|The four-unit Guohua Ninghai power plant utilises complex and efficient tangentially corner-fired boilers|
To meet China’s growing need for electric power generation, the Guohua Ninghai power plant of Shanghai has a total planned operating capacity of 4 x 600 MW.
The plant is in the Ninghai township, Ningbo city, in Zhejiang province, and serves the region’s electricity needs.
The Ninghai plant relies on coal-fired steam-driven turbines to generate electricity, with six sets of HP-983 medium speed coal mills supporting a tangentially corner-fired boiler. This makes the plant’s large production operations complex.
Coal is first fed from silos at the plant to mill pulverizers, which grind the coal into a fine powder. A fan blends and forces a primary flow of mixed ambient ‘cold’ air and pre-heated ‘hot’ air into the coal mill.
This air is mixed with the coal powder creating a combustion air supply for the furnace heating water in the boiler. The boiler produces steam to drive power-generating turbines. The plant’s sophisticated, energy-efficient boiler technology requires accurate and responsive air flow measurement for its control system.
The boiler’s steam production efficiency is controlled by adjusting a precise flow of blended primary ambient air and pre-heated air which flow from separate ducts.
Each of the duct air flows is controlled by louvered dampers located inside the ducts. The primary air, which is a precise blend of cold and hot air, flows into the coal mills and afterwards to the boiler.
Accurate, responsive and reliable air flow measurement is critical to the automated control of the dampers and the efficiency of the boiler, which also ensures safe operation of the boiler and reduces plant operation energy costs.
The temperature of the pre-heated air fed to the boiler is 270°C (518°F) and the primary ambient air temperature is 30°C.
The size of the primary air duct is 1200 x 1200 mm (47×47 inches), the pre-heated air duct has the same dimensions (1200 x 1200 mm), and the ambient forced draft air duct is 500 x 800 mm.
The air flow to the coal mills and into the boiler is controlled by the plant’s distributed control system (DCS), which requires accurate and responsive air flow measurement data to control the dampers and keep the primary air flow to the coal mill entrance at precise and efficient levels. Figure 1 shows the primary air duct at the entrance to the coal mill.
|Figure 1: Configuration of primary air duct at the coal mill entrance|
Engineers from the China Electric Power Research Institute (CEPRI) and the Guohua Ninghai power plant researched several flow meter technologies for accuracy and responsive performance, as well as the overall suitability for the unique application conditions in their air feed lines.
Then they further considered existing installations and reference applications at other power plants in China. From these criteria they narrowed their choices to two technologies for further evaluation, thermal mass flow meters – also known as thermal dispersion – and averaging pitot tube flow meter (or differential pressure, dP).
Both of these technologies were further evaluated and compared with actual on-site installed performance testing. With the large duct sizes involved, the technologies were deployed in a multi-point design.
The engineers determined that the key criteria for the flow meter technology ultimately selected would be the one that was most accurate, responsive and impervious to large temperature fluctuations that allowed them to best control the air flow automatically via the dampers in the ducts using the plant’s DCS.
These application criteria can be difficult to achieve because of the large cross-sectional area of the air ducts and the temperature stratification in the primary air duct caused by the mixing of the pre-heated and ambient air.
For the flow measurement test programme, the two different types of flow meters were installed on coal mills C and D, which feed boiler 3.
|FCI MT86 Series flow meters|
Thermal mass flow meters with multiple sensing points were installed in each of the pre-heated and ambient air ducts. This installation eliminates the large temperature fluctuations that are present in the main primary air duct because of the mixing of the pre-heated and ambient air. It also provides a more precise and responsive damper control within the air ducts.
The averaging Pitot tube flow meter was installed in the main primary air duct downstream from the preheated and ambient air ducts.
The output readings from the two thermal mass flow meters were compared to the averaging Pitot tube flow meter and to the changes in the dampers.
Based on the result of the test programme, the thermal mass flow meters provided the best system control. The thermal mass flow meters on the duct for boiler 3 provided an accurate and responsive output of the air flow rate, which was consistent with the damper positioning. See Figure 2 for the air flow measurements recorded at mill D.
|Figure 2: Test data for the thermal mass flow meter|
Both the averaging pitot tube flow meter and the thermal mass flow meter tracked the damper positioning changes in this test and their air flow readings were within 1 per cent accuracy of each other. While the averaging pitot tube flow meter provided a similar accuracy and was responsive, its accuracy was adversely affected when the air flow pressure dropped.
The tests showed the flow rate output of the thermal mass flow meter matched the set position of the cold and hot air dampers, and the thermal mass flow meter provided the response time required of the automatic control feedback loop within the DCS.
The thermal mass flow meter’s responsiveness was enhanced by an anticipator algorithm programmed into the DCS. The DCS’ PID parameter proportion factor was adjusted from 0.3 to 0.4 and the integration factor was reduced from 300 to 150. The accuracy performance of the averaging pitot tube flow meter, however, was greatly degraded by low air pressure conditions and low air flows.
Due to the low pressure in the test the averaging pitot tube flow meter installed on mill C did not track well with the damper position changes. In addition, averaging pitot tube flow meters require significantly more maintenance because their sensor design includes small air inlets, which can clog in the fly ash laden air flow inside the ducts. A plus for the thermal mass flow meters is they do not have small air inlets nor any moving parts, so they impervious to the fly ash laden air flow conditions and do not require routine or preventative maintenance.
After completion of the test programme, Fluid Components International’s (FCI) Model MT86 Series multi-point thermal mass flow meters (see image on p.71) were selected for installation at the Guohua Ninghai power station.
After testing and evaluating both flow meter technologies, the engineers from CEPRI and the Guohua Ninghai power plant solved the challenge of finding the most accurate air flow measurement technology which would be responsive enough for automatic damper control of the mix of preheated and ambient air flowing into the coal mills.
The FCI thermal mass flow meters air flow accuracy and responsiveness provided the necessary flow data to automatically control the dampers and maximise the efficiency of Boiler 3. Their non-clogging sensors and rugged design ensured the lowest overall maintenance and longest service life.
Hu Yaqi is an automation & process control engineer at Zhejiang Guohua Zheneng Power Generation Company Limited in China and Steve Craig is senior engineer at Fluid Components International LLC (FCI) in the US. For more information, visit www.fluidcomponents.com.
The solution: MT Series flow meters
These precision air flow meters are installed and operating around the world in numerous electric power generation plants in similar applications because of their accurate, stable and reliable flow measurement over wide temperatures and variable pressures.
|Thermal dispersion principle of operation|
The FCI Model MT86 Series meters can be supplied with up to eight discrete thermal mass flow sensors.
These sensors can be installed along a single probe assembly or on multiple probes, whatever is optimal for the application and installation conditions.
An MT Series flow meter is ideal for use in both thermal and nuclear power generation applications, as well as in other heavy process or manufacturing industries with large diameter pipes or large duct applications. It provides very high air and gas flow measurement performance in applications such as these coal-fired combustion air systems, as well as in HVAC systems, flues, and stacks.
MT Series flow meters are highly versatile, with a wide turndown range available from 5:1 to 1000:1 and flow sensitivity from 0.25 SFPS (0.08 NMPS) to 150 SFPS. With its smart digital flow transmitter and advanced thermal dispersion flow sensing element(s), the MT86 Series, and its sister product MT91, meet a wide range of environmental monitoring requirements worldwide, including CEMS and QAL1.
They have a no-moving parts design with no orifices to plug or foul, FCI’s thermal mass flow sensing technology places two thermowell protected platinum RTD temperature sensors in the process stream. One RTD is heated while the other senses the actual process temperature. The temperature differential between these two sensors is measured and is directly proportional to the mass flow rate of the fluid. The sensor outputs are fully temperature compensated resulting in high accuracy and repeatability in changing media and ambient installation conditions. Outputs include multiple 4-20 mA and relays and units can be powered by either AC mains (85-265 Vac) or 24 Vdc.
Power Engineering International Archives
View Power Generation Articles on PennEnergy.com