Cross Draft Gasifier

Cross draft gasifiers, although they have certain advantages over updraft and downdraft gasifiers, they are not of ideal type. The disadvantages such as high exit gas temperature, poor CO2 reduction and high velocity are the consequence of the design. Unlike downdraft and updraft gasifiers, the ash bin, fire and reduction zone in cross draft gasifiers are separated. The design characteristics limit the type of fuel for operation to low ash fuels such as wood, charcoal and coke. The load following ability of cross draft gasifier is quite good due to concentrated partial zones which operate at temperature up to 2000oC. Start up time (5 -10 minutes) is much faster than that of downdraft and updraft units. The relatively higher temperature in cross draft gasifier has an obvious effect on gas composition such as high carbon monoxide and low hydrogen and methane content when dry fuel such as charcoal is used. Cross draft gasifier operates well on dry blast and dry fuel.

Down draft gasifier

In a down draft gasifier, the 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. The main advantage of down draft gasifiers lies in the possibility of producing a tar-free gas suitable for engine applications. A major drawback of down draft equipment lies in its inability to operate on a number of unprocessed fuels. Minor drawbacks of the down draft system, as compared to up draft, 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 down draft gasifiers in a power range above about 350 kW (shaft power).

Updraft Gasifier

In the Updraft Gasifier, the air intake is at the bottom and the gas leaves at the top. Near the grate at the bottom the combustion reactions occur, which are followed by reduction reactions somewhat higher up in the gasifier. In the upper part of the gasifier, heating and pyrolysis of the feedstock occur as a result of heat transfer by forced convection and radiation from the lower zones. The tars and volatiles produced during this process will be carried in the gas stream. Ashes are removed from the bottom of the gasifier. The major advantages of this type of gasifier are its simplicity, high charcoal burn-out and internal heat exchange leading to low gas exit temperatures and high equipment efficiency, as well as the possibility of operation with many types of feedstock.

Types of Gasifiers

Gasifiers are basically divided into two major types namely fixed bed and fluidized bed. Fixed bed gasifiers typically have a grate to support the feed material and maintain a stationary reaction zone. They are relatively easy to design and operate, and are therefore useful for small and medium scale power and thermal energy uses. It is difficult, however, to maintain uniform operating temperatures and ensure adequate gas mixing in the reaction zone. As a result, gas yields can be unpredictable and are not optimal for large-scale power purposes (i.e. over 1 MW). The primary types of fixed bed gasifiers are updraft, downdraft and crossdraft.

Energy conversion efficiency and labelling

The energy efficiency ranges of few conversion devices, %
Spark ignition engine 20- 25
Compression ignitionengine 30 -45

Electric motor 80-95
Electric generator 80-95
Steam turbine 7-40(Inclusive of boiler)
Hydro turbine 70-99
Battery 80-90
Solar cell 8 – 15
Water Pump Mechanical Potential 40-60
Countries the world over have tried to promote efficient use of energy through labelling programmes. Energy Efficiency Labelling is display of a label on a product depicting data in a standard format regarding energy use and a predefined energy efficiency measure for enabling comparison with the energy efficiency of similar products. Energy efficiency label provides relevant energy-use information to the purchaser for making an informed purchase decision. It seems to be an effective way to impart knowledge to the consumer regarding efficiency and life cycle costs. The basis of acceptance of an energy efficient product is that its life time cost is less and hence it makes sense to the consumers.A graded multi-level efficiency band similar to the one to five star rating of appliances has been used by many countries.

Driving forces for biodiesel

The key driving forces for biodiesel are the directive for the promotion of biofuels and the directive on fuel quality. The former is motivated by the need to cut greenhouse gas emissions in the transport sector and increase energy security by reducing dependence on imported oil. Also encouraging the growth of biodiesel are useful properties such as less local air pollution, rapid biodegradability, low toxicity to people and the environment, and high flashpoint. The supply of biodiesel is limited, however, by the availability of oilseed crops. Any plan for biodiesel should begin with a careful study of existing experience, followed by a survey of feedstock options including recycled cooking oil.

SOFC versus MCFC

The interest in the SOFC, compared with the MCFC, might be questioned in view of the thermodynamic limitation of operation at 1000o C, which reduces the cell operating potential by 100mV, or about 15 %. The answer is that the SOFC is electrochemically simpler than the MCFC, since it does not require CO2 feedback from the anode exit to the cathode inlet. Its solid electrolyte eliminates electrolyte management problems, and it may be more resistant to contaminants such as H2S. With the correct materials, it may be capable of a lifetimes up to 100,000 hours or more (tested with no apparent degradation to almost 35,000 hours operation at Brown Boveri). At the higher operating temperature, diffusion and kinetic limitations are minimized, and the cell is essentially limited only by IR drop.