The current installed capacity of coal-fired power plants in India is greater than 60 per cent. The private sector has installed nearly 38 per cent of the total thermal power capacity. The installed renewable energy capacity is reaching over 20 per cent, and the resultant flat or low load demand has caused the operations of many baseload coal-fired power plants to become cyclical in nature. Coal-fired power plants have adapted to the requirements of frequent unit shutdowns, higher ramp rates, cyclic operations, increased lay-ups, and prolonged periods of low load operation.
To meet the changing market conditions brought about by increasing renewable energy sources, coal-fired power plants need to improve their flexibility and operating practices specifically to operate at less than 40-45 per cent turbine maximum continuous rating (TMCR) load for short periods. Power generating
companies must identify and prioritise opportunities to improve coal-fired units based on their expected returns, and the impact of operating at low loads on their business objectives.
Even though thermal power plant load factor (PLF) has dropped by 2 per cent compared to last year and is expected to drop further this year due to Covid-19, a recent report indicates that large thermal power plants can provide a competitive advantage, making existing units more flexible and safer during low load operation.
The fundamental question is: what improvements can make coal power plants even more competitive in today’s energy markets in terms of despatch, capacity, availability and emissions?
Enabling load flexibility of coal-based, baseload generating units in order to accommodate renewable energy is a major challenge in India, where power generation predominantly comes from coal.
Currently, load despatch centres are looking for a minimum MW output level that is achievable without needing costly support fuel (oil or natural gas) or a steam turbine bypass system, without compromising on the safety and reliability of the plant equipment, and with low emissions. Low load operation may not be ideal in terms of efficiency and performance.
The following key areas should be addressed by power plants:
- Operating power plants at 40-45 per cent minimum load for short durations
- Ramping up/down or cycling a unit from minimum load to full load
- Plant reliability and stability
- Flexible operation with multiple units
- Thorough analysis of coal characteristics
- Plant equipment operating practices and optimisation
- Controls tuning for flame stability during low load operation
- Reviewing fuel igniters, flame scanners, mill combinations, automatic generation control, and modes of operation
- Coal additives for combustion support and minimising emissions
- Use of plasma burners at low load for flame stability as well as reducing support oil consumption.
Operating power plants at 40-45 per cent load
Experience indicates that coal quality plays a major role in operating power plants at 40-50 per cent load without oil support. Optimising coal quantity in concert with power demands can lower costs and emissions, and generate optimum margins.
To operate at low load, a power plant should analyse coal characteristics, tune the boiler parameters, perform boiler optimisation, test the soot blower operation, and make minor control changes, including control logic modification. The following areas should be covered:
- Analysis of coal characteristics such as fixed carbon (FC), volatile matter (VM), gross calorific value, ash softening temperature (ST), fluid temperature (FT), and Hardgrove Grindability Index (HGI).
- Minimising coal costs by routinely blending different varieties of coal with similar characteristics.
- Assessing the number of mills and their parameters.
- Properly fine-tuning key boiler parameters.
- Maintaining water chemistry during cycling/load stabilisation.
- Using coal additives for combustion support and to minimise emissions.
VM comprises hydrocarbons and other gases such as carbon dioxide and nitrogen found in coal. In other words, VM is an index of the gaseous material present in the coal. The typical range of volatile matter for Indian coal is 16-30 per cent.
VM influences flame stability, secondary air requirement, reactivity, secondary oil support, and the minimum ignition temperature of the coal; supports easier ignition of coal particles; and decides the fineness of the unburned carbon in fly ash. Coal with VM less than 16 per cent may be difficult to burn, and is considered to have low volatility.
FC is the combustible matter left behind in the furnace after the VM is distilled away. It consists mostly of carbon, which burns slower than VM. FC gives a rough estimate of the heating value of the coal and dictates coal fineness and mill outlet temperatures.
Hardgrove Grindability Index
HGI measures the grindability of coal. Being an index, it has no unit, simply a number. The harder the coal, the lower the index number. Conversely, the higher the HGI, the softer and more grindable the coal and the higher the mill loading, as shown in Graph 1.
Ash softening/fluid temperature
The fusibility of coal ash is analysed by heating a pyramid-shaped ash sample, 19 mm in height and 6 mm in base width, in a gas/oil/electric furnace in a controlled atmosphere (reducing or oxidising). Various temperatures are thus obtained, as shown in Fig. 1. ST is one of the key parameters for boiler operation, deciding the slagging and fouling characteristics of the boiler.
Fixed carbon/volatile matter ratio
The FC/VM ratio is a key parameter that plays an important role in deciding boiler stability, including enabling low load operation with fewer problems. Coal quality can be determined from proximate analysis. If the FC/VM ratio ranges between 1.0 and 1.3, the coal is sub-bituminous, supporting low-load operation (VM >22 per cent). If the ratio ranges between 1.4 and 1.5, the coal will have difficulty in supporting low load operation (VM < 20 per cent).
The FC/VM ratio also influences flame stability and emission of nitrogen oxides (NOx). Mill outlet temperature influences everything from low load to full load operation with proper fineness, as shown in Graph 2. The primary airflow/coal flow ratio is another critical parameter for slagging and low load to full load operation, as displayed in Graph 3.
To operate at 45 per cent load, three mills should be in service and 500 MW units are recommended. Coal quality and heating value affect mill loading. Mill outlet temperature should be increased slightly for better combustion and flame intensity and stability, as shown in Graph 2.
Boiler key parameters for low load operation
For 45 per cent TMCR load operation, the following parameters are recommended:
- The FC/VM ratio should be analysed, and mill outlet temperature, primary air/coal ratio, coal pipe velocity, NOx emission trends and furnace-to-wind box DP should all be addressed.
- Given the less than 0.4 per cent sulphur content of typical Indian coal (as received basis), a minimum APH cold end temperature of 100 °C (uncorr) should be expected, which is sufficient for cold end due point corrosion.
- Superheater (SH) outlet steam temperature fluctuations should be restricted within 10 ºC, and those for reheater (RH) outlets within 15 ºC. A small quantity of spray may be expected.
- A minimum of three mills should be operational with the required throughput.
- The unit should be operated in sliding pressure mode to achieve proper low load stability.
- The mill inlet PA temperature should be 270-280 ºC with 8-15 per cent moisture coal.
- Control logic should be modified to be capable of 45 per cent to 100 per cent TMCR load without oil support.
- Operating ranges from 45 per cent to 100 per cent TMCR for the load ramp rates also need to be established.
- Oil igniters should be in operating condition.
- Oxygen should be greater than 5.5 per cent at 45 per cent load operation.
- The SA/PA ratio should be greater than 1.6 at minimum. The ratio depends on coal quality, corner to corner coal flow, coal/air damper operation, coal nozzle conditions, and pressure drop, which should all be at acceptable levels.
- During low load operation, the condenser back pressure can be high, leading to high exit velocity of the steam. This phenomenon leads to LP turbine blade erosion.
- SH/RH metal temperatures should be monitored for the entire period of low load operation.
Power plant operating modifications
A steam turbine/generator service provider can modify several areas on the steam turbine side, such as combining the diaphragm with rotating brush seals; installing new blades and tip seals, new rotors, and a row of new blades with conversion to full arc; or all of the above, with a new inner casing and several additional rows of blading. Original equipment manufacturers ( OEMs) should relax operational restrictions such as pre-warming criteria. On the boiler side, an OEM can identify dozens of potential improvements including economiser upgrades, furnace wall corrosion reduction upgrades, and super-heater material upgrades to improve fuel ash corrosion resistance, restore full load capability and improve availability. An OEM can also recommend strategies for coal switching and low load operation.
Coal blending to reduce coal costs
The objective of coal blending is to incur economic benefits. Blending or separately firing two different varieties of coal results in a change in the aggregate quality of coal to be fired. The main characteristics of coal that affect boiler performance are the FC/VM ratio, the gross calorific value (GCV), HGI and ash content. Indian power boilers are designed for high ash coal, and the GCV of the coal used therein is typically around 3,000-3,600 kCal per kg.
Blending or separately firing Indian coal with imported coal or coal from different Indian mines or seams requires them to have similar FC/VM ratios, HGI and ash characteristics, including ash softening and ash fusion temperatures. A GCV variation of about 500 kCal per kg is acceptable.
Low load ramp rate
Low or minimum load is defined as the lowest possible load a generating unit can deliver under stable operating conditions without costly oil support. A controlled circulation boiler permits rapid start-up, shutdown and cyclic operation.
The ramp rate indicates how fast a power plant’s load can be changed during operation. The ramp rate is defined in MW per minute (typically 3-5 MW per minute), or in the percentage of rated load per minute (say, 1 per cent per minute). In general, recommended ramp rates greatly depend on the type of the unit: 3 MW per minute for subcritical boilers with natural circulation, and 5 MW per minute for controlled circulation units.
Load stabilisation and cycling
Stabilising and optimising a power plant during low load operation is necessary for maintaining the economic viability of the plant, otherwise, the variable cost will increase, which is uneconomical for power generation. The costs of load cycling and cold, warm and hot start-ups are indicated in Figure 3. Operating at low load is more beneficial than ending up in cold start-up to minimise equipment damage.
The Electric Power Research Institute is continuously investigating large-scale coal-fired units to achieve low load operation. The process control strategies should improve the stability and efficiency of plants during low load operation. The overfire air ratio should be brought to a minimum for bumpless variations and to avoid instability during cycling to low load. Theoretically, coal-fired units should be operational without oil support above 35 per cent load.
Due to renewable energy penetration, base load coal-fired power plants are moving to cyclic operations, running at minimum loads for extended periods when the load demand is low and to maintain state load despatch centre merit orders. Mill parameters also play a major role in achieving successful operation. Alongside boiler key parameters, process control logic, flame scanners, and plasma burners should support flame stability to establish sustainable low load sliding pressure operation.
Tuning of parameters such as PA/coal ratio, coal pipe velocity, mill outlet temperature, coal fineness, SA/PA ratio, and coal oxygen/nitrogen ratio, and operational changes based on accurate data are essential for cycling to low-load operation. In conclusion, coal with an FC/VM ratio less than 1.3, VM greater than 22 per cent, and GCV greater than 3,000 kCal per kg is required for low load operation at 45 per cent load condition.
Since a unit is going to be operational at low load with high excess air, there is no need to be concerned about accumulation of unburned carbon and boiler explosion. Emissions such as sulphur dioxide and NOx are not an issue during low load operation.
Sundara Kavidass, MD, SP Energy Tek, USA and India