United Kingdom Quality Ash Association
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Environment and Sustainability Issues
Overview of coal fired generation
The burning of coal for the production of electricity supplies around 37% of the UK electricity as shown in the adjacent graph. (Click the image for a larger version).
With the well publicised issues of security of electricity supply relating to natural gas, the issues and timescales involved for the development of new nuclear power stations and slow development of renewable forms of electricity generation, it would appear pulverised coal fired power generation will be necessary for some years to come. Even in future years the development of clean coal power stations with carbon capture will still lead to the production of ash. As there are estimated to be some 200 years of coal in reserves around the world, such an energy resource cannot simply be ignored.
Ash is an inevitable result of burning coal. It results from soil and minerals laid down with the coal many millennia ago. In a pulverised fuel coal fired power station there are two types of ash produced, Pulverised Fuel Ash (PFA), also known as fly ash in many countries, and Furnace Bottom Ash (FBA). They are used in differing ways as follows.
Environmental Issues
Furnace Bottom Ash
All the UK coal fired power stations have what's known as 'wet bottom' furnaces, where the ash is flushed from the furnace using water - see picture to the left. This means the FBA is washed in copious quantities of water making it suitable for use as an aggregate. Just over 1,000,000 tonnes of FBA is produced annually and virtually all the UK production of FBA for many years has been used in the manufacture of lightweight concrete blocks. Due to its method of production all leachable materials are removed and as a result there are no known risks to the environment or the user from FBA.
The beneficial effects of the use of FBA to the environment are difficult to assess. On a weight for weight basis and in comparison with natural aggregates the overall CO2 savings of using FBA are in excess of 200,000 tonnes per annum. However, FBA is not simply replaced with commonly available naturally occurring aggregates, because FBA is a lightweight material of density between 800 and 1100 kg/m3. One natural aggregate with similar properties are pumice or alternatively there is sintered PFA aggregate with similar properties. Pumice is not found within the UK and therefore has to be imported material, this adds to the environmental impacts associated with the transport issues and virgin aggregate depletion. Sintered PFA aggregates are manufactured using heat to fuse the aggregates, again something with an environmental impact. In reality the estimated savings of 200,000 tonnes of CO2 emissions are conservative and could possibly be doubled.
Pulverised Fuel Ash 
PFA represents the largest proportion of the ash produced from a power station, with about six times the FBA being produced. This is a fine powder (like talcum), grey to dark grey in colour. It is used in a wide variety of applications as described in our Technical Datasheets. However, for a variety of reasons not all PFA is used, with about half of the annual production being landfilled. From the following pie chart one (click to view as a larger image) you will observe that large proportions of ash are used in cementitious applications, e.g. cement manufacture, concrete addition, block making, precast concrete and grouting. 
The environmental benefits of using fly ash are in some senses is easy to assess. Invariably fly ash is substituted for either virgin aggregate or Portland cement in all applications. However, it can be difficult to decide whether cementitious benefits are being achieved in some applications, for example in PFA grouting.
Typical Applications
Typical applications for PFA as aggregate include;
Producing natural aggregate has a range of environmental impacts, depending on the extraction, crushing, screening, etc operations involved. An estimate for the factory gate environmental cost of producing 1 tonne of natural aggregate is ~21kg of CO2. As ~ 2,400,000 tonnes of ash are used, predominately as filler aggregate, we can conclude a reduced environmental impact of about 56,500 tonnes of CO2 per annum. However, the situation is not as simple as explained in the following example;
PFA grouts are preferred by the contractors because;
Using PFA in grouts has been shown to reduce vehicle movements by 40% and material cost by ~50% when compared to a Portland cement and virgin sand grout. By assuming PFA
is filler aggregate only based on current usage, CO2 savings of ~12,000 tonne p.a. are realised. However, by adjusting for the technical benefits of reduced Portland cement content, the lower density and reduced haulage, these savings could increase to ~40,000 tonnes p.a.
Some benefits are also apparent using PFA as a fill material. Aerated concrete block manufacturers use PFA as both aggregate and binder, with an aerated block containing ~80% ash. As the virgin alternative to PFA is ground sand, the CO2 reductions by using PFA equate to ~17,000 tonnes p.a.
Portland cement (CEM I), by the nature of its chemistry and manufacture, has a relatively high CO2 emission of ~960 kg/tonne. When PFA (fly ash) is used in concrete environmental savings can equate to ~20% and 30% reduction in overall CO2 for 30% and 50% ash contents respectively for mixes designed with equal 28 day strength.
About 400,000 tonnes of fly ash are used in concrete production p.a. this equates to ~250,000 tonnes of CO2 prevented from reaching the atmosphere p.a. In addition, the cement industry is adding fly ash to cement as a kiln feed material, as a Minor Additional Constituent (MAC) and in the production of blended fly ash cements. This further reduces the overall CO2 emissions in producing concrete, mortars, grouts, etc.
Overall
It is difficult to estimate the overall benefits of using coal fired power station products in reducing emissions accurately, but a reduction of ~600,000 tonnes of CO2 per annum has been calculated. As only about half of the PFA produced is currently utilised, this figure could be doubled in principle.
Sustainability Issues
In most applications fly ash replaces virgin aggregate or Portland cement, both of which have an environmental impact in their production and depletion of virgin resources. Fly ash is readily available as it has been produced since the 1950’s, with most of it being landfilled. However, much of this fly ash is no longer accessible due to station closures, with the sites being subsequently redeveloped. On the remaining coal fired power stations it is estimated there is some 55,000,000 tonnes of fly ash readily available and a further 60,000,000 tonnes that may be accessible if required. As power generation based on coal is likely to continue for many years, the resulting fresh ash production plus the existing stockpiles form a readily available mineral resource for future generations.
Much of this stockpile material would need some processing, such as extraction, screening, drying and possibly beneficiation, grinding or classification for some applications, e.g. for use in concrete. Such technologies already exist and, unlike some other secondary materials, importation is not an issue if fly ash availability outstrips demand. At the current rate of utilisation existing stockpile material alone could last for at least 30 years. It must be remembered that using PFA has many technical advantages, for example in concrete by reducing the risk of ASR, chloride ingress, etc – see Technical Datasheet 1.
In practical terms fly ash concrete gains strength with time and can be far more durable than implied from the 28 day strength, thus extending the life of the structure. By making longer lasting structures the environmental impact of building is diluted. Many structures are now designed with 100 year life in mind, in which fly ash can play a major part in achieving this longevity.
To take advantage of the benefits of fly ash concrete to ensure longevity requires considerable thought on the part of the designer, who must allow for flexibility of use and ease of maintenance. However, the Romans have proven longevity is possible with fly ash concrete structures such as the Pantheon. Built between 118 and 128 AD, the dome is a volcanic fly ash concrete structure that has stood the test of time. PFA structures can do the same and reduce the overall environmental impacts substantially in comparison with traditional construction.
July 2007