bpreston
Mechanical
- Aug 10, 2020
- 3
One of our units is down for a major outage (coal fired steam turbine). Months before the unit came offline we one of our fuel sources had opened up and delivered us a bad seam of coal with high sulfur, nitrogen content, and high ash. Running it we began experiencing ID/FD fan stalling and lots of ash appearing everywhere around the unit from the PA fans, around the boiler, and out around the ID/FD fans. We continued to run worse and worse, over the course of 6 months or so we began losing PA fans running the unit at lower load until only 2 of 5 PA fans were operable where we ran for about a month.
This unit has a relatively new SCR prior to the air heaters that injects ammonia into the flue gas. To make things worse in this 6 months operations began over injecting ammonia to try and reduce our NOx from running this poor quality coal at low temperatures. Gas into the SCR at this time sat around 700°F and there's not many times there's gas leaving the SCR below 650°F, while the cold end baskets look like they are averaging about 160°F where air is coming into the heaters. Upon opening the doors on the APH we found the cold end baskets almost entire plugged of with hard build-up and the intermediate baskets filled with fly-ash.
We are typically running coal of a sulfur composition around 0.17-0.38 % by wt, and have a 1% conversion of SO2 to SO3 through the reactor. Even with the higher sulfur content coal (0.5 %) I'm not sure if how much of this deposit might be from the condensing of the product of SO3 and NH3 in the cold end baskets, from what I've been told our SO3 is almost none existent. We are required to remain below 2ppm of NH3 slip, so at most at any point that's the highest that would have made its way through the air heaters to the scrubbers. However, I suspect that it is ABS forming from condensing NH3 through the heater elements. This stuff is hard and sticky, and mixed with our ash is almost like concrete at this point. We sent out a bid to several companies who have experience cleaning air heaters. Two of them came back and told us that this is common, they were both confident they could clear the build-up with hydro blasting.
We've been blasting the baskets at 4000 psi manually with "shotgun" lances for over a week now. Yesterday we removed a few sets of the hot and intermediate baskets to see how the blasting was going. While the several feet of fly ash is removed from the intermediate baskets that buildup still remains on un-exposed side of the cold end baskets. Talking with the contractors they aren't sure 4000 psi is going to be able to cut through this material. We plan to bump up to 5000 psi and then 6000 psi to attempt to clear the build-up without having to remove all of the intermediate/hot baskets. If this does not work or begins to damage/fold our baskets, removing baskets to access the cold end elements is going to take us a month at least I would guess. Original start-up date was tomorrow, so there will be motivation to try and find a solution to this.
Does anyone have any experience clearing build-up like this from air heaters?
This unit has a relatively new SCR prior to the air heaters that injects ammonia into the flue gas. To make things worse in this 6 months operations began over injecting ammonia to try and reduce our NOx from running this poor quality coal at low temperatures. Gas into the SCR at this time sat around 700°F and there's not many times there's gas leaving the SCR below 650°F, while the cold end baskets look like they are averaging about 160°F where air is coming into the heaters. Upon opening the doors on the APH we found the cold end baskets almost entire plugged of with hard build-up and the intermediate baskets filled with fly-ash.
We are typically running coal of a sulfur composition around 0.17-0.38 % by wt, and have a 1% conversion of SO2 to SO3 through the reactor. Even with the higher sulfur content coal (0.5 %) I'm not sure if how much of this deposit might be from the condensing of the product of SO3 and NH3 in the cold end baskets, from what I've been told our SO3 is almost none existent. We are required to remain below 2ppm of NH3 slip, so at most at any point that's the highest that would have made its way through the air heaters to the scrubbers. However, I suspect that it is ABS forming from condensing NH3 through the heater elements. This stuff is hard and sticky, and mixed with our ash is almost like concrete at this point. We sent out a bid to several companies who have experience cleaning air heaters. Two of them came back and told us that this is common, they were both confident they could clear the build-up with hydro blasting.
We've been blasting the baskets at 4000 psi manually with "shotgun" lances for over a week now. Yesterday we removed a few sets of the hot and intermediate baskets to see how the blasting was going. While the several feet of fly ash is removed from the intermediate baskets that buildup still remains on un-exposed side of the cold end baskets. Talking with the contractors they aren't sure 4000 psi is going to be able to cut through this material. We plan to bump up to 5000 psi and then 6000 psi to attempt to clear the build-up without having to remove all of the intermediate/hot baskets. If this does not work or begins to damage/fold our baskets, removing baskets to access the cold end elements is going to take us a month at least I would guess. Original start-up date was tomorrow, so there will be motivation to try and find a solution to this.
Does anyone have any experience clearing build-up like this from air heaters?
