Coal fired plants apply new measurement technologies

Coal fired plants can now turn to a wide array of different technologies for measuring and monitoring applications to ensure safety and promote efficiency.

Richard Gagg, Derek Stuart and Randy Hauer, AMETEK Process & Analytical Instruments

The impressive safety and efficiency of modern coal power plants can only be maintained through careful attention to the operating conditions in all stages of the process. Instruments and systems are now available for: monitoring coal transport, storage and preparation; maximizing efficiency of coal-fired furnaces and boilers; and ensuring emissions compliance.

Many different technologies can be used for these different applications and ” while none of them can be considered ideal for all situations ” for some applications a specific technology offers a clear advantage.

This article reviews the latest technologies available for monitoring various essential operations in coal-fired power generation.

Coal Preparation, Storage and Transport

A 600 MW baseload station requires several thousand tonnes of coal per day. From the coal stockpile through to boiler pulverizers, many transfer points and storage areas must be monitored for the onset of spontaneous combustion. Unless these areas are properly monitored, the plant risks unexpected physical and monetary losses along with possible shutdown.

Coal preparation, storage and transport

To reduce sulphur emissions, there has been a move in recent years away from more traditional bituminous coal towards sub-bituminous coal, which has a tendency to react with oxygen in the air. This has resulted in fires at facilities that previously had excellent safety records. Many facilities that have switched to sub-bituminous coal are finding that increased monitoring is required to ensure safe and continuous operations.

Hot inclusions on coal or coke conveyors can go undetected until they haved cause substantial damage. Traditional methods such as visual inspection or single-point pyrometers are often unable to detect hot spots on a moving conveyor. A high-speed infrared scanning system therefore provides the most reliable and accurate method for continuous monitoring of emerging hot spots. This effectively reduces downtime and eliminates costly conveyor belt repairs.

Applications that monitor the combustion process

A high-speed scanning system samples thousands of temperature points every second. Temperature measurements are sent continually to a dedicated processor that can trigger an alarm or signal to the conveyor operator to take corrective action. At the heart of the system is a compact, high-speed line scanner that has a minimized depth and base ‘footprint’ for installation in restricted spaces. A durable sapphire window provides reliable protection for the system’s optics.

There are three places where a scanner can be used:

1) Above the belt carrying the coal to detect hot inclusions on or close to the surface;
2) Looking at the curtain of coal that falls at the end of one section of belt to another. The curtain of falling coal is more transparent to allow a scanner to see deeper into the coal and detect hot inclusions;
3) Under a belt just after a transfer point. The freshly uncovered belt will have a thermal fingerprint of any hot items that had just been resting on its surface.

These systems are small enough to be positioned under a belt just beyond the point where the material has been transferred. They will measure the entire belt surface and trigger an alarm if any areas are above a safe temperature. Alarms can be used to activate water sprays or suppression systems. Alternatively, they can be positioned above the conveyor to monitor the material to determine hot spots and prevent hot items from being loaded and transported.

Coal storage piles can be monitored by infrared thermal cameras that are typically mounted above and to the sides of storage piles and frequently are operated by pan and tilt motorized mounts. In this way, they can be programmed to patrol the surface of the coal storage piles to detect developing hot areas before they become a problem. Systems typically have embedded logic that ignores hot vehicles that may be operating in the areas of the pile to minimize the possibility of false alarms.

Inside pulverizing coal mills and silos, carbon monoxide (CO) monitoring provides earlier detection of combustion to prevent mill fires. These detection systems are specifically designed to continuously monitor the atmosphere inside pulverizing mills and silos. They respond quickly to any significant increase in CO levels and take preventative action before there is damage to a plant or injury to personnel.

The detection systems extract sample gases from the mill (often the mill outlet) or silo and continuously monitor the CO level. Alarm threshold levels can be set to best suit the plant’s individual operating conditions.

The settings can compensate for externally introduced CO, where mills use recycled combustion gases for coal feed heating. Oxygen measurement is also an option for fire prevention in oxygen-limited silos and for plants that use recycled flue gas and need to continuously monitor oxygen levels.

Combustion Process

Maintaining safe and efficient combustion requires many monitoring functions in the boiler. The dynamic nature of the process requires that the key products of combustion are monitored and controlled on a real-time basis. Parameters such as furnace exit gas temperature, oxygen and combustibles ” CO2, hydrocarbons, etc. ” are key indicators of boiler efficiency, slagging and NOx production.

Post-combustion emissions control devices and stack emissions

Gas analysis systems can be installed from the furnace entrance to the stack exit. Analyzers that utilize various technologies can be used to measure a wide range of emissions including O2, CO, CO2, CH4, NO, NOx and SO2 as well as sulphuric acid dew point and opacity/dust concentration.

Oxygen and combustibles analyzers are ideal for combustion control and NOx reduction. A side-by-side comparison of these analyzers has demonstrated their improved reliability and better response versus conventional oxygen-only in-situ probes. In addition, the hot-wire catalytic detector used by Thermox displays reduced drift and increased sensitivity to low-level CO changes, while better resisting the poisoning effects of SO2.

Infrared technology provides a proven non-contact method for furnace exit gas temperature measurement. This can be used as a tool in reducing NOx emissions and indicating possible boiler slagging conditions.

Thermal imaging inside high-temperature, refractory-lined furnaces and boilers often requires large openings in the refractory wall to view critical areas. This can cause significant heat loss as well as difficulty in keeping the opening free from debris.

Those drawbacks can be eliminated with fixed thermal imaging systems that use a rugged infrared camera to accurately profile furnace temperature through a small opening in the wall. The camera is specifically designed for high-temperature furnaces. Its lens is water cooled and air purged and able to withstand temperatures up to 1200 à‚°C (2192 à‚°F). It is designed to provide a wide angle view inside the furnace and is unaffected by hot CO2 and H2O found in most combustion atmospheres.

Post-Combustion Emissions Control Devices and Stack Emissions

Emission control to meet regulatory constraints is accomplished through a combination of combustion process control and emission reduction equipment. Monitoring analyzers provide essential feedback for optimizing the process and for demonstrating compliance with relevant emissions rules. Compliance analyzers are known as Continuous Emissions Monitoring Systems (CEMS) or Automated Measuring Systems (AMS). The term CEMS is more common in the Americas whereas AMS is generally used in Europe and Asia.

Any analyzer used for CEMS/AMS applications must be accurate and reliable. EN 14181 (Quality Assurance of Automated Measuring Systems) provides a rigorous framework with four quality assurance levels that must be met. This standard is mandatory within the European Union and is finding acceptance in South Africa and in many Asian countries.

UV-based SO2 analyzers are considered the standard for accuracy in testing for sulphur emissions. A rugged single- or multi-component analyzer can be integrated into a CEMS/AMS or used alone. It can be configured to measure most gas species that absorb UV, such as NOx, H2S and SO2.

Highly accurate and reliable non-contact opacity and dust monitors provide continuous monitoring to ensure emissions compliance. These sophisticated systems incorporate a patented “no moving parts” optical technology to ensure the most accurate and reliable stack opacity measurement and dust concentration monitoring available.

Portable sulphuric acid dewpoint monitors have been developed specifically for coal fired systems that require periodic monitoring. This measurement helps maintain boiler efficiency, prevent corrosion and evaluate the effectiveness of fuel additives and changes in combustion conditions.

These lightweight, easy-to-use instruments capture and store thousands of readings and require only simple field maintenance. They assure coal boiler operators that the optimum operating temperature is maintained ” just above the sulphuric acid dewpoint ” where no sulphuric acid is being formed and combustion efficiency is maximized.

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