5^th part

The first wood gasifier was apparently built by Bischof in 1839. The first vehicle powered by wood gas was built by Parker in 1901. This gas is a mixture of carbon monoxide and hydrogen with some traces of methane gas. Even though its technical name is producer gas all over the world the same combination of inflammable gases has many names, such as wood gas, green gas, holzgas and blue gas.

Blue gas was used as a secondary fuel source for some zeppelin designs of the early 20th century, stored in gas cells within the envelope, just below the hydrogen gas cells. The engines could use either the blue gas or liquid petroleum-based fuel for power, but the former, having a density similar to air, required little change in ballast as it was consumed.

Around 1900, many cities delivered wood gas (centrally produced typically from coal) to residences. At this time also, Rudolf Diesel and Georges Limbert were also developing their various engines. It has been theorized that all of these internal combustion engines had been inspired by observing the operation of the fire piston, fire making device which had been discovered in New Guinea and Sumatra early in the 1800s began to be used only in 1930.

Wood gasifiers are still manufactured in Singapore, China and Russia for automobiles and as power generators for industrial applications.

Producer gas is the product of thermal gasification of biomass or other carbon containing materials such as charcoal in a gasifier or wood gas generator or gas producer. It is the result of two high-temperature reactions (>700 °C): an exothermic reaction where carbon burns to CO2 but is then reduced partially back to CO (endothermic); and an endothermic reaction where carbon reacts with steam, producing carbon monoxide (CO), molecular hydrogen (H2), and carbon dioxide (CO2).

In several gasifiers, the actual gasification process is preceded by pyrolysis; pyrolysis means cracking the heavy oils at higher temperature of about 300 to 700 degrees and that gives lighter molecules of gaseous or volatile alcohols or other inflammable carbonaceous liquids. In simple gasifiers the biomass or wood chip turns into charcoal, releasing methane (CH4) and tar rich in polycyclic aromatic hydrocarbons (PAH). Other gasifiers are fed with previously pyrolysed char. Wood gas is flammable because of the carbon monoxide, hydrogen, and methane content. There are two arguments, one suggests that using vegetative carbonaceous mass (VCM) if used as in put then, the resultant gas is rich in heating power. This is because, when vegetative waste material is used, the water in the material is evaporated and the vapors are reacted upon by CO2 produced during the initial combustion giving hydrogen gas (H2). This makes the resultant gas a better fuel. Due to addition of hydrogen from moisture the gas is called water gas. This also gives polycyclic aromatic hydrocarbons (PAH) which are further processed to produce many useful organic solvents. Second argument suggests that when only charcoal is used as an in put, no hydrogen is produced and so that gas is less efficient as fuel for engines.

There are advantages of using only charcoal as an input in the sense that less material is used, very compact gasifier can be used, cheaper input cost. When VCM is used the cost of the gasifier is very high, gasifiers are very bulky and the resultant gas needs to be cleaned to separate tar and water vapor that comes with it. Many years of experience has approved the choice of VCM over charcoal. Argument says that, in spite of all the seeming defects of this approach the benefits are far more than the ones with charcoal. Recent developments in the technology have shown that the VCM is first dried in Sun to remove excess moisture then used as inputs. This makes the gasifier more efficient and less costly to fabricate. It is recommended that VCM powered gasifiers are suitable for electric power generation and charcoal powered are for automobiles.

Producer gas can be used to power cars with ordinary internal combustion engine if a gasifier is attached. This was quite popular during Second World War, in several European and Asian countries because the war prevented easy and cost-effective access to oil. In more recent times, water gas has been suggested as a clean and efficient method to heat and cook in developing countries, or even to produce electricity when combined with a gas turbine or internal combustion engine. Compared to the World War II technology, where all the controls were manually managed controlling gasifier functions was rather cumbersome. Today gasifiers have become less dependent on constant attention due to the use of sophisticated electronic control systems, but it remains difficult to get clean gas from them. Purification of the gas and feeding it into the natural gas pipelines is one variant to link it to existing refueling infrastructure, liquefaction by the gas powered car, the energy consumption has been 1.54 times more compared to the energy demand of the same car on petrol (not including the energy needed to extract, transport and refine the oil from which petrol is derived). This means, 1000 kg of wood combustible matter has been found to substitute 365 liters of petrol during real transportation in similar driving conditions and with the same otherwise unmodified vehicle. This can be considered to be a good result, because no other refining of the fuel is required. This study also considers all possible losses of the wood gas system like pre-heating of the system and carrying of the extra weight of the gas generating system. However, producer gas having less thermal power needs larger engines to produce the same amount of energy, this an important point to be considered when recommending this fuel for transport vehicles.

Burning of Producer gas and power out put of internal combustion engines

Combustion of hydrogen and carbon monoxide liberates energy. As the reaction equations below show, combustion of main gas components leads to decrease in number of gas molecules. Inert components (nitrogen N2 and carbon dioxide CO2) do not change or take part in reactions.

C + ½ O2 àCO2 energy released 283.0 kJ/mol

H2 + ½ O2à H20 energy released 242.0 kJ/mol

CH4 + 2 O2 à CO2+ 2H20 energy released 802.0 kJ/mol

Proportion of hydrogen and carbon monoxide together make about 40 % of the volume of the gas. After air addition these components make about 20 % of volume. Combustion of methane has no influence on amount of gas molecules. Total number of molecules in combustion mixture decreases by 7 % as a result of combustion, which is not favorable for power out put of internal combustion engine. Beneficial is that because of relatively large proportion of hydrogen, the combustion limit is large compared to for example to petrol. Gas which contains 21 % CO, 19 % H2 and 1.5 % CH4 the lower combustion limit is 16 % and higher combustion limit is 67 % (proportion of gas in air). This means roughly air proportion 2 to lean and about 0.5 to rich direction.

In practice the power out put is proportional to the amount of air an engine can take in. Producer gas makes about 45% of the optimal explosion mixture. This means it displaces almost one half of the volume engine could suck air in. Respectively methane displaces 10 %, propane 3% and vaporized petrol only about 1% of the volume of the explosion mixture.

C8H16 + 12 O2

à 8 CO2 +8H20 Combustion reaction for petrol

Heat value of optimal producer gas air mixture is about 2 500 kJ/m3, which is about 30 % lower compared to optimal mixture of air and petrol (3800 kJ/m3). Power loss is, however, more because there is some suction resistance in gas cleaning chain and this results as a loss of cylinder filling. Producer gas burns also more slowly. This requires earlier ignition timing results also as a power loss. Work pressure of the engine is lower because of the amount of gases is reduced during combustion. When hydrocarbons are used as a fuel, the amount of gases slightly increases, which has a positive effect on power out put.

Gasifiers have been built for remote Asian communities using rice husk, which in many cases has no other use. One installation in Burma uses an 80 kW-modified diesel for about 500 people who are otherwise without power. The ash can is used as fertilizer so this can be considered a renewable fuel. In India we have other vegetative wastes from farms such as sugar cane cuttings, coconut husk, left over of all the cereals and pulses. One estimate says that India is producing 100 million tons of such waste VCM annually which can be used in gasifiers and the gas produced be used to make electric power or like CNG, this gas can be compressed and stored to supply to vehicles. CNG is not a renewable energy source. Power from VCM is a renewable source. That makes the difference.

Against general believe, exhaust gas emission level of internal combustion engine is significantly lower on wood gas than on petrol. Especially low are HC emissions. Normal catalytic converters do perfectly well on wood gas but emission level less than 20 ppm HC and 0,2 % CO can be easily achieved by most automobile engines with out catalytic converter. Combustion of wood gas generates no particles and gas renders thus very little carbon black settles in motor oil. Normal catalytic converts are reported to suit perfectly well for wood gas operation.

This series of articles gives a fair picture of alternative fuels to replace present day mineral fuels such as petrol and diesel. Careful management of our vegetable waste both from cities as well as from farms can solve the problem of fuel for both power generation and vehicles. In Indian context this study proves more relevant because we can save our precious foreign exchange by making use of farm waste (VCM) judiciously; to produce electric power and to run our public transport vehicles economically. One financial expert suggests that if these techniques are used properly over a period the cost of these fuels can be further reduced and that will help the country in saving foreign exchange. India can become independent of foreign petroleum oils. Today farm waste is a problem before all the farmers. When this system will be available that waste will become a source of power. It is like converting a problem into a solution. Other alternatives such as making alcohol out of cereals and sugar molasses is not advisable because we convert our food producing farm land into fuel producing land and that is not advisable for our country. In USA and other countries where the population is in control such plantations may be acceptable but for India and such thickly populated countries this is not acceptable. It is observed that when one quintal of grains is produced in a farm about 2 quintal of waste is generated. In this arrangement we get food and also fuel. The farm waste is used to produce fuel through gasifiers.

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