Diary of an engineer: Control and the pollutants

Submitted by AWL on 27 October, 2020 - 2:36 Author: Emma Rickman
Engineering plant

There is talk among the operatives that the Environment Agency will be setting new emissions limits across the UK. Our daily limit for sulphur dioxide (SO2), which causes smog and lung irritation, is already lower than most power stations because we’re based in a city — but not by much. SO2 emissions may be reduced across the board, regardless of plant location.

Another monitored pollutant is Nitrogen Oxide (NOx), which irritates the lungs, affects soil chemistry and reacts to create ozone, a more powerful pollutant. The plant controls NOx emissions by spraying urea into the furnace with long probes. The urea breaks down NOx into nitrogen and oxygen, with a by-product of ammonia. The Environment Agency is considering placing an emissions limit on ammonia, which damages ecosystems, irritates the eyes and lungs and smells strongly of piss. Our plant has a problem, as our control measure for one pollutant, NOx, will be creating more of a different pollutant, ammonia.

The control room has run some tests over the last month, reducing the emissions limits and compelling the system to pump in more lime and urea to control the gases. Sometimes our waste ash has been so thick with ammonia the buyers refuse to collect it, but we’ve found that NOx can be reduced up to a point without an ammonia spike. The question will be what reductions the Environment Agency will order.

Another problem we have is with dust spikes in the air filtration system. Our maintenance team have been up at the top of the plant all week, lifting lids on cells of tubular bags that hang vertically over rows of hoppers. The bags are caked with a lime and carbon mixture that absorbs pollutants and particulates. The gas is supposed to flow through these bags and exit the chimney clean, however dust is getting onto the clean side and setting off alarms.

To get to the bags we have to break the air seal on the cells using a small crane and lots of crow-bars. Once the lid is off we shine strong torches into each bag looking for tears or build-ups of dust. We unbolt the air lines that criss-cross the bags and then prise damaged bags out — it’s an 8-meter long wire frame wrapped around with a long yellow canvas sock. B shoves a long screwdriver through the frame and then uses the crane to tug it up while we dislodge it with bars; once loose, we pull the length of it over our heads and shake white powder over everything. G and M diligently vacuum everything as we go, so that no leaks are missed.

B tells me — “The key is not to drop anything into these hoppers. We’ve dropped bags into these before and its a nightmare getting them out.”

As we’re closing up, G drops a bag into the hopper, and everyone laughs. It takes some time for the bag to fall through the hopper and onto a screw-conveyor beneath it. J puts the conveyor into manual, and we open the doors, dumping a heap of lime onto the floor. We can see the bag wrapped tightly around the screw; I lever it with a bar from one end, stepping out the way of falling lime, and M and G pull from the other while J nudges the conveyor forward in increments.

“Come on J!” G shouts, “Keeping it coming!” At the end it’s like a tug-of-war, with all three of us staggering backwards when the bag comes free. We put the lids back on the conveyor, put it back in automatic, and go to change our overalls.

• Emma Rickman is an apprentice engineer at a Combined Heat and Power plant.

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