Safe Handling of Hydrogen
MVS is proud to announce that another informative article has recently been published in the Power Watch India magazine June 2011 issue.
This informative article was written by our Vice-President (Marketing) –
Mr. Rajeev Bhalla.
You may view the actual print article here.
This article is also reproduced verbatim below.
Safety Issues Related to Hydrogen Generation Technologies
Hydrogen in Power Plants
Hydrogen’s properties of high thermal conductivity and lowest density make it an ideal choice as cooling medium in higher capacity Turbine Generators (TG). Due to lowest gas density it improves the efficiency of the TG reducing the windage losses as compared to air-cooled TG. Nearly 70% of TGs bigger than 60 MW, worldwide, are Hydrogen-cooled.
What are the challenges posed by Hydrogen?
1. Physical Properties of Hydrogen
Hydrogen is a colourless, odourless, tasteless and highly combustible gas. Presence can only be detected by Gas sensors.
2. Flammability & Explosive Nature
Hydrogen gas is highly flammable and will auto ignite when in contact with air at Hydrogen concentrations ranging from 4% to 75%. Hydrogen gas leaking into air may ignite spontaneously. Hydrogen fire is extremely hot and invisible, thus leading to severe accidental burns. Under the optimal combustion condition (a 29% hydrogen-to-air volume ratio), the energy required to initiate hydrogen combustion is much lower than that required for other common fuels.
Hydrogen leaks into closed spaces and unventilated areas can very quickly pose danger of explosion.
3. Typical reasons for accidents
Reasons for Hydrogen related accidents are similar to reasons for accidents related to other combustible gases. Human error, or overlooking to perform Gas handling operations as per prescribed safe practices, lead to most accidents.
Hydrogen is a very small molecule with low viscosity, and therefore prone to leakage. Unless dense metal forms, e.g., forgings or killed steels are used, Hydrogen being the smallest molecule, permeates out of tanks and cylinders. Many metals embrittle due to prolonged contact with Hydrogen leading to failure due to metal fatigue. Hence, inadequate equipment design considerations can lead to failures and accidents.
How Hydrogen is delivered to the TG?
1. Commercially sourced Cylinders
This is by far one of the most common methods of Hydrogen delivery at Power Plants, however it is fraught with challenges such as lower purity of Hydrogen, moisture presence in Hydrogen which in turn leads to corrosion of windings and thereby reduced life of equipment. Another major challenge is the availability of cylinders and hence Power Plants maintain a large inventory of Hydrogen cylinders on-site to allow for any supply chain disruptions.
As reported on U.S. Department of Energy incident reporting site (www.h2incidents.org), many accidents pertaining to unplanned release of Hydrogen have occurred due to improper refill connection or equipment failure (please see attached photograph of Hydrogen explosion at a Power Plant in USA in 2008). Increased number of connections and disconnections of threaded joints increases probability of Hydrogen leakage and safety failures. By international conventions, all flammable gas joints are reverse threaded. Many Hydrogen users have been caught using wrong threaded cylinders leading to disastrous consequences.
2. On-Site Hydrogen Generators
Due to challenges of getting Hydrogen delivered from outside vendors, On-site Hydrogen Generation has continued to gain acceptance in power plants. Many technologies exist for On-site Hydrogen generation including Hydrogen from Water, Hydrogen from hydrocarbons such as Methanol or Natural Gas.
Since the Hydrogen volume required in a Power Plant is very small, it is best served by Water Electrolysis Hydrogen Generators. Hydrogen from Water using Electrolysis has become a preferred mode due to the relative lower cost of the equipment and much smaller size and relative simplicity of operation and maintenance.
Comparing aspects of safety in various Water Electrolysis technologies
1. Unipolar Water Electrolysis Hydrogen Generators
This technology is the original method of water electrolysis and still used in Power Plants. Unipolar Water Electrolyzers use closed top low-pressure tanks for Hydrogen generation. Electrolyte used is DM Water and Caustic Lye solution. Safety challenges posed by such plants are:
a. Handling of hazardous chemicals such as Potassium Hydroxide.
b. Operator exposure to fumes and hot working conditions.
c. Hydrogen leakage due to poor mechanical design.
d. Higher Hydrogen-Oxygen mixing risk due to flimsy separator quality.
e. Primary generation of Hydrogen goes to water-floated gas-holders. Mechanical limit switch malfunction can lead to blowouts.
f. Increased risk due to two stages of compression systems.
g. Very high probability of KOH (caustic) traces carry over with Hydrogen gas, in turn leading to corrosion failures in compressors, cylinders and furthermore in the TG internals.
2. Bipolar Water Electrolysis Alkaline (Liquid) Electrolyte
Bipolar Water Electrolyzers are continuing to replace the older Unipolar technology electrolyzers due to their relatively smaller size. This technology, however, poses the following safety concerns:
a. Hazardous Electrolyte Chemicals
Alkaline based Bipolar water electrolysis still requires the liquid alkaline electrolyte using hazardous chemicals such as Potassium Hydroxide (KOH). Operators are exposed to these chemicals due to the requirement of electrolyte lye replacement or top up every few months.
b. Sludge Formation
The liquid electrolyte circulating through the system needs to be constantly monitored for its specific gravity to ensure there is no sludge formation. Sludge formation, in turn leads to inefficient operation of the electrolyzer and increasing requirement of power to generate same amount of product Hydrogen. It also leads to higher operating temperatures, which in turn create a very unsafe condition.
c. Balanced Pressure Hydrogen & Oxygen production
Water electrolysis produces Hydrogen from water by splitting Water (H2O) into its molecules H2 and O2. Hydrogen and Oxygen are produced at similar process pressure and hence external monitoring measures have to be implemented to prevent mixing of Hydrogen and Oxygen. Oxygen Analyzers monitor Hydrogen stream and Hydrogen Analyzers monitor the Oxygen stream for unsafe levels, thereby signaling a shutdown. However, the analyzers themselves have a limited life and cannot be trusted to provide foolproof safety. Failure to change sensors upon end of useable life can lead to disastrous accidents.
d. Zone II classification
The Bipolar alkaline electrolyzer, by virtue of its technology and internal volume containing Hydrogen and Oxygen, creates a hazardous area classification and as a result all other equipment installed in it’s vicinity have to be fit for hazardous area installation. Usage of ordinary electrical and electronic instruments or components will lead to unsafe operating conditions.
e. Asbestos Diaphragms
The diaphragm used for separation of Hydrogen and Oxygen is typically of asbestos, which is a known carcinogen.
f. Dependency on analyzers, gas leak detectors and PLC for safe operation
Purity analyzers monitor unsafe levels of gas in the Hydrogen and Oxygen streams and are supplemented with Gas leak detectors to detect any leakage of Hydrogen in the electrolyzer area. Monitors/Analyzers with expired sensors are a self-defeating safety mechanism, completely dependent on human intervention and corrective action. A PLC, responsible for safety, itself malfunctioning or program getting corrupted needs an over-seeing hot redundant PLC.
How does Proton OnSite, USA – HOGEN® SPE Technology Bipolar Water Electrolysis address safety concerns?
HOGEN® SPE (Solid Polymer Electrolyte) technology is an advancement of Bipolar technology, wherein the alkaline electrolyte is replaced by a fixed membrane coated with the electrolyte. HOGEN® incorporates numerous safety features including:
1. Solid Polymer Electrolyte – Non-corrosive and safe to human touch. The solid electrolyte neither circulates, nor gets consumed, thereby giving over 10-years of functional life of electrolyzer cell. (Please see attached photograph of Solid membrane)
2. Differential Pressure Design – This key design aspect of keeping Oxygen and Hydrogen at different pressures, ensures that physically, Oxygen cannot entrain the Hydrogen stream. This ensures safety without need for external monitoring.
3. No asbestos is used. The diaphragm (or Proton Exchange Membrane) is a commercially available product also used in Hydrogen Fuel cells and is safe to touch and for human use.
4. Hard-wired dual redundant independent safety circuit is independent of the PLC or microprocessor controlling the plant operation.
5. Dilution purge air does not ever allow Hydrogen concentration to approach unsafe levels.
6. The HOGEN® Hydrogen unit does not change the area classification to hazardous and is thus fit for installation in a normal well-ventilated industrial space.
7. Elimination of cylinder handling for daily make-up Hydrogen requirement in the TG by using a fixed pipeline for Hydrogen distribution.
To summarize, Hydrogen does pose challenges due to its combustible nature. However, with adequate safety measures taken into consideration in design stage itself and also by using advanced Hydrogen generation technologies such as Solid Polymer Electrolyte (SPE), customers can start to breathe easy when considering Hydrogen Generation equipment for their Power Plants.
About MVS Engineering Limited
MVS Engineering is a turnkey supplier of Gas generation equipment, Air and liquid drying equipment. MVS was founded in 1977 and has supplied nearly 7000 Skid mounted units worldwide. Read more about the company in the About Us section of our website.
Specifically for Proton OnSite’s products, MVS is Proton’s engineering partner in India for sales, service and turnkey supply of engineered solutions. Whether you require a single Hydrogen Generator or a complete Hydrogen solution including compression, bottle filling etc., MVS is your one stop provider.
About Proton OnSite (earlier known as Proton Energy Systems)
Proton Onsite designs and manufactures Proton Exchange Membrane (PEM) electrochemical systems to make hydrogen from water in a zero pollution process producing safe, pure, reliable onsite hydrogen to meet today’s global hydrogen requirements. Proton has been developing and manufacturing world-class electrolysis systems since 1996, with thousands of units deployed world-wide, on every continent. With a reputation for building robust, reliable, and safe systems, federal, state, and commercial partners repeatedly seek the creative solutions that Proton OnSite has proven it is capable of delivering. For more information, visit www.ProtonOnsite.com.