GAS CLEANING IN GASIFIERS

Need for gas cleaning

If the gas is to be feed to the engine, it must be cleaned and conditioned. In case of down draft gasifier the gas which is usually hot and dusty may have up to 1% tar particulates. Proper clean up system increases reliability and avoids failure of gasifier system. The total combined tar and particulates allowable for engine is less than 5 mg/Nm3 which amounts 99% removal of dust particles .For effective removal a gas cleanup system must be designed based on magnitude, size and nature of containments.

Gas cleanup goals

To meet the required cleaning goals the first step is to select a gasifier design that minimize production tar and particulates and next step is to have clean up systems that removes articulates, tar and containments effectively. The gas cleanup goals are to remove solids which are abrasive, tar mist which can cause inlet valves, rings and other moving parts to stick and hence need thorough removal for reliable engine operation. Another drawback with the tar is that it usually contains the impurities nitrogen and sulphur in the structure, which during combustion of the fuel gas produce their corresponding oxides. Successful gasifier engine system requires gas cleanliness standards from 10 to even less than 1 mg/Nm3. Tar levels of different gasifier indicate that updraft gasifers generate 5- 20% tar and downdraft gasifers upto 1%.

Tar formation

Thermal decomposition of wood does not progress at even pace but results in the production of gas, char ( the main slow proylisis product) and tar by transferring heat to surface of the particle and subsequent heat penetration by conduction. Temperature development inside the particle, and corresponding intrinsic reaction kinetics dominate the decomposition rate and product distribution. The vapours formed inside the pores are subjected to further cracking leading to formation of addition gas and/or stabilized tars. The longer residence of vapour molecules inside larger particles (at lower temperature) can explain the increased formation of tar incase of slow pyrolysis. When the vapour products enter the surrounding gas phase, they can still decompose further if they are not condensed. The degradation products from the biomass constituents include organic acids like formic and acetic acid giving oil at low pH of about 2 to 4 which can attack mild steal. With high temperatures the tar components become heavier due to the greater extent of gas phase reactions, which convert the primary formed tar components to more stable and heavier non-substituted aromatic compounds.

Composition

The composition of wood tar is an awesome mixture of phenolic and non phenolic compounds that has taxed the capabilities of several analytical laboratories for over a century. Characterization continues even today. The major molecular species identified in hard wood and soft wood tars and pyrolignous acids are reported to be Pyridine (C5H5N) and Lutidine (C7H9N).The tar which may be mixture of higher hydrocarbon and water in mist form have wide range of boiling points ranging from 100-400 C. They contain fine char particles in the exit stream from 700-6000 mg/Nm3 acting as nucleation sites for tarry vapours. Tar is usually defined as the condensable organic compounds in the fuel gas, those compounds, which at ambient temperatures have a sufficiently low vapour pressure. By this definition tar contents may vary with the sampling and analytical method. Tar is a group of organic compounds including phenol, naphthalene and PAH (poly aromatic hydrocarbons) and their substituted derivatives.

Gas purification

For operating engine, pure gas is needed. The composition and quantity of dust drawn from a producer depends on the type of producer employed and the fuel used. The first step in processing the raw gas is to cool it at the gas producer exist so that tarry liquid may condense. A gas producer, which gives tar free gas, is supposed to contain less than 10 mg of total contaminants per Nm3 of producer gas. For normal type Imbert type downdraft gasifier dust loads are reported vary from 0.5 to 5 g/m3 gas when using wood blocks of 4XX4cm. Modern gas cleaning and purifying plants are varied in type but comprise of the following units:

• One or more cyclones or centrifugal separators
• Expansion box or dust separator
• Cooling tubes or boxes
• Baffle separators
• Filtering and purifying devices

cyclones or centrifugal separators may consist of single or multiples and built over dust collecting chamber. In cooling tubes gas is led through a set of tubes with header and collecting chambers having gills for air surfaces. Baffles separators are largely employed. In cylindrical or oblong containers plates are arranged within causing a change of flow path which results in deposition of large solid particles. Filtering and purifying devices may be of different types. (i) dry filtering in scrubbers packed with coke wood wool, wire wool at the bottom and fitted with cloth bags or trays at the top, (ii) washing by light petroleum by bubbling through oil or water or light petroleum, (iii) filtration in a scrubber moistened with water or oil filled with gravel, coke, wood wool, metal turnings or cork

Producer can employ can employ a variety of combinations mentioned above. Cloth filters are rarely used where water or steam is used in gasification as these filters get quickly clogged with a type of paste formed during operation. The practical option is the need for reduced time for cleaning the purification chains. In olden day gasifiers used for mobile uses built in cleaning devices such as brushes or scrappers were provided to reduce the time required for cleaning out these filters.

Electrostatic precipitators

These have been in long use in industry to clean the gas very effectively. In principle, the dirty gas is passed through a chamber, containing high voltage negative electrode, which makes the all particles and droplets negatively charged. These are then migrating to positive electrode, which are then washed by water stream. 20 cm dia and 1 m long precipitator at 10-30kV was found to give excellent performance (99.85% collecting efficiency) in a 75 hp gas generator at SERI but suffered from short-circuiting by soot, tar and water, which was later solved. The power consumption was low, 1.5 W/hp with very low pressure drop.

Wet scrubbers:

Small difficult to capture droplets are made to grow in size with time until they are large enough to be capture by providing adequate residence time in the scrubber volume. Particles grow in size by agglomeration and condensation by particle collision . Wetted scrubbers have been used widely for cleaning and cooling the gas by creating maximum contact between gas and liquid media. There are several types such as impingement plate, packed bed, sieve plate, spray tower and venturi scrubbers.

Spray towers are simplest type having a cylinder fitted with spray nozzles. Particle collection is accomplished by counter current impact of gas with liquid droplets spray towers are well suited for dust loads over 50 g /Nm3.Cyclones spray scrubbers make use of the principle of spray towers and dry cyclone separator which improves particle capture efficiency. These are self cleaning and collect particles regardless of size with low pressure drop and efficiency more that 97%.

Sieve plate scrubbers have liquid flowing downwards over a serious of horizontal perforated sieve plates. The contact may be improved with bubble caps, impingement plates or sieve plates while the gas pass upwards. A typical scrubber has 90% efficiency.

Impingement plate scrubber is similar to sieve plate scrubber but arranged with impingement target. Gas passing through orifice produces spray droplets resulting in large between dust particles. Five to ten liters/minute of water flow is required per 1000 cfm of gas flow.

Venturi scrubbers capture large particles by impaction & impingement and rinse away the deposits. The atomized droplets present considerable surface area for fine particles to be captured by diffusion.

In ejector scrubbers, the velocity of contacting liquid scrubs the entrained gas in an ejector venture scrubber by imparting axial and tangential velocity to the liquid jet. They are well suited for very dirty corrosive or abrasive materials.

Packed bed scrubbers use saddle spheres, rings as packing to enhance gas liquid contact area. They are free draining and pressure drop is 20cm water gage for 15 cm deep bed with 12 mm spheres.

Tar cracking:

Tar can be reduced by properly cracking it either internally or externally to the gasifier. Temperature above 800 C rapidly destroys tar and promotes reaction with char. A tar cracking chamber may be added to gasifier in which small amount of oxgen or air can be added to crack the final trace quantities to as low as 50 to 500 ppm. Tarry gas can be passed over a bed of hydrocarbon cracking catalyst at temperature between 950 and 1040 C resulting in gas containing 10 to 100 ppm of tar. Using dolomite lime up to 900 C tar can be cracked as is used in Swedish gasifiers.