser connects the two stages, thus allowing the solids to move from the first reactor to the other. The t-FR showed the best performances in terms of combustion efficiency, volatile matter and char conversion, carbon-to-CO₂ conversion efficiency and loss of elutriated carbon for all the operating conditions investigated in [1]. Solids from the second stage go through the loop-seal into the AR where the oxidation capability of the OC is restored. A cyclone collects the regenerated OC that is sent through the L-valve to the first stage of the t-FR. At the exit of the second stage, water and fines were separated from flue gas. This latter is partly conveyed to a methanation unit in order to react with a hydrogen stream coming from an-electrolysis cells (EC) array, while the remnant is recycled to FR.

A coupled hydrodynamic and reactive model of the multiple interconnected fluidized bed system, developed on the basis of a previous work [2], is applied in order to evaluate solid circulation rate, solid bed levels in different parts of the multiple interconnected fluidized bed system, flue gas composition and flow rate, and power production. The hydrodynamics of the proposed MFB system is dynamically modelled by considering the different parts as separate blocks mutually interconnected. The performance of the system has been evaluated by considering chemical and physical properties of olive wood and of CuO supported on zirconia as fuel and oxygen carrier, respectively. Olive wood was selected as representative of residual biomasses diffused into Mediterranean area. CuO has been selected on the basis of the favourable thermodynamic equilibrium of oxygen release in the temperature range of interest for the chemical looping combustion. The utilization of ZrO₂ as support, instead, ensures good chemical and physical stability. The mass fraction of the active phase in the oxygen carrier has been set at 50%. A complex kinetic scheme comprising both gas–solid heterogeneous reactions taking place in the fluidized beds - involving both carbon (direct oxidation, Boudouard, and water gas reactions) and oxygen carrier (oxidation of copper and cuprous oxide, reduction of cupric and cuprous oxides) - and gas-phase homogeneous reactions occurring in the freeboard of fuel reactors (oxidation of volatiles and hydrogen) has been considered, extending the reaction scheme previously [1] developed.

The methanation unit has been modelled developing a thermodynamic calculation method based on minimization of the free Gibbs energy. Such a model does not need the application of a detailed reactions stoichiometry or kinetics model, only requiring that the system have enough time to reach equilibrium. At steady state, it was verified that the performances of the Gibbs’ model match those obtained from a detailed kinetic model. This approach ensures the evaluation of methane yield without the need of hydrodynamic and kinetic modelling of the methanation unit itself, thus greatly reducing the computational effort. In fact, several reactor concepts have been proposed and investigated in literature [3], ranging from multiple adiabatic layers fixed beds to fluidized bed reactors, in order to contain the temperature rise associated to the highly exothermic Sabatier reaction and to avoid catalyst sintering while approaching the best thermodynamic condition for the process. The performance of the system has been evaluated by considering that the CO/CO₂ stream coming from the t-FR reacts with a H₂ pure stream coming from the EC array over NiO supported on alumina catalyst.

The number of ECs arranged in the array was evaluated by considering that a constant hydrogen production able to convert the whole CO/CO₂ stream produced by the CLC process should be attained. Moreover, by considering that only energy coming from renewable sources (such as photovoltaic panels or wind turbines) was fed to the EC array, it was assessed the capability of the proposed process to be used as an energy storage system.

Under the operating conditions that enable steady state operation of the proposed process, solid circulation rate, flue gas composition, thermal power output, methane yield and mass flow rate, and electrolysis cells number, along with the number of wind turbines and/or the photovoltaic panels area needed, were evaluated.

[1] Coppola A., Solimene R., Bareschino P., Salatino P., “Mathematical modeling of a two-stage fuel reactor for chemical looping combustion with oxygen uncoupling of solid fuels”, Appl. Energy, (157) 449-461, 2015.

[2] Diglio G., Bareschino P., Solimene R., Mancusi E., Pepe F., Salatino P., “Numerical simulation of hydrogen production by chemical looping reforming in a dual fluidized bed reactor”, Pow. Tech., (316) 614-627, 2017.

[3] Rei-Yu Chein , Ching-Tsung Yu, Chi-Chang Wang, “Numerical simulation on the effect of operating conditions and syngas compositions for synthetic natural gas production via methanation reaction”, Fuel, (185) 394-409 (2016).