For my column “Put Energy Into Your Environmental Project,” Tom, a chief process engineer from California wrote, “We have had many Low-NOx burner installations at this refinery. You are absolutely right about the flames being a larger size, which can de-rate the fired heater.He cautions, though: Your thoughts on adding air-preheat to an SCR installation are a creative way to improve efficiency. However, there is one warning: If the heater is skin-temperature limited, there is no advantage to the air-preheat: When you heat the air, you increase the peak flame temperature and exacerbate the skin temperature problem. In this event you are limited on heat release.”
This is correct. I should have mentioned some alternatives like using the heat for process or steam. Then you don’t change your flame pattern. Nevertheless, I still prefer an SCR over low-NOx burners, especially in furnaces that aren’t very tall.
For the same column, Guy, a salesperson for SCR systems and low-NOx burners in Ohio, points out that certain low-NOx burners are designed to produce the same or smaller flame heights than the original furnace burners. These burners use a swirl nozzle in a firing tube to reduce the height. The design is new to me, so I stand corrected.
“Do Your Own Steam Survey,” prompted Harry from California to ask what are the biggest headaches/needs in steam. The biggest headache is increased demand during rainstorms. If your system is already loaded, a sudden downpour can cause a demand increase because of poor insulation. Most plants that have this problem have in place a steam management plan, which allows them to reduce demand by switching turbines to motors, delay high steam demand events, and divert steam away from non-essential users. However, if you have a lot of steam traps, you can instead implement a steam trap survey and monitoring program and undoubtedly uncover loads of wasted steam.
In response to “Don’t Get Heated Up by a Hot Stack,” Gopala from India wrote, “Your suggestions are well taken. I wish to point out that in these days of high cost for energy no one lets out an opportunity to maximize the use of heat. In the factory in which I was working a few years ago, they had incorporated a waste heat boiler to tap most of the heat from the combined-cycle-power-plant stack gas. However, I understand there is a limitation to extracting the heat from the stack gas. The temperature of the stack gas cannot be lower than its dew point, which would be around 360ºF. Below this temperature, the acid that condenses on the heat exchangers will corrode it. So the stack gas is allowed to vent at about 390°F to 410ºF.”
Correct! Most designs limit recovery because of CO2 production and sulfur compounds (although I have seen the limit as low as 320ºF). I know of people who have problems with air preheaters because some inferior designs can create cold pockets where air temperature drops too low, although average temperature is fine. If you actually can use the heat, I suggest a ceramic or special alloy heat exchanger that wouldn’t corrode in those conditions.
John from Arkansas brought up a hurdle not covered in “It’s Time for a Design Check,” in which I suggest NEMA premium pumps should be a part of a plant’s energy standard. “A second roadblock for our industry (and I expect many others) is at the repair or replace decision. Each facility had a maintenance budget and repairs of old inefficient motors are easy to do compared with the replacement of that motor with a new NEMA Premium design that will offer energy savings and potentially better reliability. No outside approvals are required. These new purchases come from a different capital budget requiring management approvals and many of these are very tight these days. If we speak of long-term sustainability, [repair or replace decision needs] to be addressed at the management level. Companies cannot act as they have in the past. The purchasing paradigm needs to change.”
I agree. Your plant’s energy standards should address project management as well as maintenance approvals. Any time a decision is made that involves equipment that uses energy, your standards should address how energy should affect that decision.
Keep those questions and comments coming! The better the information, the more energy we can save.
Gary Faagau is Chemical Processing's Energy Columnist. You can e-mail him your comments and questions at GFaagau@putman.net.