Pulverizer performance optimization is the first step to a successful combustion optimization program and the inter-relationships of the pulverizers must be considered when attempting to optimize combustion, overall unit performance, operability, reliability, and capacity. Pulverizer capacity seems to be an industry challenge while many units today are undergoing drastic fuel changes. Considering there seems to be a huge disconnect when correlating mill performance with such issues as fuel line distribution, heat rate, NOx and environmental control equipment performance, it is the intent of this technical paper to provide better understanding of how mechanical optimization & tuning of the pulverizers can yield overall improved plant performance. Low NOx firing and/or optimization of the burner belt combustion with a limited amount of furnace residence time is absolutely essential to optimizing plant performance. For example, when pulverizer performance is poor, it is also often related to not only high furnace exit gas temperatures, increased slagging and/or high LOI, but also degrading electrostatic precipitator (ESP) performance from the coarse particle ash. Furthermore, reliability of the boiler (ie. tube leaks, fouling, and slagging) can also be impacted negatively by secondary combustion and consequent super heater and re-heater tube metals overheating and/or wall wastage often occurs from non-optimized fuel distribution being delivered from the pulverizers. Whether the reason for improving mill performance is for the aforementioned items and/or perhaps simply to reduce power generation costs with improved fuels flexibility, the purpose of this case study is to review the basics of vertical spindle mill performance improvements. The data used to support this paper is from a compilation of actual field testing & tuning results. Furthermore, Storm Technologies, Inc. (STI) suggests the aforementioned steps as an effective approach to optimization.

Storm Technologies in cooperation with AES Westover Station implemented a total combustion optimization system approach, including a fan boosted over-fire air system on Unit 13 to reduce the emissions of NOx while also improving and/or maintaining acceptable Carbon in Ash content levels on a daily basis. Implementation of this total airflow & pulverizer performance utilized a fundamental and proven approach to performance optimization and the system has been installed now for over two years and continues to be successful. The results of this systems modifications was up to 60% NOx reduction and payback in months by reducing the need for NOx credits and simultaneously improving unit performance, reliability and fuels flexibility. All of the goals of this program were accomplished and the technical success of this project is once again the results of applying a systematic and comprehensive approach addressing fundamental opportunities for improvement. The benefit of this total combustion optimization project was not only NOx reductions, but also reliability and “fuels flexibility”. Furthermore, foresight in this system was the ability to improve boiler efficiency, heat rate and reduce rates of ammonia when and/or if SCR or SNCR is installed. Since the installation of the FBOFA System it should be noted that AES Westover has been able to consistently attain between .25–.30 lbs/mmBtu NOx and single digit carbon in ash levels with no negative effects of the system installed. The goals of this project were as follows: 1. NOx Reduction from >.54lb/mmBtu (full load) – to ≤ 0.32 lb/mmBtu; 2. Flyash Carbon Content less than 10%; 3. Minimal slagging; 4. Operations with a minimum of 2% Oxygen to maintain a “slag friendly” furnace without exceeding the NOx limits; 5. Maximum Load Capability; 6. Maximum Fuel Flexibility; 7. Total Combustion Optimization & Performance Preservation.

The traditional approach to reduce NOx has been to retrofit and install commercially available “plug-in” Low NOx burners. Typically, these use a combination of internal staging and are often used in conjunction with over-fire air to create off-stochiometric or staged combustion. That is, the complete combustion of the fuel occurs in several stages. Often, well designed Low NOx burners are installed without a comprehensive systems approach. The typical challenges associated with staged combustion are related to the fact that burner performance must be nearly perfect to complete combustion within the available residence time of the furnace. Specifically, attention to airflow measurement and control by use of reliable & repeatable venturis and with pulverizer performance optimization. To maintain or improve this unit’s excellent reliability, a focus on optimizing the inputs and completing the combustion prior to the furnace exit was implemented. The goals of this project were as follows: 1. NOx Reduction from .78lb/mmBtu(full load) – 1.0#/mmBtu(low load) to less than 0.36 lb/mmBtu; 2. Flyash Carbon Content less than 10%; 3. Combustion Optimization; 4. Minimal slagging; 5. Maintain the same as baseline FEGT or reduce FEGT; 6. Maximum Load Capability; 7. Maximum Fuel Flexibility; 8. Complete the project at the lowest cost per kW possible (with the best results). All of the goals were accomplished. The technical success of this project is the results of applying a systematic and comprehensive approach beginning with raw coal feed to the pulverizers. The benefits of this total combustion optimization project is that later when additional NOx reductions are required, they can be added as a complimentary change to the present system. For example, if this unit is later equipped with SNCR or SCR, reduced rates of ammonia will be required, there will be reduced “popcorn ash” production, and less SCR catalyst wear and overall unit improved performance and reliability.