A report on steam reformer and fuel cell cooling techniques and fuel cell testing process

 

STEAM REFORMER-

Steam reforming (SR), sometimes referred to as steam methane reforming (SMR) uses an external source of hot gas to heat tubes in which a catalytic reaction takes place that converts steam and lighter hydrocarbons such as methane, biogas or refinery feedstock into hydrogen and carbon monoxide (syngas). Syngas reacts further to give more hydrogen and carbon dioxide in the reactor. The carbon oxides are removed before use by means of pressure swing adsorption (PSA) with molecular sieves for the final purification. The PSA works by adsorbing all impurities from the syngas stream to leave a pure hydrogen gas.

Reforming  Reactions -
The reforming of any hydrocarbon is as follows:

Process  Scheme
Steam reforming can be performed for a wide range of fuels, but the process itself is similar in all cases.

Process for on-board steam reforming.

The fuel of the vehicle is pre-heated in a heat exchanger by the hot exhaust gas of the engine. Before entering the heat exchanger, hot water (steam) is separated from the exhaust gas and used in the steam reformer. Because steam reforming requires water, the usage of a water separator for the exhaust gas makes on-board water storage redundant. In the steam reformer the heated fuel and the steam are converted to syngas, which is then burned in the engine to produce duty.

Advantages-
Some advantages of steam reforming are:

1. The burning value of the fuel is increased, because steam reforming is an endothermic process, resulting in a more efficient fuel.

2. Steam reforming produces less exhaust emissions than burning the feedstock fuel

Disadvantages-
The disadvantages of steam reforming are:

1. Soot is formed in the reactor at high temperatures

2. Water sequestration from the exhaust is not easy to perform

FUEL CELL COOLING TECHNIQUES-

As we know that that the fuel cell efficiency is practically 50%. Which means the half of the energy produced is wasted as heat in the stack, which gives rise to increase in temperature. The increase in the temperature dries the membrane and reduces the conductivity so that fuel cell stops working. There are many types of fuel cell cooling techniques which depends on size and type of use of fuel cell-

1. AIR cooling- It is one of the best method of cooling. Air cooling is done by passing air at high velocity through the porous graphite flow field. Air cooling can be used for the stack of size 2KW. 

2. LIQUID cooling- liquid cooling technique basically involves the use of water. It is most used technique for cooling. In this method cold water is passed through the stack cooling plate. The cold water takes the heat produced within the stack with it and removes the heat in atmosphere and controls the temperature of fuel cell up to the desired level.

3. Heat plate cooling- Heat plate is very high heat conducting plate. Heat plate is placed after 2 -3 MEA in the stack. It absorbs the heat from the stack and releases in the atmosphere.

4. Heat pipe cooling technique- heat pipe is a also a very high thermal conducting plate. Heat pipe is placed in the contact with the stack. It takes the heat generated in the stack and put it outside. So that it controls the temperature of stack.

5. Evaporative cooling of fuel cell- the evaporation of any liquid decreases the temperature of that liquid. Evaporation is possible at any temperature above the absolute zero. In the process of evaporation the molecule of liquid leaves out the liquid by taking the internal energy (temperature dependent) of liquid. So the temperature decreases. it is latest method of fuel cell cooling which is in the research stage. This method of cooling involves the use of wicking material which has capillary action. As we know that at the cathode side water is produced so we can utilize this water for cooling of fuel cell and humidification of hydrogen.  We can put the wicking material in the flow field of fuel cell which will absorb the water produced in the cathode side and will carry it to the anode side so that it can be used for the humidification of H2.

FUEL CELL TESTING SYSTEM- 

We have seen the testing of 1 KW stack.

To test the stack we have an automatic testing system which is controlled by computer. The user can give input to the system like temperature, humidity and flow rate of different gases. The hydrated H2 is supplied from the anode side. And the hydrated O2 is supplied from the cathode side. The testing machine takes controls of flow and humidity of these gases.

As we know that the fuel cell has near about 40-50% efficiency. So half of the power produced is wasted as heat in the stack. So temperature of stack increases very much. To control the temperature rise water is circulated through the stack.

There are many temperature probes which constantly measures the temperature of different part of fuel cell stack. There is also facility to measure the potential of each cell. Each cell potential is measured by voltmeter and is displayed on the computer.

To start testing first we connect all gases connection to the fuel cell. And we connect cooling water pipe also. Fuel cell current collector terminal is connected to the testing machine which is capable to apply different load.

We supply heated H2 and O2 through humidifier. So that the fuel cell stack temperature increases and causes the activation of catalyst and the hydrated gas causes good conductivity of membrane. The PEMFC can be operated nearly 90 degree.

As the temperature of fuel cell increases the open circuit voltage of cell increases. Once the cell temperature reaches nearly 75-80 then we can apply load to take current from the fuel cell.

The fuel cell being tested had 36 cell, So that it is giving near about 35 V on open circuit. We can plot V-I characteristics (polarisation curve) of fuel cell. When we start taking current so many loses in the FC starts which increases the temperature of FC. So we have to start stack cooling system to stop the temperature increase of the stack. If we not stop the temperature rise then it will make the membrane dry and reduce the conductivity of fuel cell so FC stops working.

As we start taking current fro the fuel cell the voltage start decreasing. The rate of decrease in the voltage is not constant all over the curve.

As the current starts increasing the first loss that begin is activation loss after some more increment in the current the ohmic loos becomes significant.

Polarisation curve of Fuel cell

Further increase in the current leads to concentration polarization(mass transfer) losses.

Activation losses are caused by the slowness of the reactions taking place on the electrode surface. The voltage decreases somewhat due to the electrochemical reaction kinetics. This can be seen in the left-hand section of the current-voltage curve above.

The comic losses result from resistance to the flow of ions in the electrolyte and electrons through the cell hardware and various interconnections. The corresponding voltage drop is essentially proportional to current density, hence the term "ohm losses".

Mass transport losses result from the decrease in reactant concentration at the surface of the electrodes as fuel is used. At maximum (limiting) current, the concentration at the catalyst surface is practically zero, as the reactants are consumed as soon as they are supplied to the surface.