In Poland, nearly all the electricity is produced by burning coal, with most power plants burning either hard coal or lignite. To address environmental concerns, Poland’s electric power industry is in the process of modernizing and refurbishing its generating plants with advanced fuel-burning technologies.
Joachim Seeber, Frank Kluger, Alstom Power Boiler, Stuttgart, Germany,
Stanislaw Krupka, Rafako S.A., Raciborz, Poland, and Ireneusz Lalak, EC Zeran, Grupa Vattenfall, Poland
At the Zeran heat and power plant in Warsaw, two 450t/h circulating fluid bed (CFB) boilers are cleanly burning hard coals supplied by mines in Silesia. Both boilers were built by Rafako Boiler Engineering Co., Raciborz, Poland, under an Alstom license. Zeran A was commissioned in 1995 and was, at that time, Poland’s largest CFB boiler and the first to burn Polish bituminous coal. The Zeran B commissioning followed in 2001. Since most of the plant’s coal is delivered untreated, it often contains significant amounts of hard rock and chlorine. Due to high rock content in the fuel ash, it was difficult to achieve the proper particle size distribution of the circulating ash in the first unit ” Zeran A. This in turn resulted in high furnace temperatures ” which negatively impacted the unit’s nitrogen oxide (NOx) and sulphur dioxide (SO2) emissions ” and a loss of fine inert material, especially during full load.
As a result, when the order was placed for the second unit, Zeran B, the design was altered to include high efficiency cyclones that improved the boiler’s internal circulation by increasing the fineness of the circulating particles. Compared to Zeran A, the heat transfer within the Zeran B furnace improved significantly and the temperature profile is more even. This, in turn, reduced the unit’s NOx, SO2, carbon monoxide (CO) and particulate emissions, as well as its limestone consumption. The unit’s parasitic power consumption was reduced as well. The following is a detailed look at the Zeran B high efficiency cyclone design and the operating results it has achieved.
Both Zeran units fire Polish hard coal, supplied by different mines in Silesia, with a heating value of 18 MJ/kg to 24 MJ/kg (LHV) and an ash content of 14 to 25 per cent. As discussed, the high rock content of the fuel ash adversely affected Zeran A operations. Using a bed ash mill to reduce the particle size, which has provided positive results at other plants, was deemed unsuitable due to fears of creating erosion problems within the boiler. Instead, Zeran A was retrofitted with eccentric vortex finders in 1997, which led to an improvement in cyclone efficiency.
Figure 1. The Zeran heat and power plant in Warsaw, Poland
When the order for Zeran B was placed in 1998, it was nearly identical to Zeran A, except for the cyclone design. The intention was to improve the cyclone’s separation efficiency by combining the following features:
- Optimized arrangement of cyclone inlet ducts with respect to the furnace
- Prolongation of the cyclone inlet ducts
- Downward declination of the cyclone inlet ducts
- Decrease in cyclone vertical velocity
- Installation of the eccentric vortex finders.
Zeran B began operations in September 2001 and the effects of the improved cyclone design, including higher heat transfer within the furnace and the lower NOx emissions, were readily apparent right from the start. For example, the average particle size (d50) of the circulating material, which was in the range of 180 à‚µm for Zeran A, was reduced to less than 80 à‚µm for Zeran B. The two boilers were subsequently compared using performance test data taken from Zeran A in December 1997, and Zeran B in December 2001. While more direct comparisons are currently underway (under absolute, equivalent conditions) the advantages of the improved cyclone design can already be quantified, including:
- Significantly reduced limestone consumption
- Lower NOx and CO emissions
- Improved heat transfer in the furnace
- Reduced power consumption (due to reduced primary air, minimized flue gas recirculation, and decreased furnace pressure).
Improved cyclone design
Designed for a steam output of 450 t/h, Zeran A and B are natural circulation drum type boilers comprised of a fully water cooled rectangular suspended furnace with the upper portion divided by a partition wall. The design includes two cyclones, a cooled backpass with convective superheaters, an economizer and a tubular air preheater. Platen superheater panels in the upper furnace balance the radiation and convection characteristics of the superheaters. These platen superheaters, manufactured from double omega tubes, feature a design proven on numerous Alstom CFB boilers.
The first step to improving the Zeran B separation efficiency involved changing the cyclone inlet duct arrangement to optimize the flow into each cyclone. While the Zeran A cyclone inlet duct uses a spiral design, Zeran B uses a tangential design. The Zeran B inlet duct is also longer and arranged to prevent solids from approaching the inner side of the duct. Further separation improvement was achieved by the declination of the cyclone inlet ducts downwards and by a dislocation of the vortex finder from the centre.
The Zeran B cyclones are also steam cooled (a special customer request) to reduce the unit’s start-up time. The walls of the cyclones are arranged to provide first stage superheat duty. The tubing protected with an aluminium-based refractory, which results in total heat absorption of approximately 14 per cent of the unit’s superheat duty. As a result, the platen omega panels were reduced in size for Zeran B to balance the overall heat absorption in the furnace.
To demonstrate the effectiveness of the new cyclone design, Zeran A and B were compared using full load operating data taken in 1997 and 2001, respectively. Table 1 shows the design data for both units. Table 2 illustrates the minor differences in the fuels. The ballast content amounted to 30.5 per cent in Zeran A, while it was 27.2 per cent Zeran B. Due to this small difference, the fuels’ heating value differed slightly.
The effect of the improved cyclone separation efficiency can be clearly identified due to the finer particles in the filter ash captured in the unit’s electrostatic precipitator. The improved separation also influences the particle size distribution in the bottom ash and the circulating ash. The mean particle size (d50) of the circulating inert material was shifted from 180 à‚µm to approximately 80 à‚µm in Zeran B. The improved cyclone efficiency also reduces the share of the bottom and filter ash mass flows. In Zeran A, filter ash represents more than 70 per cent of the total ash mass flow. In Zeran B, the filter ash flow represents less than 60 per cent of the total ash mass flow. Therefore, the share of the total ash extracted by the bottom ash extracting system is higher in Zeran B than in Zeran A.
Figure 2. Cyclone arrangement of Zeran A
Due to the finer solid particle circulation for Zeran B, the overall heat transfer coefficient of the furnace and the superheater panels was increased by approximately ten per cent, meeting cyclone heat transfer predictions. The new cyclone design also improved other key operating parameters, including:
Air distribution and flue gas recirculation: In Zeran A, the primary air amounted to 55 per cent of the total air needed to fluidize the coarse circulating ash. In Zeran B, only 35 per cent primary air was required, thereby reducing primary air fan power consumption. Injecting less primary air through the Zeran B primary air nozzle grid also resulted in a more effective air staging process. That, in turn, significantly decreased the unit’s NOx emissions. In addition, the total excess air from Zeran A and B is nearly identical. However, due to the improved heat transfer in the furnace, the flue gas recirculation in Zeran B was decreased by about five per cent.
Figure 3. Cyclone arrangement of Zeran B
Power consumption: Due to a lower bed pressure in combination with the shift in air staging, the Zeran B primary air fan consumed less power. In addition, the flue gas recirculation and fly ash reinjection used on Zeran A was not needed on Zeran B. Thus, the power consumption of Zeran B is more than five per cent lower.
Furnace temperature: The cyclone design increased the heat transfer for Zeran B by about ten per cent, which directly impacted the temperature profile along the flue gas path. The profiles are the mean temperatures measured by the operational measuring points installed at the furnace walls. The temperatures measured at full load in Zeran B are approximately 30à‚ºC (86à‚ºF) lower than the temperatures measured in Zeran A.
Desulphurization: Fuel and limestone properties, as well as combustion conditions, influence a CFB’s desulphurization efficiency. As discussed, the fuel and limestone properties were nearly identical in both units. However, while both operated at SO2 levels of approximately 200 mg/Nm3, the Ca/S molar ratio was much lower for Zeran B because its furnace temperature is closer to the temperature needed to maximize limestone reactivity and sulphur capture. In addition, the cyclones’ improved separation efficiency resulted in smaller circulating ash particles, a greater desulphurization area, and longer particle residence time.
Limestone consumption: Lower combustion temperatures, higher bed material residence times, and a higher particle surface area reduced Zeran B limestone consumption by about 40 per cent. This, in turn, reduced the unit’s ash production.
Carbon burnout: It was expected that the unburned carbon heat loss would decrease by increasing the separation efficiency. But due to the substantial improvements in other operational parameters, such as a lower furnace temperature, lower primary air requirements, and the fact that the fly ash re-injection system installed in Zeran A is not needed in Zeran B. The expected effect on unburned carbon heat loss was difficult to ascertain. The total organic carbon in the filter ash was approximately nine per cent and 1.8 per cent in the bottom ash for both boilers.
NOx emissions: NOx formation and destruction depends on fuel characteristics and combustion conditions. Since the fuels were nearly identical in both units, the influence of nitrogen content, nitrogen binding form and volatile matter was negligible. However, NOx emissions decreased from 200 mg/Nm3 in Zeran A to approximately 60 mg/Nm3 in Zeran B due to improved combustion conditions brought on by the new cyclone design. These conditions included a lower furnace temperature, enhanced fuel/air mixing, and more pronounced air staging.
Figure 4. Comparison of the emissions from the Zeran A and B power plants in Poland
Carbon monoxide emissions: Carbon monoxide emissions were reduced due to the finer particle size of the bed material. Decreasing the particle size improved the combustibles/oxygen mixing and increased the density of the suspended fuel, especially in the upper furnace.
The Zeran A and B units provide a unique opportunity to evaluate the impact of cyclone efficiency on CFB boiler performance. Increasing the Zeran B cyclone efficiency led to a remarkable increase in heat transfer, while significantly reducing limestone and parasitic power consumption, as well as NOx and CO emissions. All these improvements occurred because the mean particle size of the inert material was reduced from 180 à‚µm to 80 à‚µm. The results indicate that the additional investment costs for the high efficiency cyclones will pay off in a relatively short period of time. Further, investigations are now underway to quantify the ability of the smaller particle size to reduce furnace erosion ” which has been quantified on other, smaller units using a similar cyclone design.