Partial oxidation Soot formation

Soot. Emitted smoke from clean (ash-free) fuels consists of unoxidized and aggregated particles of soot, sometimes referred to as carbon though it is actually a hydrocarbon. Typically, the particles are of submicrometer size and are initially formed by pyrolysis or partial oxidation of hydrocarbons in very rich but hot regions of hydrocarbon flames conditions that cause smoke will usually also tend to produce unbumed hydrocarbons with thek potential contribution to smog formation. Both maybe objectionable, though for different reasons, at concentrations equivalent to only 0.01—0.1% of the initial fuel. Although thek effect on combustion efficiency would be negligible at these levels, it is nevertheless important to reduce such emissions. 

The global reactions considered include the conversion by pure pyrolysis of toluene to acetylene and the conversion of isooctane to ethylene, oxidative pyrolysis of the acetylene and ethylene, and partial oxidation of the parent fuels and these hydrocarbon intermediates to CO, H2, and H2O. The specific reactions and rates for this system are given in Table II. Soot formation is assumed to be a function of temperature and oxygen and precursor concentrations. In the present study the soot precursors are taken to be acetylene and toluene, expressed as C2 hydrocarbons. 

Partial oxidation of methane (or hydrocarbons) is another option to produce syngas [4], This process, which runs without a catalyst, needs high temperatures for high CH4 conversion and to suppress soot formation. The process can handle other feedstocks, such as heavy oil factions and biomass, and yields syngas with a H2/CO ratio of about 2. The process is eminently suitable for large-scale production of syngas (e.g. for gas-to-liquids [GTL] plants). 

Partial oxidation (POX) of natural gas with oxygen is carried out in a high-pressure, refractory-lined reactor. The ratio of oxygen to carbon is carefully controlled to maximize the yield of CO and H2 while maintaining an acceptable level of C02 and residual methane and minimizing the formation of soot. Downstream equipment is provided to remove the large amount of heat generated by the oxidation reaction,... 

Not much information is published on the kinetics of partial oxidation. The methane concentration is about 8 times higher than indicated by reaction (71), but as expected increasing pressure promotes methane formation. From a mere thermodynamical point of view no soot should be present at 1300 °C and O/C > 1. However, the raw sythesis gas contains more or less soot, depending on feedstock. Gasification of heavy oils fractions yields about 1-2% soot, but with methane the soot content is close to zero. 

The occurrence of and particulate in the exhaust gas is coupled. NO is formed as a result of the high combustion temperature and pressure in combination with a superstoichiometric amount of oxygen. Particulates are formed due to local shortage of oxygen in the burning diesel spray. If better combustion is pursued to reduce the emission of particulates, this often leads to better oxidation conditions and an increased emission can be the result, [f on the other hand the oxygen partial pressure is reduced to reduce the NO, emissions, this will lead to more soot formation. This effect is called the NO,-soot trade-off... 

The overall reaction in a partial oxidation reactor is highly exothermic. The desired reactions may be accompanied by thermal cracking of hydrocarbons or oxidative dehydrogenation into nonsaturated compounds including olefins, polyaromatics, and soot. The control of the heat balance and the formation of by-products are important considerations in the design of partial oxidation reactors. 

The technologies for production of syngas from hydrocarbons are based on either steam-reforming or partial oxidation. In the former case, the hydrocarbons react with steam with considerable addition of heat to produce a syngas with a H2/CO ratio of 3 or more. Partial oxidation may be carried out either thermally or catalytically (or by a combination) to produce a syngas with an H2/CO ratio less than 2. Regardless of technology, CO2 may be added to the feed to adjust the gas composition to a low H2/CO ratio. In all cases, limits for the formation of carbon on catalysts or soot in the condensate must be considered to avoid rapid deactivation and low on-stream factors. 

Combustion processes can create pollutant emissions other than carbon monoxide and oxides of nifrogen. Unbumed hydrocarbons (UHC) is a term describing any fuel or partially oxidized hydrocarbon species that exit the stack of a furnace. The cause for these emissions is typically due to incomplete combustion of the fuel from poor mixing or low furnace temperature. A low temperature environment can be created by operating the furnace at a reduced firing rate or turndown. Particulate matter (commonly called soot) is often produced from fuel rich regions in diffusion flames. Soot becomes smoke if the rate of formation of soot exceeds the rate of oxidation of soot. Oxides of sulfur are formed when sulfur is present in the fuel. 

Major issues in steam reforming and catalytic partial oxidation or catalytic oxidative steam reforming are oxygen storage and the acidity of the catalyst support, which could be relevant in terms of soot or alkene formation. 

The non-catalytic partial oxidation [486] (POX, Texaco, Shell) needs high temperature to ensure complete conversion of methane and to reduce soot formation. Some soot is normally formed and is removed in a separate soot scrubber system downstream of the partial oxidation reactor. The thermal processes typically result in a product gas with H2/C0=1.7—1.8. Gasification of heavy oil fractions, petcoke, coal and biomass may play an increasing role as these fiaetions are becoming more available and natural gas (NG) less available. 

Thus, a replaceable tank was taken into consideration for a future system. Attempts to run the burners under conditions of partial oxidation resulted in soot formation in the reactors. 

Under the conditions of partial oxidation, carbon should not be present according to the Boudouard equilibrium [Eq. (6.2.6)], but nevertheless soot can be present in the raw gas. During partial oxidation of methane soot formation is practically zero, whereas in heavy-oil gasification of up to 4% by mass of feedstock is converted into soot. 

Partial oxidation is achieved at reactor conditions ranging from 1350 to 1600°C and pressures of up to 15 MPa (150 atm). This process is attractive because it allows utilization of hydrocarbon feeds that could not be handled in the more conventional vapor-phase processes, such as steam reforming. Particular disadvantages of the process (besides the need to furnish pure oxygen for POX reaetor injection) include cost and the inevitability of soot formation either via thermal cracking of the feedstock or through the Boudouard reaction (CO disproportionation). 

Since ATRs combine some of the best features of steam reforming and partial or full oxidation, some groups have developed compact catalyst systems to eliminate the need for a robust burner and mixer design. The catalyst system also reduces the formation of carbon and soot. Farrauto et al.6 and Giroux et al.7 discussed ATR-based systems for fuel cell applications in detail. 

Some of the substances mentioned are concerned in the formation of secondary harmful species ozone, peracyl nitrate, singlet oxygen and solid aerosols in the atmosphere, which are components of the white photochemical smog of the California variety. Sulphur dioxide in the air is partially further oxidized to sulphur trioxide and hence sulphuric acid. These substances, together with soot are components of the black smog of the London type. 

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