To comply with our permit, the plant must burn waste hot enough and long enough to ensure that everything is reduced to ashes. Once the burn has finished, the smouldering ash is quenched in waste water then sent by conveyor belt into the ash bay. A magnetic belt removes ferrous metals from the ash and piles them separately. The black-grey gravel-like substance that remains is known as IBA — incinerator bottom ash.
I’ve almost finished my apprenticeship, but in the meantime I’ve become a Compliance Technician. My job is to ensure that the plant complies with the permit issued by the Environment Agency, which means following all the laws and technical guidance on what the plant burns and how; limits and monitoring of stack emissions; bottom ash, metals and air pollution control residues; waste water discharges; noise, smell and pests.
In the first week of my new job I get a phone call from the company who purchase the IBA, informing me that it is no longer of acceptable quality, and they are sending a wagon-load back to Sheffield.
I talk firstly to the contractor and the operations assistants who load and handle the ash:
“It’s no worse than normal, I’d say these loads are ok.”
“But what about the unburned waste in the ash?”
“The company would normally take it. Sometimes they’d separate the unburned waste and return that to us, but not whole loads.”
The plant operators tell me they’ve had problems with blocked ash dischargers on the night shift, which can cause a build-up of waste on the grate. If the waste isn’t spread thin enough, oxygen pumped beneath it can’t access the top layers of waste, which dampens the combustion. Every morning we receive a shift log from the previous day, and we’ve read the reports of operators running up and down stairs with poles and hooks trying to free a blockage before the fire goes out.
“We’re coming close to an Outage,” says N “so the boiler is not in good shape. We’ve got some grate bars missing as well, which means we have unwelcome air ingress and unburned waste falling through the riddlings – this also affects the furnace temperature.”
There are ongoing complaints about the quality and quantity of the waste in the pit.
“We just have too much of it!” says B. “It’s wet, it’s been in the pit a long time — there’s so much we don’t have room to mix it — it’s starting to decompose in there and it just burns badly.”
I ring the company back and explain the plant’s problems with combustion.
“Tonight we don’t anticipate any problems with the dischargers.” I reassure their technician. “We’ll take back the unsatisfactory load, but we don’t anticipate problems with tomorrow’s ash.”
• Emma Rickman is an engineer in a combined heat and power plant