Lignite, 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. 

Properties. Pilot-unit data indicate the EDS process may accommodate a wide variety of coal types. Overall process yields from bituminous, subbituminous, and lignite coals, which include Hquids from both Hquefaction and Flexicoking, are shown in Figure 14. The Hquids produced have higher nitrogen contents than are found in similar petroleum fractions. Sulfur contents reflect the sulfur levels of the starting coals ca 4.0 wt % sulfur in the dry bituminous coal 0.5 wt % in the subbituminous and 1.2 wt % sulfur in the dry lignite. 

North America.. In the United States, lignite deposits are located in the northern Great Plains and in the Gulf states. Subbituminous coal is found along the Rocky Mountains. The western half of North Dakota has about 74% of the nation s resources, Montana 23%, Texas 2%, and Alabama and South Dakota about 0.5% each. The lignite resources to 914 m represent 28% of the total toimage of all coal deposits in the United States. The lower cost and low sulfur content have contributed to rapid growth in production. 

The lignitic coals of the northern United States tend to have low sulfur contents, making them attractive for boilet fuels to meet sulfur-emission standards. However, low sulfur content coals have impaired the performance of electrostatic precipitators. The ash of these coals tends to be high in alkaline earths (Ca, Mg) and alkaUes (Na, K). As a result, the ash can trap sulfur as sulfites and sulfates (see Airpollution control methods). 

Because of its low sulfur content, lignite is becoming mote important. The U.S. Clean Air Act Amendments of 1990 have resulted in economic premiums fotlow sulfur coal corresponding to 10/t for emission allowances at 500/t of SO2 (32). 

In 1974 a 1000 t/d ammonia plant went into operation near Johaimesburg, South Africa. The lignitic (subbituminous) coal used there contains about 14% ash, 36% volatile matter, and 1% sulfur. The plant has six Koppers-Totzek low pressure, high temperature gasifiers. Refrigerated methanol (—38° C, 3.0 MPa (30 atm)) is used to remove H2S. A 58% CO mixture reacts with steam over an iron catalyst to produce H2. The carbon dioxide is removed with methanol (at —58° C and 5.2 MPa (51 atm)). Ammonia synthesis is carried out at ca 22 MPa (220 atm) (53) (see Ammonia). 

Charcoal—sulfur processes need low ash hardwood charcoal, prepared at 400—500°C under controlled conditions. At the carbon disulfide plant site, the charcoal is calcined before use to expel water and residual hydrogen and oxygen compounds. This precalcination step minimises the undesirable formation of hydrogen sulfide and carbonyl sulfide. Although wood charcoal is preferred, other sources of carbon can be used including coal (30,31), lignite chars (32,33), and coke (34). Sulfur specifications are also important low ash content is necessary to minimise fouling of the process equipment. 

The sulfur in the subbiturninous coal or lignite is converted mosdy to hydrogen sulfide. Small amounts of carbonyl sulfide are also produced. 

Methanation Section Test Conditions. To date, the HYGAS pilot plant has been operated with Montana lignite coal feed. This lignite is a low sulfur (usually less than 1 wt % ) coal which has good reactivity for hydrogasification. The coal is nonagglomerating (thus eliminating the need for pretreatment), and it represents the lowest rank of U. S. coals that are available for conversion to substitute natural gas. 

Deer Park Complex, demonstration 1987-1991 lignite/ efficiency, 99% sulfur removal... 

GARCIA SCHOBERT Liquefaction Behavior of High-Sulfur Lignites 215... 

Table I. Ultimate and Sulfur Forms Analyses (dry, ash-finee basis) of Lignites Used in this Work...

Mechanism of Hydrogenation of Mequinenza Lignite. Our initial work focused on the hy(frx)desulfurization of the Mequinenza lignite the elucidation of the mechanism of this process then led to a further consideration of the role of organic sulfur in the liquefaction process, pesented in the next subsection. 

In most experiments the sulfur removed from the Mequinenza lignite appeared in two products, hydrogen sulfide and liquid organosulfur compounds (LOG) present in the THF-soluble extract There are three mechanisms by which these two sulfur-containing products could be produced from the lignite ... 

Figure 1. Variation of LOG and H2S yields as a function of sulfur conversion from Mequinenza lignite.

These results, particularly the indication of a sequential mechanism involving an initial production of liquids which are subsequently hydrotreated on the molybdenum catalyst, were the experimental basis for probing further into the first step - liquids formation - with particular interest in whether the unusually high concentration of organic sulfur in the Mequinenza lignite might have some effect on the liquefaction behavior. 

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