United States Environmental Protection Agency Industrial Environmental Research Laboratory Research Triangle Park NC 27711 Research and Development EPA-600/S7-81 -154 Dec. 1981 Project Summary Study of Automatic Control Systems to Maintain Constant Percentage SCh Retention in a Pressurized FBC K. J. Daniel, S. D. Finnigan, and R. M. Reinstrom The Clean Air Act Amendments of 1977 indicate that future emission standards for SO2 should be based on a percentage reduction (comparing sulfur emissions with sulfur feed). In a pressurized fluidized-bed (PFB) boiler, sulfur feed (as determined by coal sulfur content and feed rate) and sulfur removal effectiveness (as deter- mined by the reactivity and feed rate of dolomite/limestone sorbent) vary continually during PFB plant opera- tion. The purpose of this study was to assess the feasibility of using some type of automatic control system to maintain a constant percentage sulfur removal in a PFB system as variations occurred in key variables, such as coal sulfur content and sorbent reactivity. To conduct this feasibility study, a transient model of a PFB power plant was developed and validated for studying methods of controlling bed SO2 absorption characteristics. To accomplish this, the transient equa- tion for the population distribution as a function of size and utilization was solved. The model uses TGA rate data for 1337 dolomite as a function of size, utilization, and temperature. The kinetic data is integrated over the instantaneous population distribution to determine the instantaneous SO2 absorption rate constant. This transient model was used to assess the potential of alternative automatic control strategies for achieving constant percentage SO2 retention during changes in load, in coal sulfur content, and in sorbent reactivity in a PFB. Goals were to minimize costs and sorbent require- ments. Of the control strategies considered, the preferred option iden- tified in this assessment continuously monitored the sulfur content in the feed coal, and adjusted the sorbent feed rate to maintain a constant sorbent-to-coal-sulfur feed ratio. This strategy does not instantaneously change this sorbent-to-sulfur ratio to account for short-duration changes in sorbent reactivity, since the inertia of the large mass of partially spent sorbent in the bed prevents instanta- neous changes in the sorbent/sulfur ratio from being effective. However, long-term variations in reactivity are accommodated through a feedback loop which measures retention and adjusts the sorbent-to-sulfur set point accordingly. Such a control strategy minimizes, but does not eliminate, the need for excess sorbent feed to ensure that the PFB does not exceed the specified percentage SO2 reduction on a 30-day average. A prompt neutron activation tech- nique appears to be the most promis- ing for the required continuous monitoring of coal sulfur content. However, such a system is develop- ------- mental, and has not been commercial- ly demonstrated. This Project Summary was develop- ed by EPA's industrial Environmental Research Laboratory. Research Tri- angle Park, NC. to announce key find- ings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction The Clean Air Act Amendments of 1977 indicate that the future emission standards for S02 should be based on a percentage reduction. In a conventional power plant using flue gas desulfunza- tion (FGD), percentage reduction of sulfur can, in principle, easily be moni- tored and controlled. Sulfur dioxide (SOi) concentration can readily be measured before and after the FGD unit to determine the percentage of SOz absorbed by the process. Because no components have inherently slow response, control of the system is rela- tively straightforward. In the future, power plants using a fluidized bed will encounter a different situation. Rather than monitor only gas concentrations to determine per- centage reduction, coal sulfur content and feed rate will need to be monitored. Moreover, the mass of absorbent in a bed represents a large inertia that must be controlled. Changes in the reactivity of the sorbent feed also will affect the control system. This system examined the control of sulfur capture in fluidized beds with emphasis on pressurized fluidized beds (PFBs). Specifically, the study examined how changes in load and coal sulfur content affect sulfur capture in a PFB combustor. In addition, various methods of controlling bed sulfur absorption properties that minimize sorbent usage and maintain a constant percent sulfur removal were examined. The study involved developing a transient model for the PFB power plant, validating the model by comparison with experimental data, and using the model to evaluate various control strategies. Emphasis was on phenom- enological modeling of the sulfur capture processes in the PFB. However, the model also includes representations of all major equipment (e.g., gas and steam turbines) in the PFB/combined- cycle power plant. The transient rate of sulfur capture is determined from the transient distribu- tion of dolomite particles in the PFB, as a function of size and utilization. The model solves for this transient distribu- tion by integrating the distribution with chemical rate constants (also a function of size and utilization); the instanta- neous rate of sulfur adsorption is obtained. The model was validated using both transient and steady-state data from the experimental PFB Mini- plant (Reference 1); the comparison of the model and the data indicates good agreement. Results Several interesting results were obtained from the model. The response of sulfur retention to a change in a Ca/S ratio shows a strong dependence on the magnitude and direction of change. However, the response is essentially independent of whether the change is made in the calcium feed rate or the sulfur feed rate. Also, in spite of rapid changes in sulfur input to the bed, the change in retention was very slow. Retention changes slowly because the rate of the sulfur capture reaction in the bed isfirst order with respect to SOa concentration and also because the inventory of dolomite in the bed represents a large inertia. Four candidate strategies to control bed retention were identified and evaluated. The first was to change the size distribution of the absorbent feed to the bed. Because smaller particles have higher reactivity, the amount of sulfur absorbed by the bed can be controlled by changing the size distribution of the bed. However, because of the large amount of mass in the bed, the size distribution of the bed changes very slowly. Consequently, the response with this method of control is too slow to be practical. Moreover, reducing the size distribution of the feed could cause additional elutriation which would limit the maximum excursion that could be handled by this method of control. This type of control would also not be able to take advantage of downward excur- sions in coal sulfur content. This method of control, thus, had several disadvantages and no clear advantage. The second strategy for control studied was to adjust the reactivity of the absorbent fed to the bed. This method would also have a slow response due to the large bed inventory. In addition, if a highly reactive dolomite were being used to achieve low steady- state Ca/S ratio, there would be little control margin to accommodate transient excursions. Moreover, this system could not take advantage of downward excursions of coal sulfur content. Because of these shortcom- ings, this method is not recommended. The third strategy investigated con- sidered blending lowsulfurfuel with the coal feed to maintain a constant rate of sulfur feed to the bed. This system will ensure nearly constant percent sulfur retention if the reactivity of the dolomite is constant. In addition, this system minimizes the excess dolomite feed required. There are no technical bar- riers to implementing this control system; however, it is prohibitively expensive when compared to a system that uses a sufficient excess of dolomite feed to ensure that emission limits are met. The fourth strategy investigated is the preferred method. This method maintained the dolomite feed rate in proportion to the rate of sulfur entering the bed. By exercising the model it was found that 862 retention can be con- trolled within 1-1/2 percentage points by this method. Control of the dolomite- to-sulfur ratio in this manner would not, by itself, make the necessary adjust- ments if sorbent reactivity were to vary; accordingly, a feedback loop is included to measure retention and adjust the dolomite-to-sulfur set point as neces- sary to accommodate long-term variations in reactivity. Efforts to instantaneously adjust the dolomite-to- sulfur ratio, to follow short-term changes in sorbent reactivity, would not be effective, since the inertia of the large inventory of partially spent sorbent in the bed would prevent instantaneous changes in the dolomite/sulfur ratio from having a significant impact. This strategy reduces excess dolomite use to a minimum and shows a signifi- cant potential cost savings compared to a system using a constant excess dolo- mite feed to ensure compliance with emission limits. One possible disadvan- tage of this system is that elutriated fines may increase due to increased dolomite feed. While available data does not indicate that this will be a significant disadvantage, further study is recom- mended. Because the preferred strategy requires measurement of the sulfur content of the coal feed to the bed, a review was conducted of on-line coal ------- sulfur measurement techniques. This review indicated that, although many techniques are available, the one that appears most suitable is prompt neutron activation analysis of the coal feed. This measurement technique is still being developed. The added cost of this device was included in the econom- ic analysis. The methods identified for maintain- ing constant percentage sulfur capture are applicable for use with atmospheric fluidized-bed (AFB) combustors without fines recycle, and may be applicable to AFB systems that incorporate fines recycle. However, recycle of fines is not incorporated in the model and, therefore, no conclusion can be drawn regarding the effects of fines recycle on the results presented in this study. Recommendations To determine the economic attrac- tiveness of the proposed control method, the typical time-dependent sulfur variations in coal feed must be defined. Specifically, the size of excur- sion, the duration of excursion, the ramp rate of change, or frequency spectrum must be determined. This will aid in determining the margin in dolomite feed rate that is required to control variations in sulfur emissions. More accurate economic trade-offs can then be obtained. The recommended control strategy requires that the dolomite-to-sulfur ratio remain constant. This implies that' the dolomite feed be increased and de- creased in response to changes in coal sulfur content. To reduce this method to practice, experiments should be per- formed to determine the effect of changing feed rate on attrition and elutriation. Moreover, the model should be extended to study advanced AFB con- cepts that rely on the recycle of elutri- ated fines to the bed to significantly improve sulfur capture. The extension of the model could aid in the evaluation of various effects and options such as attrition rates, recycle rates, cyclone capture efficiency, freeboard height, coal sulfur contents, and load control. References 1. Hoke, R. C., et al., "Mmiplant and Bench Studies of Pressurized Fluidized-Bed Coal Combustion: Final Report," Exxon Research and Engineering Co. (EPA-600/7- 80-013, NTIS No. PB 80-188121), January 1980. K. J. Daniel. S. D. Finnigan, and R. M. Reinstrom are with General Electric Co., 1 River Road.- Schenectady, NY 12345. D. Bruce Henschel is the EPA Project Officer (see below). The complete report, entitled "Study of Automatic Control Systems to Maintain Constant Percentage SO 2 Retention in a Pressurized FBC," (Order No. PB 82-110 693; Cost: $15.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield. VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Industrial Environmental Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 * U S GOVERNMENT PRINTING OFFICE, 1981 — 559-017/7405 ------- United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OM 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 00003^9 ------- |