Sleaford ‘mods’ save costs in filter house

Sleaford Renewable Energy Plant (REP) is a multi-million pound investment in the local economy and in carbon-neutral energy production. The 38MW plant created nearly 300 jobs during construction; and now – while saving around 50,000 tones of CO2 per annum – the REP supports employment for over 80 locals: including 30 on-site operatives, and more than 50 people in the supply-chain.

And it’s not only jobs that the plant provides to the local surroundings. The REP generates electricity for 65,00 homes in the area, with surplus heat being harnessed and provided free of charge for district heating in local buildings: like Sleaford’s public swimming pool, a bowling centre, football club, primary school, and the council offices.

The plant uses proven technology aimed at the clean and efficient combustion of straw, the by-product of wheat production. The majority of this fuel is secured from farms within a 30-50 mile radius, though it can also burn woodchip biomass as an alternative. All waste ash produced by the plant is recycled as crop fertiliser.

The process of heat and power generation

Agricultural waste products, in the form of straw bales and woodchip, are delivered to the power plant. There, a specialised fuel feeding system transports the biomass fuel to the boiler’s grate where it is combusted at about 1,400°C. Water circulates through the boiler’s evaporator system turning it to steam, while internal superheaters raise the steam temperature up to 540°C. The resulting steam drives a turbine to produce electricity for the grid, and any surplus heat is harvested. VEGA products are used across the site for applications like level in woodchip silos, chute and conveyor blockage detection, and point-level switching.

Figure 1 One of the many filter bag frames being monitored

Cleaning the waste gases

After any excess heat is recovered, the flue-gas is cleaned via a grid of bag filters. This process involves the cooled combustion gasses passing through banks of hanging bags, leaving the ash residue on their surface. The system consists of 8 banks of cylinders, each bank has a row of 8 vertical bag filters, each approximately 3m long. Each row is connected to a solenoid-actuated pneumatic cylinder, which agitates the row with a short blast of air, causing the dust to fall into the hoppers below. These hoppers are then emptied into adjacent ash silos via a screw feeder.

Each of the banks of 12 solenoids operate off 8 common accumulator manifold cylinder. The pressure in this cylinder supplies the air to the bank of bag-cleaning solenoids, each of which are triggered in sequence. Any failures cause a deviation in pressure, indicating a potential bag problem or valve failure. Originally, these were fitted with very simple pressure switches, which would give an alarm output if low or high pressure thresholds were exceeded. Faults and failures are not an uncommon situation across 96 continually operating valve sets. In the event of an issue, engineers on the plant would have to closely check each pressure switch to see their status and fault-find. It was time consuming and caused delays, reducing throughput and increasing downtime for the plant.

A visible benefit

Engineers on site had the idea to modify the existing pressure switches and replace them with a new pressure sensor they had seen from VEGA, which seemed perfect for these dimly-lit and less accessible areas of the plant. The VEGABAR 38 features a super-bright display that can be programmed to change colour to show the actual pressure status. It also has pressure-switch outputs that can be calibrated to create an alarm contact for the control system to warn the operators, so it saves costs for separate local annunciator panels. When an engineer needs to investigate in the filter house, the colour easily and quickly directs them to exactly where the issue is.

Figure 2 Bright lights quickly show the sensor status and where an issue arises

“This simple idea gives us a big advantage in a small but frequently visited area on our plant” said Steve, the site engineer. “It means we can quickly assess the status of each bank and isolate it if required, so the plant can continue to operate while we carry out maintenance or repair to get the full system up and running again. The bright colour display with a readout is perfect for this application.” Since implementation, the engineers have bought a further set of sensors to install on the adjacent, parallel filter system.

Figure 3 VEGABAR Sensor/Switch monitors 12 solenoids in each bank, with easy set up and simulation via Bluetooth

The VEGABAR pressure sensor with backlit display has integrated switch outputs and a 256-colour selectable, super-bright LED. It can have up to 5 individual colour/flashing status lights and is easily set up via Bluetooth, using VEGA TOOLS App on a smartphone, tablet or PACTware on a PC. It offers pressure ranges from 100-1000mBar with a wide selection of interchangeable process fittings. Additionally, a simulation function in the app means it’s really easy to test the system operation without the need for a pressure calibrator.

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