Apart from SOx and NOx, thermal power plants release other emissions such as particulate matter (PM) and mercury. Given that the new emission standards, released by the Ministry of Environment, Forest and Climate Change (MoEFCC) in December 2015, call for limiting other emissions, a number of compliance solutions such as ESPs, fabric filters and mercury removal technologies are being planned for deployment by the thermal power generation industry. A closer look at the features of some of these technologies…
Electrostatic precipitators (ESPs) are the most commonly used particulate emission control equipment in coal-based power generation plants. An ESP electrically charges the ash particles in the flue gas stream to collect and remove the PM. It comprises a series of parallel vertical plates that contain electrodes to create the electric field through which flue gas passes and particles are separated. A well-designed ESP is characterised by high collection efficiency, high reliability, low flue gas pressure loss, resistance to moisture and temperature changes, and low maintenance. Its collection efficiency is very high at 99-99.99 per cent for a particle range of 0.01-100 micrometre; however, it does not work well for fly ash with high electrical resistivity. Since Indian coal produces high resistivity fly ash due to the burning of low sulphur fuels, it is difficult to precipitate and this often limits the collection efficiency of ESPs. In this context, the conditioning of fly ash in flue gas is an established technique that is used to restore the performance of ESPs in coal-based power plants. Most of the country’s power plants have already installed ESPs. However, with stricter PM emission norms, there are concerns regarding the efficiency and operational performance of the existing ESPs. Feasible engineering solutions for ESP augmentation are available for meeting the emission standards. For instance, increasing the collection area by the addition of pass in parallel and increasing the height of the existing ESPs by a maximum of 15 metres could be done. Another option is the use of of a moving electrode electrostatic precipitator (MEEP). An MEEP consists of movable collecting plates and rotating brushes, which enable superior collection of highly resistive dust such as coal ash and sintered ash.
Fabric filters (FFs) are very efficient particulate collectors in which dust cake is formed on the surface of bags and is removed by various techniques. The fabric provides a surface on which the dust collects through the four mechanisms of inertial collection, interception, brownian movement and electrostatic forces. A combination of these mechanisms results in the formation of the dust cake on the filter, which eventually increases the resistance to gas flow. The filter cleaning is done through mechanical shaker baghouse, reverse air baghouse or pulse jet baghouse. Baghouse, filters have a high collection efficiency, 99.9 to 99.99 per cent over a broad range of particle sizes, and around 99.7 per cent for PM 2.5. Flue gas conditioning is also implemented in fabric filters, enabling lower emissions. The commonly used conditioning agents are elemental sulphur, ammonia and sulphur trioxide. In recent times, FFs have undergone facelifts, and 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 nanofibre 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 are not available. By charging the incoming particles using a corona discharge, collection efficiency can be increased, especially for particles in the sub-micron-size range.
The primary aim of mercury control is to oxidise all metallic mercury to ionic mercury so that it can be removed from flue gas desulphurisation (FGD) systems. The basis of the control process is the oxidation of mercury, and then its removal from downstream equipment before it is emitted through the stack into the atmosphere. The most common technology involves the injection of activated carbon into the plant’s exhaust stream. Mercury can be removed through chemical adsorption on powdered activated carbon (PAC). Activated carbon is injected upstream of an ESP or FF and is removed along with fly ash. The main drawback of the PAC system is its potentially adverse impact on ESP performance at the time of particulate collection. A new market entrant, the injection of amended silicates, can potentially negate both the increased SO3 concentration in flue gas and the adverse effect on ESP performance. However, the long-term viability of amended silicate has not yet been demonstrated. Another widely used technology is halogen injection into coal using calcium bromide or sodium iodide. It is an inexpensive process and results in very high mercury oxidation. The only catch is that for halogen addition to work as mercury control technology, some sort of FGD system has to be in place. Oxidised mercury is very soluble and can be taken out from FGD systems.
In sum, a number of technologies for complying with the emission standards are available, depending on the age of the plant, the type of coal, etc. Different solutions can be implemented in combination to achieve emission control targets.