Power to your PROCESS

The philosophy of `if it's not broke, why fix it' is often good advice, but as machines reach the end of their working life, a breakdown becomes all too likely. Failure of one of the engines in the generating plant at Severn Trent's Stoke Bardolph sewage treatment works prompted some important decision making.

Gas from the sludge digestors at Stoke Bardolph has been used to generate electricity since the mid-1960s. Using the gas, a mix of methane and carbon dioxide, as a fuel has the attractions of saving money, reducing dependency on other sources and making use of something which would otherwise go to waste, with the consequent environmental advantages.

Although Stoke Bardolph is not completely self-supporting in energy, for some 30 years the plant has provided much of the power needed on site to operate equipment, and heat is also recovered from the generators to warm the digesters and keep them working at maximum efficiency.

Stoke Bardolph serves a population of about 454,000 in Nottingham and much of the surrounding area, with a contribution from industry equivalent to an additional 250,000. There are four sludge digesters, which together produce about 19,000 m3/day of gas, which is piped to the generators.

Housed in a purpose-built hall, the original plant consisted of an array of six dual fuel (digester gas/diesel) engines, which were continuously operated on the basis of four on, one on standby and one on routine servicing, generating 1.4MW in total.

Since power requirements were automatically topped up from the grid in the event of a shortfall, the plant was, in effect, an add-on to the normal treatment processes. The engines were always well maintained on a day-to-day basis, but as Brian Muggleton, assistant manager at Stoke Bardolph admits, after 30 years continuous use the engines had `more or less reached their sell-by date.'

Early in 1995 the crankshaft of one of the engines failed suddenly and catastrophically, making the question of their replacment much more urgent. Whilst the company capital investment cycle is automatically initiated on the basis of deteriorating condition and performance, it was immediately clear that one of the options was simply `nil investment' - to abandon power generation on site. Justification for a further investment would lie in the environmental and economic benefits.

The environmental benefits of maximising the use of the digester gas were apparent. The digesters would still be producing gas, but if on-site power generation was discontinued, only that burned in boilers to heat the sludge digesters would be used. A significant proportion (50 per cent) would then have to be wasted to atmosphere or burned off.

One of the first moves from the team under assistant project planning manager Dave Gillespie, was to undertake a full cost/benefit analysis of the possible financial consequences of discontinuing generation on site and taking all electricity from the grid as against the costs incurred in replacing the existing plant. The `do nothing' alternative might save on investment, but the possibility of future increases in electricity prices would have to be taken into account.

At the same time, Mike Harrington, an engineer in the project planning department, began to look into the engineering implications of replacing the existing plant and the types of engine now on the market.

There have been considerable technical advances in engine design over the last 30 years. Engines have become smaller and more efficient. The original engines required diesel pilot fuel, but since they were installed engines using electrical ignition have become available.

The amount of digester gas available can fluctuate, and new engines had to be capable of adjusting to this. It was also important that they should fit onto the existing engine bases without the need for modification. The existing generating hall was in good condition, but if the replacement engines produced more vibration, new anti-vibration pads would be needed and this could add to the cost.

However, when all the various factors were taken into account, the final result demonstrated that replacement of the engines would produce a 20 per cent return on the investment, and the project was given the go-ahead. The contract was placed on a design and install basis, using the specifications worked out by Mike Harrington. Six contractors were asked to tender, with the successful contractor being Birse. The engines used are Deutz TBG620V12K, spark-ignition, high speed, and turbo-charged, designed to fit the particular requirements of the specification

The power generation capacity of the new engines is much greater than those they replace, with the result that only two have been needed instead of the six in the old array. And they are each rated at 941MW, between them producing almost 25 per cent more than the original plant. No standby unit was provided as part of the new concept, as the view was that the revenue cost of importing more power to cover downtime would be less than the capital cost of an additional plant.

Efficient as the new engines have proved to be, they do have some drawbacks. They require gas at a higher pressure, and pressurisation equipment has had to be installed. The new engines, although smaller, are also noisier, producing something in the region of 98-105 dBA, and acoustic hoods have been erected to reduce the noise to acceptable levels.

Work on site began in summer 1997, with the old engines sequentially taken out as the new ones were installed and commissioned.

The units are fully automatic and operate continuously with one shutting down if levels of gas drop below a level. There is an electronic supervision system linked to the power management and SCADA systems, and operation is also monitored and controlled visually by staff 24 hours a day.

With the two Deutz engines now producing close on 1.9MW, compared to 1.4MW from the old plant, the amount of electricity taken from the grid has been further reduced, which Dave Gillespie estimates should produce savings of £50 - 80,000 a year.

The new generating plant has cost nearly £1.3million. `We had the advantage of a building already on site, and much of the pipework and electrical connections were already in place and just needed reconnecting, which helped to keep costs down,' points out Dave Gillespie.

Severn Trent reckons that even at present electricity charges the costs will be recovered in 3-4 years. And with a projected 15 year life for the installation, any future savings could be well worth the investment. PE