Description

The purpose of the Molten Carbonate Fuel Cell System LIBPF demo is to illustrate the process flowsheet solving capabilities of LIBPF, plus the front-end (User Interface), database repository and calculation server, based on a simplified steady-state model of a Molten Carbonate Fuel Cell (MCFC) plant similar to the one described in: Daniela Dellepiane, Barbara Bosio, Elisabetta Arato. “Clean energy from sugarcane waste: feasibility study of an innovative application of bagasse and barbojo” 10 February 2003 Journal of Power Sources.

The prototype shows basic functionality, but it could be easily extended to become a tool for off-line process diagnostic and reconciliation.

Feedstock and pretreatment

Sugar cane residues are processed to a gas with an attractive Low Heating Value (LHV) containing CO, H₂, CO₂, CH₄, H₂O which after clean-up is fed to the plant of interest.

Said fuel enters the REFORMER after it has been warmed in a cross-flux heat exchanger (REGHEX) using hot gases coming from the same reformer.

Fuel preheating is necessary because of the endothermic reforming reaction:

CH₄ + H₂O → CO + 3H₂

which is favoured by high temperatures, and, when is under way, subtracts heat and tends to lower the reactor temperature.

In the model this reaction has been assumed at thermodynamic equilibrium.

Through the reforming reaction CH₄, which is still present in the feeding stream, is transformed into CO and H₂, useful gases to feed MCFC stack.

Moreover the shift reaction:

CO + H₂O → CO₂ + H₂

also takes place in the reformer, giving H₂ and CO₂ and so enriching the biogas further. This reaction also has been considered at thermodynamic equilibrium in the model.

Reformed fuel comes out from the reformer hot and exchanges heat with the feeding stream in the heat exchanger; then it enters the anodic side of the stack.

In particular, as the stack inlet stream temperature is an important key for a proper MCFC operation; the heat exchanger area has been set in such a way to guarantee a suitable anodic inlet temperature of 600 °C.

The cathodic side on the other hand is fed with air that, before entering the electrode, is mixed with the cathodic exhaust gas in the mixer AIRMIX.

The process flowsheet is:

Fuel cell

The MCFC stack produces a direct electrical current by means of the electrochemical processes that take place inside it.

MCFCs are usually planar cells formed by a matrix (tile) filled with Li and K carbonates and coupled with two electrodes where the following electrochemical reactions occur:

• anode: CO₃²⁻ + H₂ → CO₂ + H₂O + 2 e⁻

• cathode: CO₂ + ½ O₂ + 2e⁻ → CO3²⁻

• overall reaction: H₂ + ½ O₂ → H₂O

The fuel and the oxidant are fed separately, and the tile prevents gas crossover and guarantees an adequate ionic conduction and electronic insulation.

The stack is composed of a number of superimposed cells connected in series, via bipolar plates, to supply the requested voltage.

The MCFC stack has been modelled with a concentrated parameters electrochemical model based on (3).

Postprocessing and recycle

The gaseous stream that is present at the anodic exit contains unreacted H₂, H₂O and CO₂ coming from the electrochemical reaction in the anodic zone, little quantities of unreacted CO and CH₄, and N₂.

Instead, at the cathodic exit a gaseous stream containing unreacted O₂, CO₂, H₂O and N₂ is obtained.

This gaseous stream is partly (about 40%) released into the atmosphere (SPLITOUT), and partly channelled into the catalytic burner CB (RECYCLE) in which it is mixed with the anodic gaseous stream.

The stack exhausts mixed in this way react through the following complete oxidation reactions:

CO + ½ O₂ → CO₂

H₂ + ½ O₂ → H₂O

CH₄ + 2 O₂ → CO₂ + 2 H₂O

which, in the model, involve the total combustion of the unreacted anodic gases thanks to the cathodic O₂. These reactions are exothermic and so they generate heat, therefore, steam and CO₂ created in the burner enter the reformer with the unreacted stack gases; here they exchange heat with the feeding stream and so the necessary heat for the reforming reaction is produced.

The gases leaving the reformer regain pressure in a blower and then they are mixed with the air that is fed to the cathodic size.

Main data

The FUELIN stream is the biogas received from the clean-up section after the biomass gasifier, pressure 3.6 bar, temperature 400 °C, total mole flow 10 kmol/h and composition as follows:

The STEAM stream is saturated steam at 5 bar and is fed with an excess of 75% with respect the stoichiometric amount required for complete conversion of CH₄ and CO to H₂ (this results in a total humidified feed flow of 34.6 kmol/h in accordance with the reference publication).

The AIRIN stream at 137 °C was set to 56.5 kmol/h in accordance with (1) page 52 to get a cathode inlet temperature of 600 °C.

The split ratio to SPLITOUT in SPLIT1 is set to 40%.

Modelling Simplifications

In this case study the following model simplifications were used:

1. The compressor is isentropic with ideal yield θ = 1.0

2. All the units are modelled as concentrated parameters