Charles Conroy, managing director of UK engineering firm Greenbank Group, explains how technology innovation in coal particle size analysis is an important development in improving coal fired combustion.


Charles Conroy, Greenbank, UK


The MillMaster particle size analyzer illuminates coal particles using a lazer and generating a light sheet through a specialist optic

Coal particle analysis has always been seen as a crucial element in the optimization of coal fired combustion in power generation. Since coal particle size impacts many aspects of the combustion process such as loss of ignition, coal flow characteristics, erosion and nitrogen oxides and carbon dioxide emissions, understanding coal particulate enables coal fired plants to further optimize boiler performance and increase efficiency, plus improve the maintenance strategy of the pulverizer and classifier plant.

Until quite recently, traditional manual sampling methods used to capture coal particle size data have proved both taxing and costly, and the notable time delay between collection and analysis has provided less reliable and somewhat restrictive results that have limited the benefits achieved through the measurement process.


Making progress


Current boiler optimization innovators are providing existing coal fired plants with the ability to make major improvements to operational efficiency using state-of-the-art coal flow technology that measures coal particle size online. Following a four-year research and development programme, Greenbank introduced the MillMaster, a particle size analyzer. Greenbank focused on developing the analyzer because it was aware of a growing demand for a product that could perform the highly accurate measurement of coal particulate size online, removing the need for time-consuming and much less efficient manual sampling. A power station engineer can use the MillMaster to optimize the pulverization of coal particulate, improve combustion and implement a strategic maintenance programme for the pulverizing plant.

MillMaster uses patented light sheet technology to determine the size of particles being conveyed in a lean phase pneumatic conveying environment. The size of particles that can be measured ranges from 0.5 micron to 500 microns for the standard device. It works by illuminating the particles using a lazer and generating a light sheet through a specialist optic. The illuminated flow is then captured using a high speed charge-coupled-device camera. The speed of the camera shutter ‘freezes’ the image of the illuminated flow, which enables various image processing algorithms to be applied.


Unique technology


The key to the uniqueness of the MillMaster is the application of algorithms for removing in and out of field particles, and determining the true size from the image. As the light sheet illuminates the particles it also alters their size in relation to the image, and being able to determine their true size from the image and known camera parameters is what makes this device highly effective.

Capable of performing several different measurement operations, the device counts and sizes the particles in the sample, and this size distribution can be arranged in a number of ways to the user. As a default it is arranged into bins of various sizes, indicating the approximate size distribution. The system can alternatively arrange the information as a Rossin-Rammler distribution, which is more familiar to engineers operating milling groups and classification processes.

The programme also calculates and trends the average particle size, the maximum detected and the minimum detected. In fact, through the use of Greenbank’s in-house programming engineers, the exact design of the outputs can be altered to suit a specific customer requirement.

The device itself operates on an iso-kinetic extraction system. The iso-kinetic method allows a representative sample to be taken at any point and the MillMaster is designed so that this sample is passed through the system in such a way that no obscuration takes place. Normally the sampling point is positioned in an area of good particle diffusion, well away from significant roping or clustering of particles. If such a position cannot be found then Greenbank can utilize computational modelling to select a position that would contribute the most representative sample.

The system is such that in probe sample mode it can deliver a reading every five minutes. On continuous mode it will deliver an updated reading every minute. However, the time between samples can be configured by the end user via a touch-screen panel. The results issued are averaged from several images taken of a particular sample and this is then further averaged over the time, thus improving the accuracy of the information supplied.

The particle size analysis system is built into an air-cooled IP65 industrial cabinet, which has been purposely designed to withstand industrial environments. As the system is lazer-based, the cabinet is fully interlocked, which means there is no danger of lazer exposure. The lazer diode used is of such a scale that there is no chance of any ignition from the lazer light.

The MillMaster cabinet can be set up to perform continuous sampling on a single point or timed samples on a number of points, and rotate between these ports. For example, it can be configured to sample multiple mill-classifier outlets on one or more mills.

To do this the cabinet can control motor operated valves connected to a small bore stainless steel pipe network. In this way a single system can take up to eight single-point samples. Once a sample is measured it is returned to the pipe network through either the particulate flow or to a reject line.

The system itself is controlled by an internal Programmable Automated Controller (PAC) and communicates with the plant by either Ethernet communication or by 4-20mA signals. The cabinet has a touch-screen that allows the viewing of results and enables the machine to be reset or turned off. Through an Ethernet connection, engineers can get the same interface directly on their computer allowing them to remotely view the operation. Alternatively Greenbank can set up OPC TPC/IP tags for engineers to feed into any distributed system that uses OPC.


Focus on grinding improvements


The MillMaster was originally developed for ground coal; the typical set up would be a system sampling from the ground product line from the mill to the burners. The primary function in this application is to monitor the grind of the coal leaving the mill-classifier. The probes would be place in such a position that a reasonable air: fuel mix would be present to give a representative sample. The reject line can either return the coal to the pipe, the bottom of the boiler, or a reject chute on the mill.

It has been suggested that the key focus of grind improvement should be to increase the percentage of material which is below 75 microns in size and reduce the amount that is above 300 microns. It has been shown that particles above 300 microns contribute to an increased level of carbon-in-ash due to poor combustion. The bin sizes for the size distribution are set up to monitor these fractions, and the 4-20mA signals are set up to indicate the percentage of particles above 300 microns in each sample.

With the rise in the use of dynamic classifiers in the coal industry, being able to compare the improvement of grind compared to the energy consumption of the dynamic classifier can be considered of high importance, especially as some studies suggest a case of diminishing returns on ever increasing rises in classifier rotation speeds. If the MillMaster is connected permanently it can trend the grind of a particular mill, detailing the changes in grind performance over time. This information can then be used to identify maintenance points and possible signs of catastrophic failure.

In a coal system there is a constant risk of explosion and fire. In order to counter this problem the MillMaster comes with an incident sensor that detects the leading edge of a large pressure gradient. Upon detecting this, the programme shuts down and activates a sacrificial ball valve. This ball valve is designed well beyond the standard explosive pressures, but is easily removed if damaged. The MillMaster cabinet then reports the incident via the Ethernet communication and waits to be told to restart.

This feature can be deactivated, but by default is activated to protect the device. While developed primarily for coal analysis, the technology behind this particle size analysis product has proven suitable for operation within any lean phase powder system and following the successful launch of the MillMaster, the company is introducing a sister product called the StackMaster which will be configured to measure particulate being conveyed in stack emissions.

The MillMaster is a powerful tool in monitoring the operational performance of coal fired power generators and Greennbank is confident that it that will make a real difference to accurate measurement and operational efficiency in power plants, and the StackMaster can be utilized to ensure plants particulate emissions are continuously monitored and kept under control in accordance with environmental guidelines.

Greenbank’s family of boiler optimization products, of which the MillMaster is a part, are the result of an extensive research and development programme that has included working with acclaimed UK engineering universities such as Leicester and Cambridge.



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