The emission control norms notified by the Ministry of Environment, Forest and Climate Change in December 2015 have narrowed the particulate matter (PM) emission limits for coal-based power plants considerably. As against the earlier range of 150-350 (mg/Nm3), the new PM emission limits range between 30 and 100 milligrams per Newton cubic metre (mg/Nm3)depending upon the year of commissioning of thermal power projects (TPPs). For projects commissioned before 2003, the PM limit is 100 mg/Nm3, and for those commissioned between 2003 and 2016, the limit is 50 mg/Nm3 while for TPPs commissioned in 2017 and beyond, the limit is 30 mg/Nm3. Such stringent limits call for the implementation of advanced PM control technologies and/or upgradation of existing technologies. As per the Central Electricity Authority (CEA), 220 units are required to be compliant with the new norms aggregating 63 GW by installation/upgradation of electrostatic precipitators (ESPs) by 2022.
PM emissions can be controlled pre-combustion, in-combustion, and post-combustion in coal-based power plants. Pre-combustion control can be achieved by selecting the right type of coal and by washing the coal while in-combustion control is carried out by optimising combustion and by injecting sorbents into the flame zone. There are various methods for post-combustion control of PM2.5 emissions, the most common being the installation of ESP and fabric or baghouse filters. Advancements in PM control technologies are taking place to introduce newer systems and improvise on the existing ones. Some of the new methods for PM control include flue gas conditioning, wet ESPs, agglomeration, hybrid systems and multi-pollutant control systems. Power Line presents an overview of the various PM control technologies…
Selecting a suitable coal type and washing and/or blending it with other coal types can aid in reducing PM emissions to an extent. Indian coal has a low share of sulphur (.5 per cent) and higher ash content (30-40 per cent) as compared to imported coal. TPPs based on domestic coal, therefore, have higher PM emission factors and lower SOx emission factors. Meanwhile, imported coal from Indonesia and South Africa have a higher sulphurcontent (>2 per cent) than domestic coal, and thus has higher SOx emission factors. High coal ash and sulphur content are related to high values of PM2.5/PM10 ratios. Some of the pre-coal combustion technologies that can be adopted are coal washing and blending. The usage of washed coal instead of raw coal can reduce PM emissions in flue gas by 30 per cent. Also, the particle size of coal significantly impacts PM emission as finer coal particles lead to the formation of finer PM, which is more difficult to capture. Therefore, it is important to pulverise coal to the appropriate fineness level.
Control of PM emission can be undertaken during the coal combustion process by optimising the burning time and combustion temperature and/or by injecting sorbents into the furnace. It has been observed that the emission of PM increases with increasing temperature. Also, the longer the burning time, the finer is the PM produced, which is difficult to capture.
Besides, the addition of solvents such as kaolinite, limestone, lime, silica and alumina into the flame zone helps to reduce the formation of fine PM.
ESPs have been in use by the utility industry since the 1920s to control PM emission. Other conventional systems such as cyclonic collectors, wet scrubbers and baghouse filters are also used but they have a low collection efficiency for finer PM (PM2.5).
An ESP is a device that electrostatically separates particles from the flue gas stream while imposing minimal pressure loss on the stream. Unlike PM removal devices such as cyclonic collectors, scrubbers and baghouse filters, high gas-stream pressure loss and associated high draught fan energy consumption is avoided when using an ESP. Most of the country’s power plants have already installed ESPs. However, with the narrowing of PM emission norms, there are concerns regarding the efficacy and operational performance of the existing ESPs.
Fabric filters (FFs) remove PM from the flue gas stream by cloth filtration. The dirty flue gas passes directly through FF bags, which then separate the PM from the flue gas stream. Filter bags are required to be periodically cleaned of their collected PM by methods such as shaking, clean gas backwash, and high pressure pulsing. Baghouse filters have a high collection efficiency, 99.9 to 99.99 per cent over a broad range of particle sizes, and ~99.7 per cent for PM2.5.
As per the International Energy Agency’s (IEA) Clean Coal Centre report titled “Emission Standards and Control of PM2.5 from coal-fired power plants” published in 2016, there has been a trend globally to replace ESPs with FFs in recent years. In the US, most ESPs are beingtaken out of service, and replaced with FFs. Indian power plants are also looking at replacing their existing ESPs with FFs.
Advances in PM control technology
A key issue with both ESPs and FFs is that a reduction in collection efficiency occurs as finer and finer particles are filtered. The reduction in efficiency is steeper in ESPs than FFs. The IEA report outlines various advances that have taken place in PM control technologies to overcome issues such as design changes in ESPs and FFs as well as the advent of new PM control systems with higher collection efficiency.
In recent times, ESP design, however, has undergone a facelift. Developments such as customised rigid discharge electrodes, high frequency power supplies and micromanaged ESP gas velocity distribution have advanced the technology. Other advancements in ESPs include pulse energisation wherein a high voltage pulse is superimposed on the base voltage to enhance ESP performance during operation under high resistivity conditions. Intermittent energisation is where the voltage to the ESP is turned off during selected periods, allowing a longer period between each energisation cycle and limiting the potential for back corona.
A wet ESP (WESP) system that washes the collecting electrode with liquid rather than mechanically rapping the collection plates has better collection efficiency. These systems are commercially available and have been in service for over 30 years to control sulphuric acid andparticulate emissions. Fine particulates are removed more effectively in a WESP because of the humidity in the flue gas stream.
In the case of FFs, steps have been taken to increase the number of filters and their depth in order to enlarge the filter in the same-sized space. New filter materials are being developed that use nano fibre technology and membrane-type fibres like polytetrafluoroethylene as against traditional materials such as glass, cellulose, and synthetic and polymer fibres. Also, electrostatically aided FFs that deal with low collection efficiency by providing an electrostatic charger before the FF are not available. By charging incoming particles using a corona discharge, the collection efficiency of FFs can be increased, particularly for particles in the sub-micron-size range.
Also, conditioning of flue gas with chemical agents can be carried out to reduce the resistivity of PM particles while increasing the collection efficiency of ESPs. Some of the commonly used agents are ammonia and its compounds, and sulphur and its compounds.
Further, various chemical and physical techniques can be used for agglomeration that can bind fine particulates into larger ones, thus making their collection by ESPs or FFs easier. Some agglomeration techniques are acoustic, electrostatic, magnetic, wet and thermophoretic agglomeration.
These days, hybrid PM control systems and multi-pollutant control systems are also being adopted by utilities. The hybrid systems are a combination of electrostatic precipitation with fabric filtration to benefit from the advantages of both technologies. There are several hybrid systems that have been developed, namely, the advanced hybrid collector, compact hybrid particulate collector (COHPAC), electrostatically stimulated FF-Max-9™, multi stage collector (MSC), electrostatic fabricintegrated collector (EFIC) and ESP-FF hybrid system (EFF). Of these, COHPAC, EFIC and EFF arecommercially available.
Meanwhile, multi-pollutant control systems remove two or more pollutants in a single reactor or a single system designed for the purpose. These systems have the benefit of lower capital and operating costs than a series of traditional systems to remove the same number of pollutants.
Utilities have a range of technology options to choose from for controlling PM emissions. Many of these systems may involve a moderate to high increase in capital and operating costs but the benefits in terms of better compliance with environmental norms and an improvement in the ambient air quality would outweigh the costs in the long term.