Abstract
The keys to reliable and efficient coal-fired power plant operations are combustion and performance monitoring. For this to occur, plant staff needs to have adequate tools that help them make intelligent decisions. Success in this project is ensured with proper instrumentation and vigilant operations. This paper looks at efforts made toward combustion improvements through the use of pulverizer coal flow enhancements, coal flow monitoring instrumentation and simple coal analysis trending.
Introduction
Boardman is a 610 MWe unit burning Powder River Basin coals. The steam generator was provided by Foster Wheeler, as well as the 2 MB type and 6 MBF type pulverizers. Results from routine isokinetic in situ coal pipe sampling demonstrated an unequal distribution of coal flow in all of its pulverizers. It was not uncommon for coal flow distribution to be in excess of 30 percent of the average coal flow. With this data in hand, the plant embarked on efforts to improve coal flow and subsequently, coal combustion. Further complicating
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Coal flow distribution
The plant periodically conducts coal fineness testing. This testing involves the painstaking process of obtaining isokinetic in situ coal pipe samples. The results from each test provide coal fineness, coal pipe velocity and coal pipe mass flow distribution. Figure 1 shows a typical coal mass flow distribution, but variations greater than ±10 percent were not uncommon. Typical coal fineness readings were 70-80 percent passing a 200 mesh screen, with less than 1 percent remaining on a 50 mesh screen. It was felt that these high variations in coal pipe flow were largely responsible for the slagging that was occurring in the boiler. Routine pulverizer maintenance activities also took note of the scouring patterns inside the pulverizer, which indicated roping. What is obvious from these results is that coal flow through a pulverizer is anything but homogeneous.
The plant’s first attempt to improve coal flow distribution was to weld plates to the top of the pulverizer between coal pipe outlets. The intent of this modification was to break up the coal flow, creating homogenous coal pipe flow. This modification provided little-to-no value.
Plant improvements are of no value without verification. Compounding the effort of improvements and verification is that of feedback. Isokinetic in situ coal fineness testing is absolutely necessary; results, however, are at best several hours after the fact. Immediate feedback is necessary to determine the effectiveness of pulverizer adjustments. Real-time coal flow monitoring is required.
To meet this need, the plant selected the Air Monitoring Corporation to provide real-time coal flow monitoring instrumentation. Air Monitoring uses microwave technology to determine the coal pipe mass flow and coal pipe velocity.
Figure 2 shows a pulverizer’s mass coal flow distribution taken with the installed real-time coal flow instrumentation. The graph represents each coal pipe’s flow deviation from the average of the 4 mass flow signals from each coal pipe exiting the pulverizer. Experience has shown the most meaningful displays are those showing mass and velocity deviations. Additionally, the operators use the coal pipe velocity to adjust pulverizer air flow to acceptable levels.
Being able to monitor coal conduit flow on a real-time basis allows the plant to make and immediately assess changes to the pulverizers. Figure 2 also shows the effect of a change made by the plant staff in pulverizer operation.
In this instance, the plant lowered primary air flow to the pulverizer by approximately 10 percent.
Plant staff anticipated that lowering pulverizer primary flow would improve pulverizer performance. Lowering the air flow reduces coal particle velocity, which would force an improvement in the pulverizer classification process and improve coal fineness. With improved coal fineness, a change in coal mass flow distribution was expected.
