Good Evening everyone how is we all doin tonight. I was wondering something today I passed a bunch of coal cars sitting by the journal sentinel plant today over on. Miller Park Way (43rd St) here in milwaukee and the cars were full of coal but also they were covered in snow. If the coal gets wet or anything of that nature does the coal lose it’s ability to ignite or is the coal towards the bottom of car still good and the power plant just not use the stuff that got wet from snow or rain. I have been wondering this for a while cause I would see the coal cars going down the tracks and I’d see them covered with snow or if the train is going down the tracks as it’s raining really hard. And the coal is getting all wet from top to bottom. How does the power plant use it if it is all wet. Or does the power plant still goes ahead and uses it and it dries out as it goes into the fire. Thanks for and information put into this subject.
They deal with it just like they do with dry coal. They grind it down to a powder with the consistency of baking soda or sugar, mix it with water and spray it into the firebox where it literally explodes into flame.
railfan619 the big problem with wet coal in the winter is that it can freeze in the hoppers and be impossible to get out. The Sept. 2007 L&N magazne http://www.lnrr.org/ had an article “Coal Everywhere-but none that can be used” which talks about the winter of 78/79 when this was a very real problem for the Union Carbide plant in Western KY.
Dale
Most coal contains water as it comes from the ground or from the washery, e.g., PRB coal has a moisture content of ~25% so whatever precip falls on it doesn’t make it incombustible. However, the utilities aren’t very happy when a coal train gets delayed in route for a long period and the coal gets wetter than usual. The problem with moisture is that it makes certain coals, notably subbitumnous coals like PRB coal, more subject to spontaneous combustion. Coal that is stockpiled is carefully compacted to reduce oxygen content in the coal – the combustion occurs as coal oxidizes (absorbs oxygen from the air).
drafterdude’s note about freezing in the car also applies – big problem!
RWM
That’s right. So next time you see that big 15,000 ton coal train, consider that the train is actually hauling about 4,000 tons of water.
I’ve heard that some coal plants have actually turned down shipments of coal due to excessive moisture content.
Is precipitation during transportation really that big a deal as long as it doesn’t freeze? Afterall, when the car is unloaded at the power plant, in most cases, the coal is placed on top of the stockpile that is sitting outside in the snow and rain.
Yes, it really is that big a deal.
There’s “long-term stockpile” and “working stockpile.” Power plants do not want to turn over the long-term stockpile, or touch it, because it is compacted, monitored, and carefully stacked to minimize the risk of spontaneous combustion. The power plant only digs into long-term when the railway is falling seriously behind on its service committment. The “working stockpile” is the running tally of the regular coal train delivery less the plant daily consumption plus a small amount of float. The power plant already knows how often it expects to turn over the working stockpile and has an expectation of average moisture content in that pile including whatever falls on it in the course of ordinary events. What they don’t like is when the coal train is a few days late and the average moisture content goes up. They they have to pay more attention to reducing spontaneous combustion.
RWM
I have personally never heard of a power plant refusing a shipment and have never seen an loaded coal train being returned to me the biggest problem would be thawing out the cars to get the coal unfrozen so it can be unloaded
I have. It was an Illinois plant (name withheld) and the train took weeks to get there. We couldn’t take it back to the mine, we couldn’t just dump it at trackside, and we didn’t want to pay $$$ to put it in a landfill, so finally we found a steel mill that took it off our hands for free. They never would tell us what they did with it.
I also recall being in the office of a VP Marketing the morning after we had a big coal-train derailment. The newspaper sent up a helicopter to get a picture, and there it was, in full color, giant-size, on the front page of the newspaper – showing a big pile of tangled steel and spilled coal, and leading away from it dozens of underailed cars with a heavy blanket of snow on top. I brought the paper in and gave it to the VP. His eyes bugged out, he exclaimed an oath at the top of his lungs, and said “Now the next call I’ll get is from the powerplant bitching about the snow on their coal.”
RWM
I had heard from someone in the coal analysis biz - we were working on a proposal for an improved coal moisture measurement system. He said that the power companies often had moisture content specs as part of the contract.
As for PRB coal, I’m taking a wild guess that the extra moisture content can cause small scale fracturing of the coal and thus generating a lot more surface area for oxygen to react with the coal. I also seem to recall a power plant in Indiana having an explosion after switching over to PRB coal and not taking appropriate safety precautions with dealing with the stuff.
Moisture content is specified in every coal purchase contract I’ve seen, along with BTU, ash, sulfur, ash fusion temperature, and in some cases sodium too.
Turns out that affect of moisture on spontaneous combustion is complicated and not well understood. There appears to be a difference between intrinisic moisture and surface moisture, and surface moisture has a catalytic effect on the oxidation reaction that leads to temperature increase that leads to combustion initiation.
This paragraph from the document below helps explain:
"Ignition criteria. The model calculations show that partial wetting of coal or condensation of moisture exerts two competing influences on the overall rate of oxidation. On the one hand, a portion of the coal fills up with liquid moisture. In this region the rate of oxidation becomes negligible since the oxygen has to dissolve in the moisture before it can gain access to an active site on the coal surface and the solubility of oxygen in water is low. On the other hand, condensation also leads to the release of the latent heat of vaporization. This heat effect raises the temperature of the particle and enhances the rate of oxidation in the dry region of the coal. For ignition to occur, the latter influence must predominate such that the rate of heat generation exceeds the rate of he
The biggest problem with frozen coal is it sticks to the cars. When it is wet it sticks to the chutes in the transfer houses and at the bottom of the coal bunker above the coal mill feeder.
Once the coal gets into the pulverizer the hot air dries the coal and warms it as it is being pulverized and blown into the steam generator. Water is not used for this, air is used.
A lot of times in the winter, coal off the stock pile is frozen, is sent to the bunkers and after sitting in the warm bunker starts to thaw than becomes a stickey mess and will not go thru the feeders, creats a lot of problems
K. Borg
BNSF and UP regularly wet down their coal loads as they leave the PRB to reduce the amount of coal dust generated in transit. This means that they must have a fair idea how much additional moisture is acceptable, how much drying occurs in transit under various weather conditions, and so on.
I would say that moisture content is just one more factor with which operations planners have to contend.
Interesting, I hadn’t heard about spontaneous combustion in stored coal. In the past, I have surveyed a power plant’s stockpile a couple of times to provide them with the current, accurate, volume. They wanted to correct their estimated deliveries minus burn calculations. This plant only had one pile and that is what I based my comment upon.
They didn’t have scale records?
Only one stockpile. It goes to show that no practice is uniform. Every time I think I’ve seen the most different thing possible, I stumble over something new. A mine-mouth powerplant or one that draws from several mines close by may have no need for a large stockpile. My experience is mostly with large power plants (6-20 million tons/year annual consumption) that are running between 30 and 90 days inventory on hand at any given time, which is a pretty big coal pile. I have a new plant I’m working with now and we will put 6 trainsets into service for up to nine months before the first match is struck, in order to build up a sufficient inventory.
Different coals have different vulnerability to spontaneous combustion. In broad terms, the lower rank the coal, the more vulnerable it is. Lignite is thus the most likely to have issues, then sub-bituminous, and that’s where I run into it the most.
RWM
The one I worked with was MUCH smaller than the ones you describe. It did have multiple sources for coal and a lot (most?) of it was trucked in. Maybe they were correcting their moisture lost/gained due to evaporation/precipitation in their calculations. I didn’t need to know, I just worked there.[;)]
There’s a local cement mill that has problems with smoldering stockpiles of coal on a pretty regular basis, in the summer especially. This past summer the conveyor belt bringing the coal from the pile to the furnace caught fire and burned up as a result of spontaneous combustion.
Shipments do get refused. Not because of moisture content, but because the coal delivered does not meet the specifications of the coal purchased. All coal is not the same with differing chemical makeups all of which affect the burn characteristics. Boilers are designed to burn coal that is within a range of specific characteristics. Coal is tested before unloading to insure that the proper product is being delivered.
Open top coal cars must pick up a lot of unexpected water when on the road in the spring and summer. What is specified in the delivery contract is internal moisture content. The coal plant I toured mixed the ground up coal with water to increase the pressures inside the firebox. They said it was better than just blowing in the powdered coal and was part of the pollution control process and also gave them better BTU output from the coal.
Coal will suffer from spontaneous combustion while in coal cars, in loading silos and while on the ground. Local fire departments will be called out for coal car fires and they just flood the car with water until it literally over flows the top chord of the car. I wonder how much overweight those cars are. After a derailment, the hottest cars on line are the mty coal hoppers going back to the mine. Far more priority than the loads headed to the power plant. Once the coal has been in the silo for just a few days the mine will have to unload it into trucks and dump it on the ground so it can be rolled over and cooled. If there is spontaneous combustion within the closed silo the fire can be explosive, and catastrophic to the structure. The railroad pays a penalty for the dropping of the silo load and reload which is very expensive. Coal on the ground is carefully monitored and rolled or compacted as needed to keep hot spots from forming.
All these proceedures have been learned through experience. I am pretty sure the first power plants did not predict hot spots but quickly learned about them. Coal fired steam ships also learned how a hot spot in a coal bunker in the ship could quickly burn right through the bottom of thewood or steel hull if it did not explode through the deck. There are interesting naval stories about the transition from sail power to steam power. One theory is a coal bunker explosion was the cause of the sinking of the USS Maine in Cuba. &n