Hydrogen in power plants

The high thermal conductivity and lowest density properties of hydrogen make it an ideal cooling medium for high capacity turbine generators (TG). Due to the lowest gas density， it increases the efficiency of the turbine generator and reduces wind resistance losses compared to air-cooled turbine generators. Nearly 70% of the world's turbine generators above 60 MW are hydrogen cooled.

What are the challenges posed by hydrogen?

1. The physical properties of hydrogen

Hydrogen is a colorless， odorless， tasteless and highly flammable gas. Its presence can only be detected by gas sensors.

2. Flammability and explosive properties

Hydrogen is highly flammable and will automatically ignite when in contact with air at concentrations of 4% to 75%. Hydrogen that leaks into the air may spontaneously combust. Hydrogen fires are very hot and invisible and can therefore lead to serious accidental burns. Under optimal combustion conditions (hydrogen to air ratio of 29% by volume)， the energy required to start hydrogen combustion is much less than that required by other common fuels.

Hydrogen leaks into enclosed spaces and unventilated areas can quickly create a risk of explosion.

3. Typical causes of accidents

The causes of hydrogen-related accidents are similar to those of other combustible gas-related accidents. Human error， or neglecting to follow prescribed safe practices for gas handling operations， causes most accidents.

Hydrogen is a very small molecule with a low viscosity， which makes it easy to leak. Unless a dense form of metal is used， such as forged or killed steel， hydrogen gas， being the smallest molecule， can seep out of tanks and cylinders. Many metals become embrittle due to prolonged contact with hydrogen， leading to metal fatigue and failure. Therefore， inadequate consideration of equipment design can lead to failures and accidents.

How is hydrogen delivered to the TG?

1. Commercially sourced gas cylinders

This is by far one of the most common methods of hydrogen delivery to power plants， but it is fraught with challenges such as low hydrogen purity and the presence of moisture in the hydrogen， which in turn can lead to corrosion of the windings， thus reducing the life of the equipment. Another major challenge is cylinder availability， so power plants maintain a large inventory of hydrogen cylinders on site in case of any supply chain disruptions.

As reported on the U.S. Department of Energy's accident reporting website， many accidents related to accidental hydrogen releases have occurred due to improper filling connections or equipment failures (see attached photo of a hydrogen explosion at a U.S. power plant in 2008). The number of connections and disconnections of threaded joints increases the likelihood of hydrogen leaks and safety failures. According to international conventions， all fittings for flammable gases are reverse-threaded. Many hydrogen users were found to be using the wrong threaded cylinders， leading to catastrophic consequences.

2. On-site hydrogen generators

On-site hydrogen generation has continued to gain acceptance in power plants due to the challenges of obtaining hydrogen from outside suppliers. There are many technologies for on-site hydrogen generation， including hydrogen extraction from water and hydrogen extraction from hydrocarbons， such as methanol or natural gas.

Since the amount of hydrogen required for power plants is very small， water electrolysis hydrogen generators are preferred. Hydrogen production using electrolysis of water has become a preferred mode due to the relatively low cost of the equipment， its smaller size， and its relatively simple operation and maintenance.

Comparing the safety issues in various water electrolysis technologies

1. Monopole water electrolysis hydrogen generator

This technology is the original method of water electrolysis and is still used in power plants. The monopole water electrolyzer uses a closed top low pressure tank to produce hydrogen. The electrolytes used are DM water and caustic soda solution. The safety challenges posed by this equipment are

a. Handling of hazardous chemicals， such as potassium hydroxide.

b. Exposure of the operator to fumes and high working conditions.

c. Hydrogen leakage due to poor mechanical design.

d. High risk of hydrogen-oxygen mixing due to unstable separator quality.

e. The main generator of hydrogen gas is the cylinder floating on the water surface. Failure of the mechanical limit switch may lead to well blowout.

f. Increased risk due to two-stage compression system.

g. Traces of KOH (caustic soda) are likely to be carried with the hydrogen， leading to corrosion failures in the compressor， cylinder， and inside the TG.

2. Bipolar water electrolysis Alkaline (liquid) electrolyte

Bipolar water electrolyzers are continuing to replace the older monopolar technology electrolyzers due to their relatively small size. However， this technology poses the following safety issues.

a. Hazardous electrolyte chemicals

Alkaline based bipolar water electrolysis still requires the use of liquid alkaline electrolytes such as potassium hydroxide (KOH) and other hazardous chemicals. Operators are exposed to these chemicals as the electrolyte lye needs to be replaced or added every few months.

b. Sludge formation

The liquid electrolyte circulating in the system requires constant monitoring of its specific gravity to ensure that no sludge is formed. Sludge formation in turn leads to inefficient operation of the electrolyzer and increases the power requirements to produce the same amount of product hydrogen. It also leads to higher operating temperatures， which in turn creates a very unsafe condition.

c. Balanced pressure hydrogen and oxygen production

Water electrolysis produces hydrogen from water by splitting water (H2O) into H2 and O2 molecules. Hydrogen and oxygen are produced at similar process pressures， so external monitoring measures must be taken to prevent the mixing of hydrogen and oxygen. Oxygen analyzers monitor the hydrogen flow and hydrogen analyzers monitor unsafe levels of oxygen flow to signal a shutdown. However， the analyzer itself has a finite life and cannot be trusted to provide foolproof safety. Failure to replace the sensor at the end of its usable life could result in a catastrophic accident.

d. Two-zone classification

Bipolar alkaline electrolyzers， due to their technology and the volume of hydrogen and oxygen contained inside， create a hazardous area classification and therefore all other equipment installed in their vicinity must be suitable for hazardous area installation. The use of common electrical and electronic apparatus or components will result in unsafe operating conditions.

e. Asbestos diaphragms

Diaphragms used to separate hydrogen from oxygen are usually asbestos， a known carcinogen.

f. Rely on analyzers， gas leak detectors and PLCs for safe operation

A purity analyzer monitors unsafe gas levels in the hydrogen and oxygen streams and is supplemented by a gas leak detector to detect any hydrogen leaks in the electrolyzer area. The monitor/analyzer with expired sensors is a self-defeating safety mechanism that is totally dependent on human intervention and corrective measures. A supervised， hot redundant PLC is required in the event that the PLC responsible for safety itself fails or the program is corrupted.

How does Proton OnSite USA - HOGEN?SPE technology bipolar water electrolysis solve the safety problem?

HOGEN? SPE (Solid Polymer Electrolyte) technology is an advancement in bipolar technology where the alkaline electrolyte is replaced by a fixed membrane coated with electrolyte.HOGEN? incorporates a number of safety features， including.

1. solid polymer electrolyte ? Non-corrosive and safe for humans. The solid electrolyte is neither recycled nor consumed， resulting in an electrolytic cell with a functional life of over 10 years. (Please refer to the attached photo of the solid membrane).

2. Differential pressure design? This key design keeps the oxygen and hydrogen at different pressures， ensuring that physically， the oxygen cannot entrain the hydrogen stream. This ensures safety and does not require external monitoring.

3. No asbestos is used. Diaphragms (or proton exchange membranes) are a commercially available product that is also used in hydrogen fuel cells and are safe to touch for human use.

4. Hardwired dual redundant independent safety circuits are independent of the PLC or microprocessor controlling the operation of the equipment.

5. Dilute purge air does not allow hydrogen concentrations to approach unsafe levels.

6. The hydrogen unit does not turn area classification into a hazard and is therefore suitable for installation in normally well-ventilated industrial spaces.

7. Eliminates cylinder handling for routine hydrogen replenishment in the TG by using fixed piping to distribute hydrogen.