Biogas-Fueled Generators Can Go Boom Without Proper Precautions. Find a Remedy Here

A solution that combines hardware with software simulation can help biogas-to-energy plants minimize the risk of damaging explosions.

Biogas-Fueled Generators Can Go Boom Without Proper Precautions. Find a Remedy Here

HOERBIGER EVT and EVM overpressure relief valves are flameless and reclosable solutions for wastewater treatment facilities. 

Interested in Dewatering/Biosolids?

Get Dewatering/Biosolids articles, news and videos right in your inbox! Sign up now.

Dewatering/Biosolids + Get Alerts

Biogas-fueled combined heat and power systems are increasingly common at wastewater treatment facilities.

Recent studies indicate that the United States has more than 2,200 operating biogas power systems and has potential to add some 13,500 more. However, the use of green and sustainable biogas energy brings some often overlooked risks in the form of explosions caused by backfire: the ignition of unburned gaseous fuel in intake and exhaust manifolds, exhaust lines and downstream piping.

These backfires can have devastating effects on the engine, exhaust ducts or even buildings and personnel. Now, practical and innovative solutions can mitigate this threat by using a self-resetting, closeable, spring-loaded valve with an integral flame arrester.  The valves can be used indoors or outdoors and can be activated multiple times without plant downtime or maintenance. 

Variable fuel

The explosion risk is vividly demonstrated when viewed in terms of the intake manifolds or exhaust lines of biogas-fueled engines, where all five elements of the explosion pentagon are potentially present during operation.

Biogas, when compared with natural gas, has more variable quality and a lower methane content. This poses an engineering challenge when setting up (mapping) the engines and acquiring the necessary air/fuel settings to achieve the optimum performance.

Biogas consists mainly of methane (CH4) and carbon dioxide (CO2), but also contains impurities in amounts that depend on feedstock used. Therefore, it is necessary to conduct engine mapping during startup and after any significant changes to the process, conditions, feedstock or fuel to ensure the plant operates efficiently and safely.

For example, if an explosive mixture develops in the exhaust during startup — methane falling between the upper and lower explosive concentration — an ignition source in the form of engine backfire or contact with hot components can generate an explosion, releasing a massive pressure wave and flame front, with devastating consequences.

Since engines powered by conventional fossil fuels and many alternative fuels do not exhibit this same exposure to variable fuel quality and methane content, there is low awareness of explosion protection techniques in exhaust ducts at the initial design engineering phase.

Fitting the application

However, previous incidents in biogas plants confirm that there is a real explosion risk during the routine operation of the plant; therefore, proper consideration of mitigation measures is necessary.

Such mitigation and protection measures need to consider the specifics of the application, including the explosive characteristics and variability range of the fuel and the exhaust duct pressure rating, length, diameter and geometry.

The initial assessment of any potential biogas engine explosion protection scheme requires design calculations to be made to determine the size, type, number and placement of explosion relief devices capable of mitigating the effects of any incident. A Windows-based engineering simulation software program is available to fulfill that requirement and to enable the selection of the optimum level of explosion protection.

The simulation has been independently tested by FTZU, an internationally recognized Notified Body Test Institute, using a variety of installations with different pipeline lengths, diameters and added components to create a range of realistic physical layouts.

After testing, FTZU identified a very close match between simulation and test. The software accurately predicted not only the right qualitative behavior but also the size and timing of the pressure peak.

Deploying the solution

Once the simulation results have been generated, the design of the protection system and devices required to mitigate any explosive/overpressure incidents can be formulated. The hardware solution consists of a series of flameless and reusable explosion-relief valves, designed in consultation with leading engine manufacturers specifically to protect personnel and plant equipment.

The device consists of a circular explosion-relief vent with stainless steel flame arrestors and a reclosable, spring-loaded valve plate with seal. If an internal explosion should occur, the pressure wave causes the valve plate to open and directs the explosive forces and fireball through the flame arrestors, quenching the flame and cooling the hot combustion gases as they pass through.

This enables the devices to be used indoors where other equipment may be situated or where personnel are working, and where conventional venting devices would not be permitted. Furthermore, a self-resetting design means that the units can be activated multiple times without plant downtime and without the need for maintenance.

Available in a range of connection and venting area sizes with a compact profile, the explosion-relief valves are either designed for high opening pressures typically fitted to the engine intake and exhaust manifolds, or for lower opening pressures with a large effective vent area and located on the exhaust piping.

These devices are third-party certified by ATEX and IACS. Units are individually tested for function, opening pressure and leak-tightness before shipment.

As the movement intensifies to reclaim and use biogas as a sustainable fuel for power generation and as renewable source of energy for many processes, it is vital to ensure that engines and combined heat and power plants are equipped to mitigate against the risks of explosion. Specifying engineers should ensure that appropriate explosion protection relief devices are designed into biogas-fueled engines; this should also be considered in any plant upgrades or retrofits to existing equipment.

About the author

Fred Callahan (fred.callahan@hoerbiger.com) is regional sales manager, engines and generators, with HOERBIGER Safety Solutions, a subsidiary of IEP Technologies, a company specializing in explosion protection technologies.   



Discussion

Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.