What is producer gas? With a schematic diagram explain the working of a downdraft type gasifier?by Madan Mohan
Producer gas is fuel gas that is manufactured from material such as coal, as opposed to natural gas. In the USA, producer gas may be wood gas produced in a gasifier and used to power cars, town gas, originally produced from coal, for sale to consumers, and syngas, used as a fuel source or as an intermediate for the production of other chemicals. In the United Kingdom, producer gas, also called suction gas, specifically means a fuel gas made from coke, anthracite or other carbonaceous material. Air is passed over the red-hot carbonaceous fuel and carbon monoxide is produced. The reaction is exothermic and proceeds as follows:
- 2C + O2 + 3.73 N2 → 2CO+ 3.73 N2
The nitrogen in the air remains unchanged and dilutes the gas, giving it a very low calorific value. The concentration of carbon monoxide in the “ideal” producer gas was considered to be 34.7% carbon monoxide (carbonic oxide) and 65.3% nitrogen. After “scrubbing”, to remove tar, the gas may be used to power gas turbines (which are well-suited to fuels of low calorific value), spark ignited engines (where 100% petrol fuel replacement is possible) or diesel internal combustion engines (where 40% – 15% of the original diesel fuel is still used to ignite the gas). During World War II in Britain, plants were built in the form of trailers for towing behind commercial vehicles, especially buses, to supply gas as a replacement for petrol (gasoline) fuel. A range of about 80 miles for every charge of anthracite was achieved. In old movies and stories, when describing suicide by “turning on the gas” and leaving an oven door open without lighting the flame, they were talking about coal gas or town gas. As this gas contained a significant amount of carbon monoxide, the gas was quite toxic. Most town gas was also odorized, if it did not have its own odor. Modern ‘natural gas’ used in homes is far less toxic, and has a gassy odor added to it for identifying leaks.
Downdraught or co-current gasifiers
The downdraft gasifiers can be of two types. Those having, throat type design (including choke plate) and thosewith open core design. Throat type gasifiers are used for biomass fuels with low ash and uniform size, while open coregasifiers can tolerate more variation in fuel properties like fuel moisture, size and ash content. Also smaller throat diameter means higher gas velocities at the oxidative and reduction zones. This reduces tars but increases dust loading. Large throatdiameter causes an increase of tar in the gas stream due to by passing of the hot zone. Fuels with high ash content (e.g. ricehusk -21.3%) create, problems by ash clogging and slogging at the combustion zone in downdraft gasifiers. The choke plates and throat type combustion regions used in downdraft gasifiers work well with lower coking tendency fuels(e.g. wood), but when high coking fuels (e.g. cotton stalk) are used they cause bridging in and above the pyrolysis zone. A solution to the problem of tar entrainment in the gas stream has been found by designing co-current or downdraught gasifiers, in which primary gasification air is introduced at or above the oxidation zone in the gasifier. The producer gas is removed at the bottom of the apparatus, so that fuel and gas move in the same direction, as schematically shown in Fig.
On their way down the acid and tarry distillation products from the fuel must pass through a glowing bed of charcoal and therefore are converted into permanent gases hydrogen, carbon dioxide, carbon monoxide and methane. Depending on the temperature of the hot zone and the residence time of the tarry vapours, a more or less complete breakdown of the tars is achieved. The main advantage of downdraught gasifiers lies in the possibility of producing a tar-free gas suitable for engine applications. A major drawback of downdraught equipment lies in its inability to operate on a number of unprocessed fuels. In particular, fluffy, low density materials give rise to flow problems and excessive pressure drop, and the solid fuel must be pelletized or briquetted before use. Downdraught gasifiers also suffer from the problems associated with high ash content fuels (slagging) to a larger extent than updraught gasifiers. Minor drawbacks of the downdraught system, as compared to updraught, are somewhat lower efficiency resulting from the lack of internal heat exchange as well as the lower heating value of the gas. Besides this, the necessity to maintain uniform high temperatures over a given cross-sectional area makes impractical the use of downdraught gasifiers in a power range above about 350 kW (shaft power).
Processes occurring in the down-draught gasifier
In the down-draught gasifier, schematically illustrated in Fig., the fuel is introduced at the top, the air is normally introduced at some intermediate level and the gas is taken out at the bottom. It is possible to distinguish four separate zones in the gasifier, each of which is characterized by one important step in the process of converting the fuel to a combustible gas. The processes in these four zones are examined below and the design basis will be discussed in the following section.
a) Bunker Section (drying zone)
Solid fuel is introduced into the gasifier at the top. It is not necessary to use complex fuel-feeding equipment, because a small amount of air leakage can be tolerated at this spot. As a result of heat transfer from the lower parts of the gasifier, drying of the wood or biomass fuel occurs in the bunker section. The water vapour will flow downwards and add to the water vapour formed in the oxidation zone. Part of it may be reduced to hydrogen and the rest will end up as moisture in the gas.
b) Pyrolysis Zone
At temperatures above 250°C, the biomass fuel starts pyrolysing. The details of these pyrolysis reactions are not well known, but one can surmise that large molecules (such as cellulose, hemi-cellulose and lignin) break down into medium size molecules and carbon (char) during the heating of the feedstock. The pyrolysis products flow downwards into the hotter zones of the gasifier. Some will be burned in the oxidation zone, and the rest will break down to even smaller molecules of hydrogen, methane, carbon monoxide, ethane, ethylene, etc. if they remain in the hot zone long enough. If the residence time in the hot zone is too short or the temperature too low, then medium sized molecules can escape and will condense as tars and oils, in the low temperature parts of the system.
c) Oxidation Zone
A burning (oxidation) zone is formed at the level where oxygen (air) is introduced. Reactions with oxygen are highly exothermic and result in a sharp rise of the temperature up to 1200 – 1500 °C. As mentioned above, an important function of the oxidation zone, apart from heat generation, is to convert and oxidize virtually all condensable products from the pyrolysis zone. In order to avoid cold spots in the oxidation zone, air inlet velocities and the reactor geometry must be well chosen. Generally two methods are employed to obtain an even temperature distribution:
- Reducing the cross-sectional area at a certain height of the reactor (“throat” concept)
- Spreading the air inlet nozzles over the circumference of the reduced cross-sectional area, or alternatively using a central air inlet with a suitable spraying device.
d) Reduction zone
The reaction products of the oxidation zone (hot gases and glowing charcoal) move downward into the reduction zone. In this zone the sensible heat of the gases and charcoal is converted as much as possible into chemical energy of the producer gas. The end product of the chemical reactions that take place in the reduction zone is a combustible gas which can be used as fuel gas in burners and after dust removal and cooling is suitable for internal combustion engines. The ashes which result from gasification of the biomass should occasionally be removed from the gasifier. Usually a moveable grate in the bottom of the equipment is considered necessary. This makes it possible to stir the charcoal bed in the reduction zone, and thus helps to prevent blockages which can lead to obstruction of the gas flow.
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