Excess Air and The Kenora Paper Mill

My fight with the U of W has been really heating up lately, so I've been distracted from the physics. But something reminded me of a story I'd like to tell, so I thought now is as good a time as any. It's about a paper mill where I used to work in the mid-90's.

I was hired in 1993 as a Mechanical Maintenance Engineer in the Boise Cascade Paper Mill in Kenora, Ontario. I only worked there two years; in that time, the mill changed hands three or four times. It was Abitib-Price, Stone Consolidated, and for all I can remember it might have been Abitibi Consolidated as well. (This despite the fact that it was losing money every single month. For reasons I've never understood, there seems to be more money to be made in buying and selling paper mills than in actually making paper.) The Kenora plant was actually a pulp and paper mill. My responsibilities were mainly on the pulp side. The Maintenance Engineer was supposed to provide engineering backup to the Maintenance Trades (millwrights, welders, pipefitters etc.) and also to identify chronic maintenance issues which could be corrected by redesign or other measures. It was a fantastic job that I went into without really having any qualifications. But that's another story...

Today's story is about the Steam Plant. Steam production is HUGE in the pulp and paper industry, and our boiler plant was fired by natural gas, supplemented by bark and other wood byproducts. If I remember the cost structure of the mill, it was a $500,000,000/year facility with costs fairly evenly divided into five parts: consumables (wood and other material costs), transportation (mainly shipping), energy, wages, and capital depreciation. So Energy was about a hundred million dollars a year, and steam production was maybe thirty percent of that. That's the best I can remember.  (Actually, as I think back on it, it wasn't natural gas at all...it was oil..."Bunker C" they called it. Where did I ever hear of "Bunker C" if not in the Kenora Mill? It was oil, not gas.)

As Pulp Side Maintenance Enginer, the Steam Plant was part of my territory. Since I didn't know anything about boilers, I started hanging around the control room quite a lot, asking questions and generall getting to know the guys. Well, to be truthful, there was just a lot of hanging out going on. But now and then I'd glance at the control screen and ask a question or two. And there was a parameter on the display called "O2", and I asked what it was. "Excess Air", they told me. It was usually sitting around 10%. If it got much higher, they would throttle back the intake vanes on the combustion air blowers.

I did a little research on my own, and learned that to run a boiler efficiently, you needed to use just enough air to completely combust the fuel. It's not hard to see why. Remember that air is 80% nitrogen, which is inert. So the job of combustion is mostly to heat up that nitrogen. Let's say you put in the exact right amount of fuel to consume the oxygen, which is 20% of the air. Then your combustion products (CO2, water vapor, and Nitrogen) will go up the smokestack at whatever temperature....let's say, 1200 degrees. On the way up, they pass through the boiler, where heat is extracted via the heat exchangers. The heat exchangers can only draw down so much heat, because you need a temperature gradient to drive the heat flow. So after leaving the heat exchanger, the best you can hope for is that you've cooled the flue gasses to, let's say, 400 degrees. From a reference point of zero degrees (for the sake of argument), that would be a thermal efficiency of 67%. That would be your theoretical optimum for this process.

But what if you are running with excess air? For the sake of argument, let's say you are putting in twice as much air as you need to combust the oil. Then your combustion products are only going to be 600 degrees, not 1200 going into the heat exchanger. And since they leave the heat exchanger at 400 degrees, you've only extracted 33% of the available energy. You're running your boiler at half the optimum efficiency. And if you're spending 30 million on fuel, that means your sending 15 million dollars up the smokestack.

So where does the 10% excess air figure in on all this? Well, you'd like to run at what the chemists call "stoichiometric conditions"...zero excess air. But there are obviously practical problems with this, which I don't need to get into here. Suffice it to say that 10% excess air is considered a good target for efficient boiler operation. And that's what we were reading on our console.

Except...it said "O2". But that's ok...O2, is air, right? Not exactly. It's oxygen. It took me a while, but eventually I figured out that what we had was an oxygen sensor in the smokestack, and it was reading 10% oxygen. Now, when you bring air into the furnace, it's 20% oxygen. So if you're sending it up the stack at 10% oxygen, you've only burned half the oxygen. You used twice as much air as you needed. And that's what we were doing. We were running at 100% excess air, and sending fifteen million dollars a year up in smoke.

You'd think I would have been a hero for figuring this out, but it didn't work out that way. It's hard to get people to change the way they do things, especially if you're a quirky newcomer who looks more like a mad scientist than a hard-boiled engineer. And to tell the truth, there were some people there who already had a vague notion that the boiler was a bit off, but previous half-hearted attempts to improve it had led to other problems, so it was thought best to let well enough alone. Bottom line, I didn't have the credibility to get the necessary people onside to do what needed to be done.

Fast forward ten years. I was driving home from a trip to Thunder Bay where I had taken part in the local Fringe Festival, performing my Yiddish/English comic operetta "The Ballad of Monisch". And suddenly I was back in Kenora. I thought I'd go visit the paper mill and see if I had any friends left there.

I pulled up near the Wood Plant entrance (the building where they have the debarker and wood chipper), and made my way straight inside where I quickly grabbed an orange safety helmet. (Because a bareheaded visitor would stick out like a sore thumb.) And then I wandered around to some of my old haunts, meeting some former friends and just chilling out. I ended up in the Steam Plant, where the guys were quick to pour me a cup of coffee and enjoy reminiscing about old times. While I was there, I glanced at the console...yes, the "excess air" was still running at 10%.

I thought about it on the way home, and figured out a plan of action. I myself never had the credibility to get this taken care of, but there were some consulting engineers in town who regularly worked out at the mill, and if they approached management with the idea, surely it would be taken seriously. On Monday morning, I phoned Dave MacMillan, senior parter at KGS Engineering, and told him I had figured out a way we could make about half a million dollars each in commisions by fixing the steam plant in the Kenora Mill.

"Marty, have you read the papers this morning", Dave asked.

"No, what difference does that make?" I answered.

"I think you should read the papers, Marty." Okay. I went downstairs and looked at the front page. "Kenora Mill to Shut Down". It was over. No one was going to be making any more money out of Boise Cascade, or Stone Consolidated, or Abitibi whatever they were calling themselves.

The mill was in fact a dinosaur, with oudated equipment and a built-in cost structure that could never compete with the brand-new mills being built in the southern US. But I still wonder sometimes how much longer they might have stayed in business if I'd had the moxy to fight for my ideas back when it might have made a difference.