|By Denis Osowski, Babcock & Wilcox Power Generation Group|
Sometimes power industry professionals get lucky when a solution developed for one complex challenge provides coincidental benefit for an unforeseen issue that arises in the future.
Such is the case with a process originally developed to reduce acid gas formation when operating a selective catalytic reduction system (SCR) during turndown on coal-fired boilers which led to an unplanned (but very welcome, in today’s regulatory climate!) improvement in heat rate, and a corresponding reduction in greenhouse gas emissions.
When SCRs were introduced in the early 2000’s, it was recognized that the equipment’s minimum operating temperature posed a limitation on unit turndown. Turndown limits were reached when boiler load was reduced and economizer exit gas fell below the minimum SCR inlet temperature. In many cases this limitation was so severe that units could essentially only run at full load.
Back when coal units were base loaded, this was not a serious restraint. But as the economics of the power market changed, units without turndown capability became liabilities. Multiple solutions were implemented to address this, including economizer gas bypasses, economizer water bypasses, variable temperature economizers and even duct burners. All these devices were intended to maximize economizer exit temperature at low loads and return some of that valuable turndown back.
But with the rollout of U.S. EPA’s MATS rules, limiting acid gas emissions became the hot topic of the day – and all that hot gas going to the SCR was found to unwittingly enable the production of acid gas through the conversion of SO2 to SO3. To some, the solution for acid gas control was to add a dry sorbent injection system and reduce SO2 entering the SCR. But for some plant operators, relying on DSI would create an undesirable increase in operating cost. It was clear that not only was low temperature at low loads an issue for SCR operation, but high temperature at high loads needed to be controlled as well. It’s decidedly cheaper for an operator to control the conditions that cause the acid gas production in the first place, rather than spend money on reagent for acid gas control.
The solution to economizer exit gas temperature control was an over-surfaced economizer with water bypass or water flow bias control. The same systems used to modify the amount of water fed to the economizer to maintain higher temperature at low load can be used in conjunction with an over-surfaced economizer to control economizer exit gas temperature nearly to a setpoint from full load down to about 50 percent load. This number could be a little lower or a little higher, depending on the boiler design.
The additional economizer surface installed to knock down full load temperature has been successfully added to existing economizer spaces by redesigning the surface geometry, adding fins or extended surface, moving equipment (i.e., sootblowers) to make room for additional surface, or even adding a new standalone horizontal economizer in the downstream flue work. Through the application of flow biasing within a specially designed economizer or water bypassing of an entire economizer – or both used together – operators can gain great flexibility to control exit gas temperature to maximize SCR performance, while minimizing SO2 to SO3 conversion and dry sorbent reagent cost.
Then came a pleasant surprise. Since adding surface to an economizer also provides a significant improvement in heat rate by capturing additional energy in the final heat trap of the boiler, we find we have to combust less fuel for the same load, thus producing less CO2. This heat rate improvement offers a significant head start toward a compliance strategy w for the currently contemplated EPA CO2 rules for existing sources and, in this case, the heat rate improvement “building block” in the EPA’s plan.
Depending on the existing full load economizer exit gas temperature, the extra heat absorbed by the additional economizer surface can provide for an improvement in full load heat rate of up to 1%, meaning less fuel burned, and an associated reduction in CO2 emissions. Such a reduction in CO2 emissions represents a significant piece of the 6-percent heat rate improvement target that the proposed existing source CO2 rule describes as one of the four primary building blocks to achieving the Agency’s greenhouse gas reduction targets.
In a world where emissions control technologies are seen as ‘parasitic’, this tool for acid gas control contributes to a healthier bottom line today and a possible aid to regulatory compliance tomorrow.
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