Precombustion processes

FluidiZed-Bed Combustion. Fluidized-bed combustors are able to bum coal particles effectively in the range of 1.5 mm to 6 mm in size, which are floating in place in an expanded bed (40). Coal and limestone for SO2 capture can be fed to the combustion zone, and ash can be removed from it, by pneumatic transfer. Very Htfle precombustion processing is needed to prepare either the coal or the sorbent for entry into the furnace (41). 

In the case of C02-selective membranes in precombustion processes, the fuel heating value remains at the high-pressure retentate side (H2 and unconverted fuel) in the WGS separation process. The power-cycle efficiency for natural-gas-fuelled GTCC including C02-selective membranes in the WGS reactor appears less pressure dependent compared to hydrogen-selective membranes, due to the lower amount of C02 produced in the MSR + WGS reactions [24]. A relative simple simulation for precombustion capture made (based on C02/H2 selectivity of 50) [24], suggests that... 

Precombustion and postcombustion pollutant removal technologies should not be considered to be in competition. Rather, they are complementary technologies that should be used together for maximum benefit. Many precombustion processes are low in cost, but cannot remove all of the pollutants, while... 

Desulfurization of fossil fuels was the subject of an authoritative review by J. B. Hyne (Alberta Sulphur Research Institute). This is a topic of increasing importance as Canada relies more and more on sulfur-containing fuels such as tar sands and heavy oils. Hyne reviewed the present state of the chemistry and technology for both precombustion desulfurization of natural gas and crude oils and postcombustion tailgas clean up of coals and cokes. He clearly identified areas of possible future research such as the high temperature-high pressure chemistry pertaining to in-situ desulfurization processes. 

The first major attempt at precombustion desulphurisation was in the coal gas industry and a number of efficient and effective techniques for removal of H2S, COS, CS2, mercaptans and other volatile sulphur containing products of the gasification process were developed. Many of these techniques found application in the subsequent development of sour natural gas processing where large volumes of hydrogen sulphide had to be removed from the hydrocarbon component. 

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. 

The German COORETEC CO2 Reduction Technologies) concept favours coal gasification with precombustion capture to introduce COj capture into coal fired power plants. The same capture process is suited to produce hydrogen from coal in an environmentally-friendly way. This technical option is outlined in the recently drawn up national vision on hydrogen technologies. The first projects in this area are to be funded in the near future. 

Schematic of a simple, singe-shaft gas turbine process (1) inlet, (2) comprssed state, precombustion, (3) post-combustion, pre-compressor, (4) post-turbine to exhaust.

The following example illustrates the potential of membrane-separation processes for precombustion carbon capture in an IGCC. This approach avoids using an expensive air-separation unit (asu) or a difficult-to-implement high-temperature mixed-ion conducting membrane process however, it still enables capture of C02 at purities suitable for commercial use or sequestration. 

Figure 9.1 Basic schematic diagram of (a) postcombustion capture, (b) precombustion decarbonization, and (c) oxy-fuel processes.

Precombustion Decarbonization Processes Including Hydrogen and Carbon Dioxide Membrane Separation... 

Membranes for Oxygen Separation in Precombustion Decarbonization and Oxy-Fuel Processes... 

Precombustion control involves removal of sulfur compounds from fuel prior to combustion. Control during combustion employs techniques to minimize the formation and/or release of SO2 and N0X during the combustion process. Finally, SO2 and N0X can be removed from the combustion flue gas using various postcombustion control methods. This chapter discusses the potential of mitigating acid deposition through precombustion cleaning of coal to remove sulfur compounds. 

The full potential for removing pyritic sulfur from various coals by physical coal cleaning is significant but difficult to achieve. However, SO2 control by precombustion removal of pyrite could be an important S02-emissions reduction strategy. The cleaned coal produced could be used in coal-fired utilities, constructed both pre-and post-NSPS, as well as in industrial boilers. To realize the potential for coal cleaning in actual practice, however, new techniques must be demonstrated in the laboratory and then at the "proof-of-concept" scale (approximately one ton of coal per hour). These new coal beneficiation techniques could be advanced physical-coal-cleaning (PCC) processes, or they could employ microbial desulfurization or chemical desulfurization to remove organic sulfur. These latter processes could be used by themselves or in concert with PCC processes. 

There are several approaches available to a utility to construct a boiler that will meet New Source Performance Standards. These approaches can be classified according to the position in the combustion system at which pollutant control technology is applied. Precombustion control involves removal of sulfur, nitrogen, and ash compounds from the fuel before it is burned. For coal combustion this approach involves the application of coal-cleaning technology. Combustion control relies on modifications to the combustion process itself or the addition of material to the combustion process to reduce pollutant formation or capture the pollutants formed in the combustion chamber. Examples of combustion control include staged combustion, boiler limestone injection, and fluidized-bed combustion with limestone addition. Post-combustion control involves removal of pollutants after they have been formed but before they are released into the atmosphere. Traditionally, flue gas desulfurization has meant the application of postcombustion control either alone or in conjunction with another... 

The processing scheme for the 53-jLtm Nitex samples was similar to that described previously (10) where each filter was transferred to the PVC stand, rinsed with deionized water under suction to reduce sea salt, folded once onto itself to prevent loss of large particles, and then dried at 60 °C for >24 h. Where samples larger than 53 pm have been required for lipid analysis, precombusted, 53-pm stainless steel mesh of identical specifications to the 53-jLtm Nitex was used. In this case a subsample was dried for the regular chemical analysis the remaining sample was specially preserved. 

There are three main ways to capture C02 in the NGCC process post-combustion C02 capture, oxyfuel combustion, and precombustion C02 capture. 

If the technology is accessible and the cost of applying it is not too great, precombustion removal of potential air pollutants has substantial merit. At this stage the offending substance (or substances) present in the raw material has not as yet been burned or reacted to form the process exhaust gases, which should make recovery easier. 

This approach is known as precombustion CO capture [4], Solvents for H S removal processes are often selected based on their ability to selectively separate H S from COj [1, 2], In chemical solvents, selectivity for H S can be achieved throng preferential reactions favoring rapid reaction of H S with amines but disfavoring its reaction with CO through steric hindrance or slow kinetics [1,2]. 

The authors have stated that in the IGCC process, it is desirable to treat the fuel gas streams at higher temperatures than typical of current processes so as to maximize process efficiency and minimize the impact of precombustion CO capture [3]. Commercial aqueous amine and physical solvent processes respectively require the gas stream to be cooled to 40°C and to -40°C or lower [3]. Minimization of the cooling requirement through warm treating ( 200°C) is an optimal temperature range which enables not only the removal of CO and H S but other contaminants such as sulfur, ammonia, chlorides, and heavy metals (Hg, Cd, etc.) whose emissions are... 

Precombustion of sulfur from coal by selective oxidation is an important step in coal cleaning, without the need for a postcombustion cleanup step to remove sulfur oxides. This process, known as oxydesulfurization of coal, is a three-phase gas-liquid-solid system. As an example, a sparged reactor wiU be designed to oxydesulfurize 100 tons/day of coal in an aqueous slurry of coal with air. 

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