Coal and lignite are the principal fuels used in many power plants world wide. Although their characteristics appear to be almost same, some differences do exist which set them apart totally. Variations in boiler design also present many challenges and system design has to take care of these issues.
Lignite and coal handling systems are two most commonly adopted fuel handling systems in power plants. Depending on whether it is coal or lignite that is burned in the power plant, the intricacies of the material handling and processing system design vary. In certain aspects, coal is friendlier to handle as opposed to lignite. These differences set the two types of fuel apart. Also the system requirements vary with the type of boiler, which, for example, results in crushing and screening systems being different for each type of boiler. The various issues related to the equipment and design differences in coal and lignite handling systems are covered in the following sections.
The most important characteristics that influence the handling system design are size of coal or lignite particles, bulk density, calorific value, angle of repose, moisture content, tendency to consolidate, clay content, Hard Groove Index (HGI), whether material is friable or not, property of spontaneous combustion and ability of small size material to get air borne.
The calorific values and some chemical properties of different classes of coal are given in Table 1. For an actual system design, all properties have to be determined using specimen of the fuel to be burned and the appropriate testing methods. The effect of the individual characteristics is elaborated under relevant aspects of the discussions.
Lignite Characteristics and Types of Boilers
Lignite is at the lower end of the natural process of metamorphosis of the fossil material and hence has lower calorific value (approx. 29 000 kJ/kg max.) compared to coal. It also has a higher inherent moisture content of 30 %. In rainy season, the total moisture can increase to more than 50 %. The hard groove index of lignite is about 160, and its bulk density varies between 0.8 to 1.0 t/m3.
Lignite deposits in south India contain small percentages of Marcosite which is a very hard material. Sometimes this material damages crusher and screen parts. In Gujarat, lignite contains clay and sulphur pyrites, and large sized stones. Lignite with sulphur pyrites cannot be crushed. The clay content helps screens to get chocked and affects crushing as well. During dry seasons, dust nuisance will be quite high due to the fact that lignite is friable and the crushing process generates high amount of fines. Furthermore, it has high volatiles which will aid spontaneous combustion.
Two types of boilers are mostly used in lignite as well as coal fired power plants, i.e. Pulverized Fuel (PF) Fired boilers and Circulating Fluidized Bed Combustion (CFBC) boilers. The aspects related to the boiler technology are same for both lignite and coal and are hence not discussed further. In case of PF Boilers the screening and crushing systems to be adopted are less complicated involving only single stage crushing and screening whereas in case of CFBC boilers, the screening has to be in two stages and crushing in three stages which makes the system more complicated.
Effect of the Low Calorific Value of Lignite
The necessary capacity of the lignite handling system will be higher than of a coal handling system for the same unit capacity due to its lower calorific value. Hence, the capacity of the crushers, screens, conveyors, etc., needs to be higher compared to those of a coal handling system. This also puts up some restriction considering the availability of the equipment with the desired capacity. For example, the maximum capacity for hammer-mill-type crushers is 1100 t/h. If the necessary capacity of the system is higher than 1100 t/h, the system will have to be split into a number of streams which is not desirable considering the flow problems. Further, in lignite powered stations, lignite is received directly from the mines via conveyors. The capacity of the incoming conveyor will be higher than the plant requirements. Hence, the feed rate has to be reduced to match the plant requirement. Based on these concepts some of the design features required to meet the challenges posed by lignite are furnished in the following sections.
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