Hydrogen-Fuel Cell

Flammability range of gas could be defined in term of its lower flammability limit (LFL) and upper flammability limit (UFL). The LFL of a gas is the lower gas concentration that still support a self-propagating flame when mixed with air and ignited. Below the LFL, there is not enough fuel to support the combustion reaction. The UFL of a gas is the upper gas concentration that still support a self-propagating flame when mixed with air and ignited. Above the UFL, there is not enough oxygen to support the combustion reaction.

Between this two limits is the flammable range for the mixture of gas and air to burn when ignited. The consequence of ULF is that when you store hydrogen (as gaseous or liquid) on a cylinders tank, it will not explode or flame as long as there is no leak in your peripheral devices. When there is a leak on your hydrogen cylinder tank, hydrogen will mixed with air and when there is ignition, hydrogen will explode.

From the graph, we know that the flammability range of hydrogen is about 4 – 75%. This mean when there is small leakage on hydrogen storage, this has big risk on explosion accident. When the leakage happen on enclosed environment, the risk of explosion will increase. But, hydrogen more safe compare with gasoline when on small concentration. That because, gasoline easy ignited to burn on small concentration compared to hydrogen.

Color of hydrogen flame is very pale blue and invisible in daylight because there is no smog that released when you burn hydrogen gas. It clean energy. Visibility of hydrogen flame enhanced by present a impurities or moisture (like sulfur) on the air or night season. The other way is by feeling the heat radiation of hydrogen flame that release from hydrogen storage leaking. In some case, heat radiation from hydrogen flame make materials that around on burning area release smog.

In many case, hydrogen fires safer than gasoline fires. Hydrogen gas quickly go to atmosphere because hydrogen gas has high buoyancy and diffusivity. Consequently hydrogen fires are vertical and localized. When hydrogen storage on fuel cell car rupture and ignited, fire burns will away from the car and the interior will not get very hot.

Otherwise with gasoline. Gasoline has physical characteristic to form a pool, spreads laterally (horizontal) and the vapors form a lingering cloud. So gasoline fires are broad and encompass a wide area. When gasoline tank rupture and ignited, then fire burns will engulfs the car within a few seconds and this condition will increase temperature of interior car quickly. In some case, high temperature on the inside of car could ignite the second explosion.

In general case, burn of hydrogen always emit non-toxic emission and gasoline emit toxic emission. Moreover, burn the hydrogen safer than other conventional fuels. Hydrogen danger just when there is leaking on hydrogen storage in air and ignited on concentration range 4 – 75%.

References:

• Hydrogen, 09/10/2006
• Hydrogen Basics, Catherine E. Gregoire Padro, Los Alamos National Laboratory, First Annual International Hydrogen Energy Implementation Conference, Santa Fe, NM, February 17, 2005
• Revision O, Module 1: Hydrogen Properties, Modul 1: Hydrogen Properties, December 2001

Tags: Hydrogen