Emission monitoring benefits for power plants

Monitoring and measuring the emissions of all process plants can give an indication of the levels of pollution being exhausted into the atmosphere and, as importantly, the degree of efficiency of the plant. In many cases, if the plant efficiency is improved, the emission of pollution is significantly reduced; win-win for everyone.

Although each state and territory has its own anti-pollution agency, the overarching national guidelines emanate from the National Environment Protection (Ambient Air Quality) Measures (NEPM). The desired environmental outcome of this Measure is “ambient air quality that allows for the adequate protection of human health and well-being”. The Measure specifies the pollutants; carbon monoxide, nitrogen dioxide, photochemical oxidants (ozone), sulfur dioxide, lead and ‘large’ particulates (PM10). Additionally, the Measure specifies what reductions are desired by what date.

Other pollutants that are under review are water vapour, oxygen, HCl gas, ammonia, carbon monoxide and more. Typically, water vapour and oxygen are monitored as a measure of plant efficiency and not as an environmental pollutant.

Also in the crosshairs are smaller particles (dust) - PM2.5 - emitted into the atmosphere, normally from the stack. This monitoring is also a direct reflection on the efficiency of the plant, as well as an emission issue.

Local regulators and interested groups

Because pollution through emissions is such an emotional and controversial global topic, there are many local and international interest groups. Greenhouse gases, also in Australia, which make up the majority of the pollutants, are tracked by international institutions like WWF (World Wide Fund for Nature) and Carbon Monitoring for Action. Localised regulators such as each state and territory Environmental Protection Authority (EPA) monitor the greenhouse gases, as well as particulate emissions.

Penalties and fines for transgressions vary from region to region and are enforced with differing levels of urgency, but are generally expensive and time-consuming to resolve.

It is strongly advised that every producer that could cause emission pollution implement a strategy to monitor and measure the emission levels and work with the authorities towards a reduction before the fines are enforced.

Industries of interest

All process and manufacturing industries pollute the atmosphere to a certain degree, some significantly more than others. At the lower end of the scale are the food and beverage producers, water and wastewater plants and various manufacturing facilities. Conversely, the higher end emission polluters are paper and pulp mills, cement plants, iron and steel mills, chemical producers, refineries and thermal power stations. In general, coal-fired thermal power stations are the worst polluters and are under constant pressure from the community and regulators to ‘clean up’.

Coal-fired thermal power stations

According to the Australian Bureau of Resources and Energy Economics (BREE), coal continues to be the major fuel source for electricity generation, comprising about 64% of the fuel mix in 2012-13. However, this share has decreased from 77% in 2003-04, accounting for much of the decline in total electricity generation. In 2012-13 coal-fired generation declined across all states in Australia, with black and brown coal-fired generation dropping to their lowest levels since 1997-98. This decline in 2012-13 is most likely due to the increase in the relative costs and uncertainty of coal-fired electricity generation under carbon pricing.

Nevertheless, there are 25 coal-fired power stations nationally, varying in capacity from 2880 MW base-load generators down to 150 MW localised generators, and they each require increased emission monitoring and measurement to reduce emissions and increase efficiency.

What coal-fired power plant processes should be monitored?

Coal-fired power plants comprise similar sections/processes, some more sophisticated than others, but the emission measurements are similar for all. The sections that interest emission monitoring are:

• coal bunker, hopper, coal mill, pulverised fuel (PF) mill, coal conveyers
• steam generator, boiler drum, combustion control, superheater
• flue gas denitrification (DeNOx plant)
• precipitator, bag-house, de-dusting
• flue gas desulfurisation, scrubbers (FGD plant)
• stack, emission monitoring

Figure 1: Emission monitoring points in a coal-fire power station (Image courtesy of Sick).

For a high-resolution image, click here.

Coal bunker, PF mill, conveyors

Pulverised coal (PF) is typically used to fire the boiler of power plants. For safety reasons, monitoring of CO in coal bunkers and coal mills is a critical measurement. CO is an odourless and very toxic gas and poses a serious explosion threat at levels above 8 vol.% in air. Elevated CO concentrations may indicate a source of smouldering and require immediate counter measures. In addition, O₂ concentrations provide significant information for coal pulverising plants which are operated under inert purging conditions: an increasing oxygen concentration value monitors the entrance of false air into the system and thus protects against the risk of explosion.

Situated in the process, an in-situ O₂ measuring sensor, using laser spectroscopy, provides immediate warning of pending problems. To monitor both CO and O₂, a remote sampling system is preferred.

Steam generator, boiler drum, combustion control, superheater

Power plant efficiency requires continuous monitoring and optimising of the combustion process. Supply of combustion air is a primary task because it delivers the required amount of oxygen, which must be optimised and controlled carefully to ensure safe and efficient combustion, to minimise fuel consumption as well as reduce the emission of pollutants like CO, CO₂ and NO_x. Reliable and accurate monitoring of primary combustion air volume flow at the boiler inlet is therefore a very important responsibility.

Due to the harsh conditions in this process, the O₂ is best measured with an in-situ zirconium dioxide sensor and the CO and CO₂ with a cross-duct in-situ gas filter correlation probe.

Flue gas denitrification (DeNOx plant)

Environmental regulations demand efficient reduction of the NO_x content of the flue gas before it is released into the atmosphere. The selective catalytic reduction (SCR) process is typically implemented using an added reagent such as ammonia (NH₃), which converts nitrogen oxide (NO) into water and nitrogen over a catalyst at approximately 400°C. NO concentration is measured at the inlet of the DeNOx plant to determine and control the required amount of ammonia. At the outlet of the DeNOx plant NO and NH₃ are measured: the NH₃ concentration (ammonia slip) indicates the efficiency of the denitrification process while the NO concentration is controlled to ensure compliance with the environmental regulations.

Typically, the NO_x content is monitored using UV spectroscopy technology as a direct measurement. The NH₃ concentrations can also be measured with this technology, but experience has shown that diode laser spectroscopy is the preferred in situ method.

Precipitator, bag-house, de-dusting

Flue gases from coal-fired combustion processes are loaded with particulate matter (PM) and, due to environmental regulations, must be cleaned before release into the atmosphere by passing a de-dusting device. Baghouse filters, where the particles are collected by passing through a tightly woven fabric, are common practice. Alternatively, electrostatic precipitators can be used, in which particles are deposited on electrodes when passed through an electric field. The correct operation of the dedusting plant and compliance of the residual dust content with the regulations is ensured by continuously monitoring the dust concentration after the filter.

Opacity is a measure of the dust concentration and the most cost-effective technology is cross duct transmittance measurement for medium to high concentrations.

Flue gas desulfurisation, scrubbers (FGD plant)

Wet scrubbing systems are typically used for flue gas desulfurisation. After leaving the dust removal plant, the flue gas enters a tower where it is sprayed with a calcium-based slurry (scrubbing liquid, eg, ground limestone in water) that is fed from a tank. The gaseous pollutants such as SO₂ are dissolved in the liquid and react with the liquid to form calcium sulfite or sulfate, which is removed by dewatering and settling into a thickener. Alternatively, calcium sulfite is oxidised to form gypsum by bubbling compressed air through the sulfite slurry.

SO₂ can also be measured using UV spectroscopy technology.

Stack, emission monitoring

Depending on the type of fuel and local environmental regulations, a number of gas components, predominantly CO, NO_x and SO₂, are to be monitored continuously in the flue gas at the stack along with dust (particulate matter, PM), gas flow, temperature and O₂ (frequently H₂O as well). In case of co-incineration of alternative fuels, additional components such as HF, HCl, Hg and VOC may be required to be monitored as well (emission monitoring in waste incineration). Measuring data are transferred to a specific data acquisition system for further processing and reporting to the authorities.

Since the stack is the final interface between the power station processes and the environment, these measurements are critical for avoiding penalties and ensuring optimum plant efficiency. It is important to quantify the volumetric flow rate to establish the amount of discharge. This is best achieved with a time-of-flight ultrasonic flowmeter. All other parameters to be measured have been monitored earlier in the process so the same technologies are suitable for these stack measurements.

Time to act

The thermal coal-fired power station has been used as an example of what can be measured and what benefits can be derived through investment in the most suitable probes and sensors. But the same strategy can be applied to all process and manufacturing industries since they all pollute the atmosphere to a certain degree, some significantly more than others. The pressure to ‘clean up’ will continue to increase and if it’s not measured it cannot be managed.

Now is the time to implement an emission pollution strategy sanctioned by top management.