Influence of Coal Properties on Gasification

Coal properties have a major influence on the process and gasifier design and include moisture; ash, volatile matter, and fixed carbon content; caking tendencies; reactivity; ash fusion characteristics; and particle size distribution. Some of these properties are listed in Table 5-7, and each is briefly discussed in the following sections.

Moisture

Fixed-bed gasifiers can accommodate moisture contents of up to 35%, provided the ash content is not in excess of about 10% [2]. Predrying may be performed if the moisture and ash contents are above these amounts. Entrained-flow or fluidized-bed gasifiers require the moisture content to be reduced to less than ~5% by drying to improve coal handling. In the entrained-flow system, the residual moisture contributes to the gasification steam but requires heat to evaporate it.

Ash should be kept at a minimum because provisions must be made for introducing it to and withdrawing it from the system, provisions that add to the complexity and cost of the overall system. Ash can be used as a heat transfer medium, either by its flow countercurrent to the products of gasification and gasifying agents in fixed-bed systems or by provisions in entrained-flow systems [2]. In fixed-bed systems, ash accumulates at the base of the fuel bed and is withdrawn by a mechanical grate if unfused or through a taphole if it is a liquid slag. In the entrained-flow system, it is removed as a liquid slag. In fluidized-bed systems, the ash is mixed with the char and the ash is separated either by sintering and agglomeration of the ash or circulation from the bed through a fully-entrained combustor to melt and separate the ash as a liquid slag. Fluidized-bed and entrained-flow gasifiers tend to have higher losses of carbon in the ash than the fixed-bed systems. Ash constituents are important in the selection of materials of construction, particularly in slagging com-bustors. In addition, proper ash composition, or its chemical manipulation through the addition of fluxing agents, is necessary for desirable slagging operations.

Volatile Matter

The volatile matter from the coal can add to the products of gasification without incurring steam decomposition or oxygen consumption. The volatile matter, which can vary from less than 5% (on a moist, ash-free basis) for anthracite to over 50% for subbituminous coal or lignites, can consist of carbon oxides, hydrogen, and traces of nitrogen compounds [2]. The volatile matter composition, the type of coal, and the conditions under which the volatile matter is driven off affect the nature of the residual fixed carbon or char that remains.

Fixed Carbon

The nature of the fixed carbon, which is the major component of the char after the moisture and volatile matter are driven off, is important to the performance of the gasifier and can vary physically and chemically. Properties such as density, structure, friability/strength, and reactivity depend primarily on the original coal but they are influenced by the pressure, rate at which the coal is heated, and its final temperature [2].

Caking Tendencies

The caking tendencies of the coal—strongly caking and swelling, weakly caking, and noncaking—must be considered in the gasifier and process design [2].

Some gasifiers can be designed to handle caking and swelling coals, but others will require the coal to be pretreated. Table 5-7 lists the acceptability of caking coal to the generic types of gasifiers [44].

Ash Fusion

The ash fusion temperature is a measure of when the ash will melt and transform from a solid to a liquid state. This temperature is an important parameter for the design and operation of gasification systems—for those that operate below the ash fusion temperature so as not to incur fusion, sintering, or clinkering of the ash, as well as those gasification systems that operate above the ash fusion temperature to promote slag production. Typical preferred ash melting temperatures for the generic types of gasifiers are listed in Table 5-7. Another important ash characteristic is the relationship between temperature and ash viscosity as the flow characteristics of the slag are critical. The importance of slag viscosity was discussed earlier in the chapter and is as relevant to slagging gasifiers as it is for slagging combustors.

Reactivity

Coals vary in their reactivity to steam and to a lesser extent to hydrogen [2]. Reactive coals, or their chars, decompose steam more rapidly and sustain that decomposition down to a lower temperature than do less reactive coals. Reactivity has three important influences: (1) it favorably influences methane formation; (2) it reduces oxygen consumption by allowing steam decomposition down to a lower temperature; and (3) it allows less steam to be used per volume of oxygen or air than with less reactive coals without incurring ash clinkering.

Coal Size Distribution

The coal size limits are important gasifier system design considerations. In fixed-bed gasification systems, provisions have to be made for the fines that are generated from mining, transportation, and processing. This may include steam and power generation or briquetting, extrusion, or injection to allow them to be supplied to the fixed-bed gasifier [2]. The size distribution is less critical with fluidized-bed and entrained-flow gasification systems.

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