Gas Clean-up

The process is carried at moderate (slightly above atmospheric) pressures, but at very high temperatures that reach a maximum of 1900°C. Even though the reaction time is short (0.6—0.8 s) the high temperature prevents the occurrence of any condensable hydrocarbons, phenols, and/or tar in the product gas. The absence of Hquid simplifies the subsequent gas clean-up steps. 

The use of hot gas clean-up methods to remove the sulfur and particulates from the gasified fuel increases turbine performance by a few percentage points over the cold clean-up systems. Hot gas clean-up permits use of the sensible heat and enables retention of the carbon dioxide and water vapor in the... 

LOCAT units can be used for tail-gas clean-up from chemical or physical solvent processes. They can also be used directly as a gas sweetening unit by separating the absorber/oxidizer into two vessels. The regenerated solution is pumped to a high-pres.sure absorber to contact the gas. A light slurry of rich solution comes off the bottom of the absorber and flows to an atmospheric oxidizer tank where it is regenerated. A dense slurry is pumped off the base of the oxidizer to the melter and sulfur separator. 

Kr is extracted either by stepwise heating or melting of a sample followed by usually several gas clean-up steps and by separation of Kr from other noble gases by selective adsorption on charcoal at cryogenic temperatures [11]. Kr is analysed in ultra-clean static mass spectrometers which allows recycling of... 

Graham, R.G. and R. Bain, 1993. Biomass Gasification Hot Gas Clean-up. Report Submitted to IEA Biomass Gasification Working Group, Ensyn Technologies/NREL, 44 pp. 

Gas clean up methods can be classified into two distinct routes wet low temperature cleaning and dry high temperature cleaning. 

As costs of precombustion hydrodesulphurisation and post combustion flue gas clean-up have escalated and as environmental regulations have further limited the sulphur dioxide emission rates, there has been a growing interest in technology designed to effect fuel desulphurisation during the combustion process. Desulphurisation during fluidised bed combustion of coal has been a leading technique in these developments. 

In Japan, there is a project aimed at capturing the considerable volume of hydrogen gas which can be obtained as a by-product steel production. R D will focus on the purification process of fuel from coke oven gas to an acceptable level for fuel cell utilisation. METI, the Japan Research and Development Centre for Metals and Nippon Steel are working on the project with a 2003 budget allocation of 549 million. Japan also operates the 4C/.f project which aimsto develop an optimum coal gasifier for fuel cells and the establishment of gas clean-up system for purification of coal gas to the acceptable level for utilisation for MCFC and SOFC. The budget allocations for 2000-2003 total 4.6 billion. 

The efficiency of the Claus Process, long the means of conversion of H2S to sulfur, has been increased through improvements in reaction furnace, catalyst bed and computerized feed composition control leading to recovery efficiencies in excess of 98%. Recent development of a Claus Process under pressure may yield further important improvements. Add on tail gas clean-up processes have further reduced plant effluent in response to environmental protection requirements. 

Furnace temperatures have also been shown to be important in controlling the formation of COS. While COS has little effect on the downstream Claus catalyst efficiency its presence in the gas stream leads to higher loading of the reductive tail gas clean-up processes (e.g. SCOT, BSR, see environment) or to higher SO2 emissions in the stack gas. The recent developments regarding the control of its formation in the front end furnace are thus a significant contribution to the improvement of environmental quality control. 

In this new process the H2S/SO2 reaction is carried out in liquid sulfur at pressures in excess of five atmospheres. Typical Claus catalysts are still employed but temperatures are lower (below the dewpoint of sulfur) and thus the redox reaction occurs in the liquid sulfur phase at the surface of the catalyst. Vapor losses due to sulfur mist entrainment are reduced and interstage condensers in the tradition Claus train are not required thus avoiding wasteful heat transfer problems. The authors claim that overall sulfur recoveries in excess of 99% are possible without the use of tail gas clean up units. 

Using essentially the same sub-dewpoint Claus reaction principle the Cold Bed Absorption (CBA) process of Amoco (47) achieves the same level of tail gas desulfurization. The low temperature high efficiency swing converters can be in line rather than as a tail gas clean up add on unit. 

Residua from various processes are the preferred feedstocks for the production of hydrogen-rich gases. Such fractions with high sulfur and/or high heavy metal contents are difficult to handle in upgrading processes such as hydrogenation or coking and, for environmental reasons, are not usually used as fuels without extensive gas clean up. 

While the development of flue gas clean-up processes has been progressing for many years, a satisfactory process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubber is the most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically attractive. In spite of this, it is expensive and accounts for about 25-35% of the capital and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based selective catalytic reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. coal-fired power plants because of reliability concerns and other unresolved technical issues (1). These include uncertain catalyst life, water disposal requirements, and the effects of ammonia by-products on plant components downstream from the reactor. The sensitivity of SCR processes to the cost of NH3 is also the subject of some concern. 

In the U.S. there are only two commercial users of Low-Btu Gasifiers operating today. In both cases, no sulfur removal and only limited gas clean-up is involved. At one time, (1920 s) in the U.S. there were over 10,000 similar small gasifiers in use. 

Once removed from the raw gas, the question arises as to what should be done with the acid gas. If there is a large amount of acid gas, it may be economical to build a Claus-type sulfur plant to convert the hydrogen sulfide into the more benign elemental sulfur. Once the H2S has been converted to sulfur, the leftover carbon dioxide is emitted to the atmosphere. Claus plants can be quite efficient, but even so, they also emit significant amounts of sulfur compounds. For example, a Claus plant processing 10 MMSCFD of H2S and converting 99.9% of the H2S into elemental sulfur (which is only possible with the addition of a tail gas clean up unit) emits the equivalent of 0.01 MMSCFD or approximately 0.4 ton/day of sulfur into the atmosphere. Note that there is more discussion of standard volumes and sulfur equivalents later in this chapter. 

The costing of sulfur recoveiy units is outside the scope of this book. However, the acid gas injection scheme described would produce approximately 20tonne/d of sulfur and the capital cost of such a sulfur plant (including tail gas clean up) would be about 8 million. 

Figure 11.3 Layout for post gasifier synthesis gas clean-up...

The separator produces a water phase, a hydrocarbon liquid phase (which can be regarded as a synthetic crude oil) and a recycle gas. Part of the synthesis gas is purged to stop the build up of inert materials such as nitrogen. The recycle gas contains light hydrocarbon gases, unconverted synthesis gas and carbon dioxide produced in the process. This is sent to a gas treatment plant for recovery of synthesis gas. This operation may be integrated into the gas clean-up operation of the fresh synthesis gas from the gasifier. 

Glass-Combustion Gas System. Certain combustion gas components can promote alkali vapor transport in glass systems. Such transport is important in glass melting. Also, glass had heen suggested as a medium for trapping particulate material in combustion gas clean-up processes, such as for pressurized fluidized bed combustion ( ). ... 

The gasiflcation process is particularly effective for the treatment of plastics. The reducing atmosphere and the presence of hydrogen leads to an instant breakdown of the molecular structure of the plastic to form CO and H2 while any halogen compounds are released for capture in the gas clean-up system [72]. 

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