Feb 21, 2009

Biofuel: Fuel for the future

MOLECULAR hydrogen has the highest energy content per unit weight among the known gaseous fuels with heating value of 143 GJ per tones, and is the only carbon-free fuel which when burned, only produces water as a combustion product.

Burning hydrogen not only has the potential to meet a wide variety of end use applications but it also does not contribute to greenhouse emission, acid rain or ozone depletion.

Currently hydrogen is used basically as primary feedstock in industrial processes, including petroleum refining, petrochemical manufacturing, glass purification and in fertilisers. It is also used in the semiconductor industry and for the hydrogenation of unsaturated fats in vegetable oil.

Although abundant on earth as an element, hydrogen combines readily with other elements and is almost always found as part of some other substance, such as water, biomass and hydrocarbons like petroleum and natural gas.

Approximately, 90 per cent of the hydrogen produced annually worldwide is from fossil fuels, mainly by steam reforming of natural gas and petroleum derivatives. Other industrial methods use coal gasification and water electrolysis. Each method of hydrogen production requires a source of energy — thermal, electrolytic or photolytic (light) energy.

Hydrogen production requires all the energy we get from burning the gas and a bit more on account of inefficiencies. Hydrogen produced by steam reformation costs approximately three times more that the natural gas per unit of energy produced. Also, producing hydrogen from electrolysis using electricity will cost slightly less than two times the cost of hydrogen from natural gas.

Worldwide hydrogen is being considered as a fuel for the future. It is an environmentally benign replacement for gasoline, diesel, heating oil, natural gas and other fuels in both the transportation and non-transportation sectors. However, producing hydrogen using conventional methods defeats the purpose of using hydrogen as a clean alternative fuel. The production of hydrogen from non-fossil fuel sources, such as solar, hydropower, wind, nuclear, etc., has becomes central for better transition to hydrogen economy.

Unicellular green algae offer an attractive choice in the production of hydrogen from water via the process of photosynthesis. Biological processes, unlike their chemical or electrochemical counterparts, are catalysed by micro organisms in an aqueous environment at ambient temperature and pressure. Biological hydrogen production has advantages, when compared to photo-electrochemical or thermo-chemical processes, that include low energy requirement and investment cost.

Light absorption by the photosynthetic apparatus is essential for the generation of hydrogen gas. While regular green algae absorb most of the light falling on them, engineered algae have less chlorophyll and let some light through. In University of California, Berkeley, Melis and his colleagues are designing algae that have less chlorophyll so that they absorb less sunlight. When grown in large, open bioreactors in dense cultures, the chlorophyll-deficient algae will let sunlight penetrate to the deeper algae layers and thereby utilise sunlight more efficiently.

The critical enzymatic component of this photosynthetic reaction is the reversible hydrogenase enzyme, which reduces protons with high potential energy electrons to form hydrogen. During normal photosynthesis, algae focus on using the sun's energy to convert carbon dioxide and water into glucose, releasing oxygen in the process. Only about three to five per cent of photosynthesis leads to hydrogen. Because hydrogenase is sensitive to oxygen, this hydrogen production must be carried out in an anaerobic environment

Photosynthetic hydrogen production by green algae involves water splitting to produce hydrogen and oxygen. Unfortunately, hydrogen production by this process is quite ineffective since it simultaneously produces oxygen, which inhibits the hydrogenase enzyme.

Thus, during light reaction, hydrogen evolution ceases due to an accumulation of oxygen. Therefore, the prerequisite for photohydrogen production by green algae is that they have to adapt to an anaerobic condition.

By exposing the cells to specific conditions, scientists are able to modify photosynthesis so that oxygen will not act as the final electron carrier of the electron transport chain; rather hydrogen will allow the cells to release molecular hydrogen as opposed to molecular oxygen.

Melis estimates that if the entire capacity of the photosynthesis of the algae could be directed toward hydrogen production, 80 kilograms of hydrogen could be produced commercially per acre per day. The yield of hydrogen production currently achieved in the laboratory corresponds to only 15 to 20 per cent of the measured capacity of the photosynthetic apparatus for electron transport.

In a laboratory, Melis worked with low-density cultures and have thin bottles so that light penetrates from all sides. Because of this, the cells use all the light falling on them. But in a commercial bioreactor, where dense algae cultures would be spread out in open ponds under the sun, the top layers of algae absorb all the sunlight but can only use a fraction of it.

In Pakistan, hydrogen is largely produced in the fertiliser industry from natural gas, which is used for the production of anhydrous ammonia — the building block for nitrogen fertiliser. On an average, the fertiliser sector has consumed 15.6 per cent of our natural gas.

The government provides an indirect subsidy to fertiliser manufacturers. As on January 1, 2008, the balance recoverable natural gas reserves have been estimated at 31.266 trillion cubic feet. With the present consumption rate of 3,826 million cubic feet per day, these resources will last only 20 years at the most.

According to The Energy Security Action Plan of the Planning Commission, Pakistan will be facing a shortfall in gas supplies rising from 1.4 billion cubic feet (BCF) per day in 2012 to 2.7 BCF in 2015, and escalating to 10.3 BCF per day by the year 2025. It is, therefore, a matter of economic security to develop alternative hydrogen resources to avoid mid century energy crises in the country.

The advent of hydrogen will bring about technological developments in many fields, including power generation, agriculture, the automotive industry and other as yet unforeseen applications. It will increase employment, stimulate the economy and will have a positive impact on the environment in which atmospheric pollution is all but alleviated and the so-called greenhouse effect is mitigated.

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