Hands-on testing required
Comprehensive diagnostic testing is commonly used to supplement plant instrumentation monitoring because of the accuracy desired. For example, sampling representative ductwork cross-sectional areas is important to accurately measure combustion airflow, and fuel conduit traverses are needed to optimize pulverizer and burner performance. Fuel fineness also has a huge impact on the flow distribution across the fuel conduits, because there is a point where very fine fuel mixed with air behaves much like a homogenous gas (Figure 4).
4. The key is coal fineness. Poor coal fineness often yields poor distribution as large coal particles and air mix to form a two-phase mixture that will not homogenize (left). Good fineness creates a homogenous and balanced mixture that behaves more like a gas than a mixture of air and fuel. Source: Storm Technologies Inc.
Temperature, CO, oxygen, and NOx profiles were measured with an HVT (high-velocity thermocouple) traverse, providing an indication of imbalances in air and fuel originating in the burner belt zone. Economizer exit flue gas was also profiled in conjunction with the furnace exit HVT traverses.
The furnace may experience secondary combustion when fuel flow, airflow, or overall stoichiometry control is lost. This situation can result in reduced lower furnace heat absorption and a corresponding reduction in the life of the superheater and reheater pendant sections because of increased gas temperatures leaving the furnace. High furnace exit gas temperatures coupled with high sulfur (which is typically high in iron) can increase the potential for slagging and fouling of the furnace and radiant surfaces and can increase waterwall tube wastage.
Finally, a system of semiannual combustion airflow device calibrations and periodic air heater performance tests were instituted to optimize the performance of the steam generator. Continuous attention to the daily details of inspecting and maintenance of key components is a must for Unit 2 to remain at peak performance (Figure 5). SEC has those tasks well in hand.
5. Stick with the system. Typical maintenance activities required during an outage related to fuel-flexible boiler operation. Source: Storm Technologies Inc.
Reduce the slag
The SEC plant operations team strives for reliable operation with peak environmental performance. In fact, the OUC tagline is “The Reliable One,” reflecting the company’s emphasis on reliable plant operations. However, reliability comes at the cost of increased O&M expense when greater fuel flexibility is needed.
One of the most significant drawbacks of firing higher-sulfur coals containing high levels of iron is increased slagging, fouling, and fireside corrosion. The combination of staged combustion for low-NOx firing and operations with high-sulfur coals has long been a challenge for plant operators battling fireside corrosion and the thinning of boiler tubes (Figure 6).
6. Low-NOx burners have evolved. The complexity and difficulty of tuning low-NOx firing systems continues to increase. Source: Storm Technologies Inc.
Typically, using higher-sulfur coals correlates with increased iron in the ash. At SEC, sulfur content varies from 1.5% to 3%, with 20% ferric oxide. Operators have found that slag formation and the resulting reliability problems correlate well with these constituent levels. To reduce slagging and fouling to acceptable levels, the plant adopted the following operating requirements:
- Reduce the furnace exit gas temperature below ash fusion temperature to reduce the potential for slag formation.
- Combust coal in an oxidizing environment, since the ash fusion temperature is higher in an oxidizing environment and less char is formed with complete combustion.
- Avoid combustion in the superheater that forms slag due to secondary combustion and reduced ash fusion temperatures, as it is a reducing environment.
- Closely review the slagging characteristics of the purchased fuel. Coals with higher sulfur, iron, and lower ash fusion temperatures will produce more slag.
- Improve coal fineness to inherently improve fuel distribution.
- Improve fineness to reduce the formation of char, incompletely combusted carbon, and slag from ash in fuel. Doing so results in more complete combustion and less slag to form popcorn ash. (The popcorn ash increases in size as sulfates form, causing flyash to bind to the popcorn ash, which leads to SCR screen plugging and perhaps even air heater plugging.)
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