Re-Electrification Of Hydrogen:-Why? and How?

Due to heavy restrictions on the emission of greenhouse gases by governments, solar and wind would play a significant role in power production in the near future, contributing to about 60% of power production by the year 2050. When wind and solar play a major role in renewable energy systems, why do we need to re-electrify hydrogen to produce electricity? The main reason is that the electricity generated by them is based on weather conditions and mainly covers only half of the total energy demand. The figure below shows the electricity consumption in Germany for a day and the sources contributing to it. If green H2 is produced and re-electrified, this could fill the gap between the power produced through renewable energy sources and the actual power demand. This not only bridges the gap in-between but also reduces the emission of carbon and other greenhouse gases into the environment. If the hydrogen supply chain is large enough, then the burning of fossil fuels to produce electricity in a conventional powerplant can also be evaded entirely.

The chemical and combustion properties of hydrogen gas can be used to re-electrify hydrogen or to convert hydrogen into electricity. This can be done either by fusing H2 with O2 on a series of connected fuel cells in a fuel cell power plant or by combusting H2 in a large powerplant. In either case, it requires a large-scale electrolyzer powered by a renewable energy source to produce hydrogen and a pipeline system built with suitable materials to transport the H2 produced to the storage area. The hydrogen storage mainly employed could be underground storage tanks where hydrogen is stored as pressurized gas or a liquid depending on the time duration for hydrogen storage.

Re-electrification Of H2 In Fuel Cell Powerplant

A hydrogen fuel cell works on electricity generation through an electrochemical reaction between hydrogen and oxygen. The end product of this reaction is mostly water, along with some heat. These devices are so powerful that when combined could produce a massive power output equal to or greater than a conventional powerplant. A simple layout of a hydrogen fuel cell power plant is shown in the figure below.

The three main components of a fuel cell are the anode, cathode, and electrolyte membrane. The hydrogen molecules get split into electrons and protons at the anode. These electrons pass through an external circuit generating electricity, and the protons diffuse through the electrolyte membrane, which then combines with oxygen and the electrons from the external circuit to form water [82]. Re-electrifying hydrogen in a fuel cell is more cleaner and more efficient way of producing electricity. The only disadvantage is that they are highly expensive due to the platinum catalyst used. Hanwha energy’s hydrogen fuel cell plant is the world’s largest fuel-cell powerplant in size and capacity and is located in the Daesan industrial complex in Korea.

The electricity from renewable energy sources is used to produce hydrogen in an electrolyzer. In this way, green hydrogen can be produced, then compressed and stored in underground tanks. A small amount of oxygen produced during the electrolysis is then directed to another storage tank, where it gets mixed with the purified air coming from the microfilters. This increases the oxygen concentration in the air, which will speed up the reaction at the cathode in the presence of a catalyst, and hence an increased efficiency is possible. Then the H2 and O2 get reacted inside a series of fuel cells to produce electricity.

Re-electrification Through Combustion Of H2

Combustion is the process in which a substance decomposes in the presence of oxygen, producing heat and oxides as the products of combustion. This substance that is combusted is called fuel. A fuel can either be in a liquid, solid, or gaseous state but mostly liquid. Combustion plays a significant role in energy transformation today like combustion of coal produces electricity, combustion of petrol and diesel enables transport, and so on. Similarly, hydrogen as gas can also be combusted to produce electricity or mechanical energy to do work.

Conventional Steam Power Plant Using HFSG and CHB

A conventional steam power plant works on the principle of an ideal thermodynamic cycle called the Rankine cycle, where steam and water are the working fluid. The heat energy produced by burning a fuel is used to produce steam, which is expanded in a steam turbine that runs a generator to produce electricity. The main components of a steam turbine power plant are:

Steam boiler (HFSG)

In the steam boiler, fuel is burned to produce heat which is then used to boil the water inside the boiler and convert it into steam at a required temperature. Typically coal is used as the primary fuel for steam power plants, but we use HFSG for producing steam. HFSG refers to Hydrogen Fuelled Steam Generation, in which hydrogen is burned in the boiler to produce heat. The combustion of a stoichiometric mixture of hydrogen and air/oxygen is an exothermic reaction producing heat that can be used to generate steam. This reaction itself produces steam at high temperatures in the range of 1000-1500 °C, which can then be used in a heat exchanger to superheat the steam further. If pure oxygen is used, then the steam temperature would be very high for its use in the steam turbine; hence, water is injected inside the steam generator to reduce the steam temperature. A schematic design of a hydrogen-oxygen steam generator is shown in the figure below. Due to high operational temperatures, NOx emissions are very likely to develop, which should be removed from the exhaust vapors.

Steam turbine

Depending on the temperature and pressure of the working fluid, turbines are used in stages, i.e., more than one steam turbine mounted to the same generator shaft. These include:

1. High-pressure turbine.

2. Medium/ Intermediate pressure turbine.

3. Low-pressure turbine.

A high-pressure turbine converts the heat energy from the superheated steam to mechanical energy as the steam travels through the turbine blades. This turbine is smaller compared to the other two, and the steam expands over multi-stage blades, reducing the temperature of the steam. The steam leaving the high-pressure turbine is then passed through a re-heater which increases the temperature of the steam but at lower pressure in order for it to get further expanded in the intermediate steam turbine. A medium-pressure turbine is similar to a high-pressure turbine but is a bit bigger, allowing low-pressure steam to expand further. Then the steam is passed through a low-pressure turbine that extracts the remaining available energy. LP turbines are even bigger in size and have one or two rotors.

Reheater

A reheater, also called a heat exchanger, normally adds more heat to the output steam coming from the high-pressure turbine in a conventional Rankine cycle power plant. This thermodynamically increases the efficiency of the entire system. This is done by using the heat from the boiler. But in an HFSG boiler, the exhaust from the boiler could be used for this purpose. As the exhaust temperature is very high, this can be used to superheat the steam before it passes through the high-pressure steam turbine. However, there is another possible concept to reheat the output stream from the HP turbine. It’s called catalytic hydrogen-burning, which is described below.

Catalytic hydrogen burning (CHB) This method uses a chemical reaction between hydrogen and oxygen in the presence of a catalyst to produce heat. This process is also called the flameless combustion of hydrogen. This method is safer than flame combustion because it operates at a lower temperature of around 300 °C [93], and hence the formation of harmful nitrogen oxides (NOx) is avoided, and consequently, the only by-product of this reaction is water vapor which can be released into the environment. These reactions take place inside a catalytic burner. The schematic diagram of a catalytic burner is shown in the figure below. These burners consist of a channel of catalyst, mostly palladium or platinum. These catalysts promote the oxidation of hydrogen with oxygen in the air, producing heat that can then be extracted from the surface. Due to low operational temperature CHB is still a possibility that requires further research for use in powerplants.

The other components include the generator, condenser, and several auxiliary systems like air filter hydrogen storage tank, pumps, etc. All the turbine shafts are connected to the generator that converts the mechanical energy into electrical energy. The air filter supply purified air to the boiler for combustion. Hydrogen tanks are used to store the hydrogen produced through electrolysis. A part of the steam coming from the low-pressure turbine at the end can be used to perform high-temperature electrolysis that produces furthermore hydrogen. This H2 can be stored and used to power the CHB heat exchanger, which would reheat the expanded steam from the high-pressure turbine. The remaining steam left can be condensed inside the condenser and passed back to the boiler. The layout of a simple hydrogen-powered steam power plant is shown in the figure below:

Combined Cycle Gas Turbine Power Plant

A combined cycle refers to the combination of different thermodynamic cycles in a way that power can be produced more efficiently. A layout of a simple hydrogen-powered gas turbine power plant is shown in the figure below. Electrical efficiencies of up to 60 % can be achieved through combined cycle power plants. A combined cycle power plant consists of a gas turbine and a steam turbine connected by a heat recovery steam generator (HRSG). The main components of a combined cycle powerplant are as follows.

Heat recovery steam generator (HRSG)

The HRSG is a heat exchanger that uses the heat from the gas turbine's exhaust to produce steam. Hence HRSG is also called the boiler of the CCGT power plant. The exhausts from the gas turbine are made to pass over sections of tubes that circulate hot water using pumps. This heat enables the formation of steam inside the tubes. An HRSG consists of three modules, namely the economizer, evaporator, superheater, and preheater. The economizer preheats the water to the boiling point. The evaporator uses the heat from the exhaust to create steam inside the tubes. This steam is passed through the superheater that produces dry steam, which is then expanded in the steam turbine, and the preheater is used to preheat the heat exchanger fluids and is located at the cold end of the HRSG. The superheated steam from the HRSG expands in a steam turbine and then is sent to a condenser. The other main component is the steam turbine which is already explained above.

Conclusion

It can be seen that re-electrifying hydrogen can certainly fill the gap between the power demand and the power produced from renewables. Of course, there are several challenges in doing so and this idea would become a reality only when a strong hydrogen supply chain is developed. Countries rich in renewable energies can produce green hydrogen and transport it all over the globe just like it's done with fossil fuels today thereby making renewable energies a more sustainable source.  

-Afrin Hewitt Alban.