Coal conversion, high sulfur

Conversion of carbon in the coal to gas is very high. With low rank coal, such as lignite and subbituminous coal, conversion may border on 100%, and for highly volatile A coals, it is on the order of 90—95%. Unconverted carbon appears mainly in the overhead material. Sulfur removal is faciUtated in the process because typically 90% of it appears in the gas as hydrogen sulfide, H2S, and 10% as carbonyl sulfide, COS carbon disulfide, CS2, and/or methyl thiol, CH SH, are not usually formed. 

Partial oxidation of heavy Hquid hydrocarbons requires somewhat simpler environmental controls. The principal source of particulates is carbon, or soot, formed by the high temperature of the oxidation step. The soot is scmbbed from the raw synthesis gas and either recycled back to the gasifier, or recovered as soHd peUitized fuel. Sulfur and condensate treatment is similar in principle to that required for coal gasification, although the amounts of potential poUutants generated is usually less (see Coal conversion processes, gasification). 

Other applications of microparticles include spray drying, stack gas scrubbing, particle and droplet combustion, catalytic conversion of gases, fog formation, and nucleation. The removal of SO2 formed in the combustion of high-sulfur coal can be accomplished by adding limestone to coal in a fluidized bed combustor. The formation of CaO leads to the reaction... 

Hydrogen sulfide is a by-product of many industrial operations, eg, coking and the hydrodesulfurization of crude oil and of coal. Hydrodesulfurization is increasing in importance as the use of high sulfur crude oil becomes increasingly necessary (see Petroleum, refinery processes). A large future source of hydrogen sulfide may result if coal liquefaction attains commercial importance (see Coal CONVERSION processes). 

Eastern coal conversion development may come to be favored because of market proximity, water availability, and coal sources which, because of their high sulfur content, are currently unuse-able and, hence, largely decoupled from other energy prices. Proximity reduces transport costs and allows an increased use of low and medium Btu syngas processes. Reactivity and swelling problems may be overcome by technology. 

Plants to process alternate sources of energy will make low-sulfur fuels from high-sulfur feeds such as coal, oil shale, tar sands, or heavy oil. Elemental sulfur will be a normal byproduct. Nearly all processing schemes first produce H2S from the sulfur compounds in the raw material, and then convert H2S to elemental sulfur. Usually this conversion step is labelled "Claus Process". 

The sulfur distribution from the coal conversion plant is shown in Figure 4. The wt% of sulfur remaining in the ash depends on several factors among which are the relative distribution of organic and inorganic sulfur in the coal and the chemical composition of the ash. High alkaline ashes will capture sulfur as sulphide or sulfate. 

Large-scale demonstration of the SRC-II process is currently being pursued as the next step in establishing the capability for the conversion of our high-sulfur coal reserves into a spectrum of hydrocarbon products to displace imported petroleum. 

Biomass is similar to some coals with respect to total ash content as discussed in Chapter 3, but because of the diversity of biomass, several species and types have relatively low ash and also low sulfur contents. Woody biomass is one of the feedstocks of choice for thermal gasification processes. The ash contents are low compared to those of coal, and the sulfur contents are the lowest of almost all biomass species. Grasses and straws are relatively high in ash content compared to most other terrestrial biomass, and when used as feedstocks for thermal conversion systems, such biomass has been found to cause a few fouling problems. 

Low sulfur fuel oils were prepared from a high volatile bituminous coal by hydrogenation under high temperatures and pressures. At a coal conversion of 80%, the ratio of oiU to-gas yields was about three, and 23% of the coal sulfur was contained in the oil. Sulfur content of the oil, however, remained the same at different coal conversion levels. The data obtained in the semi-continuous, dilute phase hydrogenation system showed that the whole oil can be directly used as a fuel oil where 1% sulfur is tolerated. Fuel oils containing 0,5 and 0,25% sulfur were produced by desulfurization of the whole oil, A preliminary economic evaluation indicated that low sulfur fuel oils can be produced from coal by hydrogenation at a manufacturing cost of about 5-6 per barrel. 

Since the bulk of the adsorption is accomplished in the second phase under stationary conditions, the adsorbent was developed to obtain high sulfur dioxide-to-sulfuric acid conversion rates for a large portion of its inner surface. The relationship between pore structure and sulfur dioxide adsorption is shown in Figure 1. The ordinate is the time, in hours, after which 10% of the inlet sulfur dioxide will pass through the carbon without being adsorbed. The mean pore diameter of adsorption pores was selected for the abscissa as the parameter to characterize the adsorbent structure (3). Adsorbents produced from bituminous coal with and without catalyst impregnation were tested. In both cases, the sulfur... 

Coal/oil slurries were used in the instrument performance tests. The coal was finely ground Ohio 9, a high-ash highly volatile bituminous coal, the major constituents of which are 59 wt.% carbon, 4 wt.% hydrogen, 4 wt.% sulfur, and 24 wt.% ash. A sieve analysis disclosed that = 86% of the coal particles were of 63-125 om in diameter. The oil was a representative organic liquid used in the feedlines of some pilot coal conversion plants. Oil density at 20°C was 0.868 g/cm3. 

This process involves the conversion of (high-sulfur) coal to a solid residue and a synthetic crude oil by extraction of coal with a coal-derived solvent which has been previously hydrogenated (Nowacki, 1979). In this process (Figure 19.8), crushed coal (0.375 in. [10 imn]) is partially dried and heated to 230°C (450°F) before being mixed with the coal-derived solvent. The coal is then extracted in a reactor at 405°C (765°F) and 150-400 psi. The liquid product and the solid residue (which also contains any... 

The solvent refined coal (SRC) process has been conveniently described in two forms the SRC 1 process and the SRC 11 process (Schmid, 1975 Baughman, 1978). In the SRC I process, high-sulfur, high-ash coals are converted to a low-ash solid fuel whereas the SRC II process results in a liquid product (rather than a solid product) from a recycle of the product slurry, thereby increasing the conversion of the coal to lower molecular weight species (Figures 19.11 and 19.12). 

---