Waste & recycling - Rubbish as a resource06 June 2005
Rubbish is just waste, isn't it? Rubbish! It has significant value as a source of energy, according to a report from the Institution of Civil Engineers and the Renewable Power Association. The report says that if all residual waste were to be sent through a waste-to-power process, this could meet as much as 17% of the UK electricity needs in 2020.
Looking at just UK municipal solid waste, the report Quantification of the Potential Energy from Residuals (EfR) in the UK says the gross thermal value of this waste is 304,000TJ for 2005, with a theoretical absolute maximum electrical yield of 27.8 Terawatt hours. The total electricity consumption in the UK for 2005 is estimated at 342.8 TWh. This is quite apart from the energy savings available by recovering secondary metals and using the residue as aggregate for construction, instead of having to crush rocks and transport them over long distances.
"Nowadays, we tend to think of rubbish as a source of value we can recover," explains Matt McGeehan, finance director of LondonWaste, one of the two waste-fired power generation and recycling facilities in London.
Each year, in comes 1.3m tonnes of residual waste - from which householders have already hopefully removed cans, paper, plastic bottles in some cases, and other recognisable recyclables. Out goes about 250,000MWh of electric power, 15,000 tonnes of scrap iron, a quantity of aluminium, and an increasing range of other products.
The plant was set up in the 1960s, so the base equipment is mostly old. A glance at equipment name plates shows many great British engineering companies of yesteryear - Vaughan Cranes, AEI which made the turbines and generators, and Yarrow which supplied the roller grates and boilers. The business - and its approach to new opportunities - however, is very much up to date.
As a fuel, residual waste has a calorific value of about 11MJ/kg, about half that of coal. Martin Sides, the power plant operations manager, adds that he sometimes blends in high value commercial waste. The waste is delivered into vast concrete-lined bunkers, from where it is fed by overhead cranes into the furnaces whose fiery hearts run at more than 850º C gas temperature. The heat energy from the waste is converted into steam at 40bar, originally at 450 to 460º C, but now reduced to 390 to 400º C to improve superheater life, which is evidently more important than the slight consequent reduction in power output.
The turbines convert the steam energy into electrical energy and produce a maximum output of 50MW. Between 15-18% of this output is used internally in the plant. The steam from the turbines is condensed and returned as feed water, with losses made up from a demineralised water plant. The water for the cooling towers to condense the steam also runs in a closed loop, with make up provided from a nearby brook. The bottom ash from the furnaces goes to a plant on site that makes graded aggregate for construction.
About 10% of the company's revenue comes from electricity. McGeehan said this proportion would be higher were it not for electricity deregulation which had caused prices "to plummet". The rising price of fossil fuels in the UK could, however, cause this situation to change just as dramatically.
Scrap metal is recovered in four ways. Source segregated aluminium cans are delivered from the London boroughs to be bulked before being sent on to secondary aluminium smelters. Mixed, bulky material is sorted when it is received on site. Vat magnets remove iron from the bottom ash residues from the furnaces. And metal is also recovered from the graded aggregate.
LondonWaste is a public private partnership. Half the non executive directors are appointed by the North London Waste Authority, made up of seven North London local authorities that send their waste to the plant. Half are private, appointed by SITA (UK), the owner of the other half of the business. The operation has stringent health and safety and pollution control policies - McGeehan says: "We monitor our flue gas all the time and keep detailed records which are available on the Public Register."
Flue gas from the furnaces passes first through an electrostatic precipitator followed by the injection of lime and powdered active carbon (pac). The lime absorbs hydrochloric acid, gas and sulphur dioxide, and the pac traps heavy metals. The operation is undertaken in a quench tower at 140º C, where the gas temperature is reduced to 120º C before the lime and pac is injected. The resulting air pollution control (apc) residue is recovered in bag filters. And dioxins ? often a criticism of waste incinerator projects - are not a problem. Emission levels, including dioxins, are strictly regulated and most pollution parameters are continuously monitored with results available on the Public Register. The apc residue has a pH of 11 to 12 because of the lime. This material still goes to landfill, but the aim is to lower the pH by mixing it with acid waste.
LondonWaste occupies a 43-acre site and plans to use all of it. The European Union, the UK government and the London boroughs are all setting increasing targets for recycling. Indeed, the London borough of Barnet looks to be the first on our shores to follow the German example of fining householders who put recyclable items into general rubbish instead of separate containers.
LondonWaste already treats medical waste and incinerates sensitive documents, and issues certificates as evidence it has done so. Latest ventures include shredding wooden pallets, with the shredded wood used to make block board and biofilters. The company has just obtained planning permission for a composting plant that will reduce 30,000 tonnes of green waste per year and produce 20,000 tonnes of compost. Other ventures still in the pipeline include using the adjacent canal to transport waste and/or recyclate.
Getting up to date
Opened in 1994, is another plant in Deptford, southeast London, owned by South East London Combined Heat and Power (SELCHP) and operated by Onyx SELCHP. Like LondonWaste's facility, it delivers up to 230,000MWh of electric power and can process up to 420,000 tonnes of waste per year, but occupies only a five-acre site. It has just two boilers and one steam turbine, plus a plant control system supplied by ABB, which is being upgraded to meet the latest emissions reporting requirements of the European Union Waste Directive.
Being a more modern plant, it uses Martin reverse acting stoker grates from Martin Gmbh of Munich, instead of roller grates. This patented design is used all over the world. It succeeds because it is able to apply a controlled amount of agitation, sufficient to keep the charge in motion, but not so much that parts of the charge can be carried up in the flue gas.
Facility manager Stephen Tower says that fluidised bed grates work very well provided that the charge is very carefully prepared. The charge has to cover a fairly narrow particle size range, otherwise fine particles get carried off in the flue gas stream, while coarse particles have insufficient motion and are liable to fuse together, clogging the whole thing up. (Anecdotal unofficial reports of user experiences with refuse-fired fluidised beds elsewhere confirm this, but Plant Engineer would be interested to hear from any readers who might have a different story to tell.)
Tower speaks highly of his Martin grates, which, he says, achieve a very high degree of combustion, with only tiny amounts of carbon monoxide and unburned hydrocarbons in the flue gas, while enabling responsive automated control. This is achieved by varying the amount of grate movement and fuel feed rate in response to measured oxygen concentration in the combustion gas, and furnace temperature as determined by a pyrometer.
Apart from this, the flow sheet is not dissimilar to that operated by LondonWaste and many other companies, with one or two exceptions. The ash from the grates is not treated on site but goes to a Cleanaway site in Rainham, Essex, where it is aged for two or three months, prior to grading. The iron is then removed by magnets and the non-ferrous metals by inducing eddy currents. This is a joint operation between Onyx, one of the owners of SELCHP, and Hanson. In addition to injecting active carbon to trap heavy metals and lime to react with acid gases, SELCHP also injects ammonia to react with nitrogen oxides resulting from the high combustion temperatures used.
Another conceptual difference, as the company name implies, is that the plant was established not only to generate electric power, but also to supply heat to domestic housing, although this is yet to transpire. The building intended to house the heat exchangers to supply heat to a large area of social housing nearby is being used as a store room.
However, Tower says that it may still happen. The social housing is currently heated by burning fossil fuel. However, consultancy PB Power has been commissioned by the Greater London Authority to study the feasibility of creating a heat grid for London. Like McGeehan of LondonWaste, Tower complains about the low prices and trading arrangement for selling electricity, but does concede that the selling price of power has been increasing over the last 18 months and looks set to stay higher. Since failure to deliver the contracted amount of electricity leads to a penalty related to the cost that London Electricity has to pay to purchase electricity on the open market, producing and selling heat would complicate the operation. While some of the heat would be otherwise wasted, some might normally be used to raise steam to generate electricity. Hence a high heat demand in cold weather could lead to the electricity cost penalty, and in warm weather, surplus heat would be hard to sell.
Facts and figures
Emissions from the plant are "miniscule" and during our visit Tower was happy to show Plant Engineer the figures for the previous day. In mg per cubic metre, they were: HCl, 16; nitrogen oxides 319; sulphur dioxide 10; carbon monoxide 4.5; ammonia 2.7; and organic carbon 0.3.
These figures are to be further reduced and extensive work is underway to increase bag house capacity by 50%. This will have the effect of reducing pressure drop, and allowing more time for contact between the injected lime and the acid gases - HCl and sulphur dioxide. This should bring emissions down to well below the latest, stricter European Union Waste Directive requirements, which require HCl to be below 10 and nitrogen oxides to be below 200.
One way to drive efficiency of the operation still higher would be to improve the software in the model-based control loop. Since ABB is continually making strides in this area, doubtless further improvements will be incorporated into the plant as soon as they are sufficiently well proven and commercially available.
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