(26c) Power Cycle Design for Optimal Cogeneration

Authors: 
Nielsen, C., Process Systems Engineering Laboratory, Massachusetts Institute of Technology
Barton, P. I., Massachusetts Institute of Technology
At over 250 million tonnes per year, more sulfuric acid is produced worldwide than any other chemical, mostly in double-contact, double-absorption processes. In these processes, the catalytic conversion of SO2 to SO3 is highly exothermic, requiring heat to be removed after each converter stage at temperatures up to 1000 C. These large quantities of high-temperature, high-quality heat make sulfuric acid production an ideal case for cogeneration, where the excess heat is used to supply power cycles that generate electricity to run the rest of the plant or to sell back to the grid. In fact, due to the significant potential for power production, many plants that require large quantities of sulfuric acid, including OCP’s Jorf Lasfar platform, the world’s largest phosphate-processing facility, opt to produce their own to generate cheap electricity. However, these cogeneration systems rarely take full advantage of the high-quality heat provided by the sulfuric acid process and usually use Rankine cycles that operate with boiler temperatures far below the gas cooling temperatures, lowering the exergetic efficiency of the process. Therefore, in collaboration with OCP, we have utilized new tools in process integration to study and optimize these cogeneration systems.

For this study, we used sulfuric acid production at the Jorf Lasfar platform as a test case for optimizing power produced by cogeneration. To perform this optimization, we proposed a superstructure of distinct power cycles with fixed working fluids, including both traditional Rankine cycles and CO2 Brayton cycles that can operate at high turbine inlet temperatures to better utilize high-temperature heat from sulfuric acid production. To optimize the usage of heat, both from sulfuric acid production and cascaded between power cycles, we applied a new generalized, nonsmooth approach for process integration. This method allows us to solve efficiently for the design parameters, including the size and operating conditions of each power cycle, while enforcing conditions for maximum heat reuse by introducing a simple nonsmooth system that consists of only two equations regardless of the complexity of the proposed power cycles. [1]

We will present this general methodology for optimizing power production from cogeneration and demonstrate its use on a sulfuric acid production unit in the Jorf Lasfar platform. Our results show that for this system, the addition of a Brayton cycle increases potential power production by upwards of 20 percent by discharging heat at temperatures high enough to supply an additional Rankine cycle. This increase in power production can decrease operation costs and energy use by powering additional processes and reducing grid-dependency, being sold back to the grid, or freeing additional heat to be used in other processes. These improvements suggest that approaching cogeneration problems using heat integration methods could identify simple strategies to improve energy use across different industries and provide a systematic approach to decreasing emissions and improving sustainability in chemical processes.

References

[1] C. J. Nielsen and P. I. Barton. “A Generalized, Nonsmooth Operator for Process Integration.” Accepted for ESCAPE-29 conference proceedings. Eindhoven, Netherlands. June 2019.