requires accurate estimation of parameters such as moisture in coal, HGI of raw coal and mill output fineness.

Collection of a representative coal sample from pressurized gravimetric feeder had been a major constraint in characterizing the mill performance because of unrepresentative sampling location and inappropriate equipment for sample collection. The performance of the mill is governed by the following operating parameters that need to be set accurately if a reliable database is required to be generated.

a)Mill outlet temperature

b)Mill air flow

c)Accuracy of gravimetric feeder

d)Raw coal size

3Pulverizer Testing

Pulverizers are selected to provide a required grinding capacity, based upon a set of design criteria that include the required coal particle fineness and coal characteristics. The required pulverizer capacity is determined by the boiler heat input requirement, with some additional allowance added to account for wear of the mechanical parts. The required fineness is a function of specific coal properties such as the volatiles in coal and the rank of the coal. It is also a function of the type of burners, firing system and furnace size. Coal characteristics that determine the mill performance are the grindability and the moisture in the coal.

Pulverizer capacity shall vary according to the current operating conditions. There is a defined relationship between mill mechanical capacity, coal grindability and coal particle fineness. Higher grindability means that mill can grind more coal to same fineness or the same amount of coal to a greater fineness.

Pulverizer capacity is affected by such factors as moisture in coal, and the available primary air inlet temperature and flow. To a lesser extent, the ambient temperature and raw coal temperature will also affect the mill performance.

High ash content in coal results in greater loading on pulverizers and subsequent erosion causes performance degradation. Internal wear parts life varies from 2500 hrs for erosive coals to 5000 hrs for softer coals, and within the same pulverizer, the life of different components can differ by as much as 100%.

3.1 Process Description

This procedure provides guidelines for conducting the performance checks on pulverizers. Implementation of this procedure in conjunction with associated prerequisites shall result in identification of pulverizer system deficiencies and possible improvement of the coal pulverizer systems.

Performance guarantee tests conducted on the pulverizers for their acceptance stipulate most of these tests and base line data are available with most of the stations. However, the testing techniques have improved due to availability of modern instruments, which are capable of giving a better insight into pulverizer system performance. This procedure describes in detail the methodology of pulverizer testing using the dirty pitot testing kit.

3.2Objective

The objective of this procedure is to establish a standard method for the following performance tests related to the pulverizer system.

(1)Clean air balancing between coal transport pipes.

(2)Dirty air balancing between coal transport pipes.

(3)Fuel balancing between coal transport pipes.

(4)Fineness testing of pulverized coal using isokinetic sampling.

(5)Fineness testing of pulverized coal using non isokinetic sampling.

(6)Calibration of feeders.

This methodology is suitable for the following conditions only.

•For pulverized fuel sampling from vertical circular pipes.

•Maximum particle size to be sampled should be less than one-third diameter of the sampler tip aperture.

•Sampling to be done in pipes with an internal diameter between 250mm and 700mm.

•The air/coal ratio in the pipe is with in the normal range of direct fired pulverized system.

The numbers of sampling ports are determined based on the available straight lengths of coal pipes, upstream or downstream of the nearest bends. The guidelines for locating testing cross sections are described in ASME PTC 4.2.