Castle Cement may have had the highest profile in the alternative fuel project, but it was not the first UK producer to consider alternatives to fossil fuels.
'Blue Circle experimented with municipal waste and old tyres back in the 1970s,' says Castle production director Peter Weller. 'And of course cement companies all over the world have been using all sorts of waste- derived fuels in vast quantities for many, many years.'
Weller's first real inquiry into the potential of alternative fuels began in 1990. 'It was obvious a recession was on the way, so the pressure was on to reduce energy costs,' he says. 'We started to pull together the work that had been going on in several of our works, and in April 1991 I asked the board for money to look at alternative fuels.'
Castle's drive to reduce costs was triggered by more than the need to match efficiency with other UK producers. Competition from foreign manufacturers is a constant threat even in the UK, and many of these were burning very high percentages of much cheaper alternative fuels.
Many national governments were encouraging the use of these fuels, seeing this as a more efficient and environmentally-friendly way of disposing of certain waste products than landfill or even incineration. As it turned out, many in the UK felt very differently.
At first, says Weller, Castle considered both solvent-derived and refuse- derived fuels, but laboratory trials soon showed that the practical problems with RDFs outweighed their theoretical advantages. Low alkali contents were no compensation for the need to mill the available fuel pellets down to a usable size at each works before they could be burned in the kilns.
'And there was a long history of the successful use of SDFs in Norway, which was very comforting, ' Weller adds. 'The only real problem seemed to be availability.'
It was initially calculated that only about 120,000t of suitable SDF could sourced in the UK every year. Long discussions with the waste recovery industry - and the then Her Majesty's Inspectorate of Pollution - eventually produced a detailed specification.
'By the end of 1991 we had a pretty clear idea of what we wanted, although at that time we believed we would only be able to replace a maximum of 20% of the normal petroleum coke,' says Weller. 'And US experience suggested that wet process kilns would be more effective than dry.
'Later, of course, we discovered that wet and dry kilns were different, but we had no particular difficulty with either.'
By early 1992 discussions with the waste recoverers had borne fruit, and Castle was ready to begin trialling the new fuel, by now known officially as Cemfuel. 'This was never meant to be its name,' Weller confesses. 'It was just the working title of my file on the project.
'The PR department was always meant to come up with a sexier name, but somehow Cemfuel stuck.'
In fact it was a very tentative toe that Castle stuck into the Cemfuel water. During the whole of the first year less than 3,000t of Cemfuel was burnt at the guinea pig Ribblesdale works. But the results were very promising, Weller reports.
'Clinker alkali levels reduced slightly and clinker quality improved - that is, it became somewhat more reactive. The only downside was that the kiln dust became stickier and harder to handle.'
Replacement levels were up to 35%. More importantly, emissions from Ribblesdale's wet kiln were well within the limits set by HMIP - effectively the same or better than those for fossil fuels.
Further trials on the dry process kiln at Ribblesdale were promising enough for Castle to plan to begin to use Cemfuel at its Ketton works. Then, in 1994, the company suddenly found itself embroiled in a major environmental dispute.
'Back in 1993 some of us couldn't even spell the word dioxin,' Weller says, 'Now we're all environmental experts.'
Castle was particularly hurt by accusations that the original Ribblesdale trials had been carried out in some sort of furtive, underhand way that kept the local community in the dark about potential risks.
'This is a total myth,' Weller insists. ' On 6 May 1992 we held a meeting with local councillors at the Ribblesdale works and told them exactly what we were planning to do.'
Castle had always relied on this liaison with local councillors as its primary method of communication with the general public. 'This time it failed,' Weller admits.
'At Ketton we did it differently, adopting a much more proactive approach, with more public meetings to establish an open dialogue with the local population.
'This didn't remove all opposition - but it did help a lot.'
Part of the problem, he says, was a common conceptual mistake. 'A 50% increase sounds like an enormous change to some people. But if a very minute fraction of the emissions increases 50% it's still a very minute fraction.'
Most protesters also apparently failed to understand that almost all the environmentally sensitive heavy metals actually came from the coal rather than the Cemfuel, Weller adds.
None the less, Cemfuel use at Ketton went ahead, successfully. Kiln performance was more stable, clinker quality more predictable. The success lead to other alternative fuels being considered, not least surplus tyres.
'More than 30M tyres have to be disposed of in the UK every year,' Weller points out. 'And calorific value is high - around 20% better than most coals.'
At first, he admits, the potential benefits of tyres seemed to be outweighed by handling problems. Trials at Ketton in 1997 resulted in Castle choosing whole tyres over shredded, even though this meant installing a complex tyre pre-weighing system to maintain a constant calorific feed rate. Results, however, 'are beyond my personal expectations' and in time all Castle's works could be burning tyres.
Also under consideration is factory waste paper and plastic. Calorific value is 'about the same as Russian coal', Weller reports, but the material would have to be processed down to a 3mm particle size on site. He adds: 'There is a huge volume of industrial paper, plastic and timber wastes available, far more than solvents.
'Trials of this alternative in Sweden have been very successful, and we should start experiments here in the UK early next year.'
Such fuels can be very cheap - less than zero in the case of tyres at the moment - but Weller stresses that efficiency is still more important than cost.
'We are a cement maker first, last and always,' he say. 'The key factors are always product quality, safety and emissions. We now know that Cemfuel is actually a better fuel for cement kilns than coal, and every other alternative will be judged against it.'
However, Weller points out that the high temperatures and long dwell times of cement kilns can be the best way of disposing of some difficult or potentially dangerous wastes. 'There are a lot of old tar and sludge lagoons around, quietly leaking into watercourses and aquifers. In a few years we could be burning these, to everyone's benefit.'