Methanol from Biogas

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Most mammals (including humans), as well as termites, produce a flammable gas termed "biogas" when they digest their food. Biogas is also generated in wetlands, swamps and bogs where large amounts of rotting vegetation may accumulate. Biogas is formed when anaerobic bacteria break down organic material in the absence of oxygen. Anaerobic bacteria are some of the oldest forms of life on Earth, having evolved before the photosynthetic processes of green plants were able to release large quantities of oxygen into the primitive atmosphere.

Biogas is in fact a waste product of those microorganisms, composed mainly of methane and CO2 in variable proportions, and trace levels of other elements such as hydrogen sulfide (H2S, rotten egg smell), hydrogen, or carbon monoxide. The biological process for biogas generation by anaerobic methanogenic bacteria, termed "methanogenesis", was discovered in 1776 by Alessandro Volta, and has been used since the 19th century in so-called "anaerobic digestion reactors". The process was used quite extensively in Europe when energy supplies were reduced during and after World War II. Interest in biogas was then revived after the oil crisis of the 1970s. Most common feedstocks are animal dung, sewage sludge and wastewater or municipal organic waste. The use of anaerobic digesters to treat industrial waste water has also rapidly gained popularity during the past decade. Wastewaters treated include those from food, paper and pulp, fiber, meat, milk, brewing and pharmaceutical plants. In Brazil, anaerobic digesters are used to treat the waste liquid (called vinasse) which is co-produced during ethanol manufacture from sugar-cane. Anaerobic digestion is thus an important method of reducing the impact of waste disposal on the environment [217].

Anaerobic digester can be constructed from concrete, steel, bricks, or plastic, they may be shaped like silos, basins or ponds, and they may be placed either underground or on the surface. In large-scale operations, organic materials are constantly fed into a digestion chamber, and the gas produced is allowed to exit through a gas outlet placed on the top of the reactor. The sludge left behind in the digester, rich in nutrients, can be used as a soil fertilizer or conditioner.

The gas produced in anaerobic digesters, depending on the feedstock and effectiveness of the process, consists of 50-70% methane with the remainder being mostly CO2. At present, the gas is mainly used to produce electricity or heat. Compressed methane obtained from biogas (basically the same as CNG) is also used as an alternative transportation fuel in light- and heavy-duty vehicles, notably in Brazil. Today, Sweden runs some 800 buses on biogas and has recently unveiled a small biogas train powered by two biogas bus engines. After the removal of impurities (especially hydrogen sulfide), biogas could also be used directly for the production of methanol in the same way as natural gas. A fraction of the CO2 already present in the gas could react with the excess hydrogen generated during methane steam reforming in order to form more methanol. Using this technology, the manure of 250000 pigs is currently planned to be used in Utah to produce some 30 000 L of methanol per day.

Another source for biogas production is landfills which, worldwide, is the dominant method for municipal solid waste disposal. As in anaerobic digesters, the organic material present in the landfills is decomposed in the absence of oxygen by anaerobic bacteria. The resultant gas comprises 50-60% methane, 40-50% carbon monoxide, and traces of H2S as well as other volatile organic compounds. The landfill gas, which for safety reasons is otherwise vented to the atmosphere or is flared, can also be collected through elaborate underground piping for use as a fuel or feedstock for methanol production.

In the United States, 240 million tonnes of municipal solid waste were produced in 2003. Of this, 30% was recycled or composted, 14% incinerated, and 56% landfilled. From the more than 1700 landfills currently in operation, close to 400 have operational landfill gas utilization units in place [218]. Most of the gas produced is used to generate electricity or produce heat. The production of methanol from landfill gas is also possible, and a plant using this technology, with a capacity of 15 000 tonnes per year, is planned to be constructed in Ohio. A molten carbonate fuel cell (MCFC) that can operate on methanol from municipal organic waste of the city of Berlin, Germany, has also been in use since 2004 (Fig. 12.8).

It is important to note, however, that in 2004 the combined production of electricity from all landfill gas energy projects presently installed in the United States was only 9 billion kWh [218] - similar to the output of a single large-scale nuclear reactor, or 0.2% of the national electricity production! While landfill gas utilization provides a good means of controlling methane greenhouse gas emissions from municipal solid waste, it has a very limited capacity for energy or methanol supply.

Figure 12.8 The HotModule from MTU: The first bi-fuel molten carbonate fuel cell (MCFC) went into service at Vattenfall Europe AG in Berlin in September 2004. It is fuelled with natural gas or methanol or any mixture of these two. The methanol used is derived from waste generated locally. (Courtesy: MTU Friedrichshafen GmbH 2004.)

Mtu Hotmodule

Figure 12.8 The HotModule from MTU: The first bi-fuel molten carbonate fuel cell (MCFC) went into service at Vattenfall Europe AG in Berlin in September 2004. It is fuelled with natural gas or methanol or any mixture of these two. The methanol used is derived from waste generated locally. (Courtesy: MTU Friedrichshafen GmbH 2004.)

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