AU6279394A - An environmentally sound method of handling and utilizing effluent from the burning of coal in large industrial operations - Google Patents

Description

An Environmentally Sound Method of Handling and Utilizing Effluent from the Burning of Coal in Large Industrial Operations.

This invention is an improvement in the current process of handling effluent (flyash) from the burning of coal to generate electricity or for other industrial operations. The effluent from the burning of coal in large industrial operations like electricity generating power stations causes enormous uncontrolled air and water pollution from where the ash is dumped because it is not currently known, in an economically viable manner, how to separate the flyash, inside the industrial coal burning operation itself, into air polluting flyash, water polluting flyash and non air nor water polluting flyash.

The invention is irected to first keeping the light weight air polluting flyash separate rather than remixing them with the main flyash and or bottom ash as is the current process, then magnetically treating the main flyash without the remixed light weight flyash to remove the water polluting trace elements, which occur as contaminants of the magnetic as opposed to the non magnetic portion (subsequently converted to aerated concrete) of the main flyash. The current process involves the remixing of the light weight flyash with the main flyash and or bottom ash and then sluicing or water conditioning the remixed flyash and or bottom ash for disposal into ash ponds or ash disposal pits. The invention is described in detail in relation to effluent from coal burning from coal burning electricity generating power stations, but the invention is equally applicable to effluent from coal burning from other large industrial operations. CURRENT STATE OF THE TECHNOLOGY:

TECHNICAL & ECONOMIC ASPECT:

In modem coal burning power stations, around 20% of the coal burned ends up as residual effluent known generally as flyash and bottom ash. All over the world, coal burning power stations are organised to dispose of their residual ash rather than utilise it. This is primarily for economic reasons. In the state of New South Wales, from the approximately 6 million tonnes of ash produced annually, only about 300,000 tonnes are used and this use is mainly in the cement substitution market . The NSW Electricity Commission receives an average of around $5 per tonne or a total of some $1,500,000 per year for the sale of the said flyash. The use of flyash in the cement substitution market is likely to decrease. It is not good economics for the NSW Electricity Commission to design 7 power stations in the state for the purpose of supplying the total Australian cement substitution market of some 300,000 tonnes (30% of flyash produced at one of the 7 power stations) for a total return which is irrelevant in the context of disposing of the remaining 95 % of the flyash produced in the state. Thus power stations around the world are designed to dispose of their effluent as cheaply as possible usually in large ash ponds because of the large tonnage of unwanted, difficult-to-handle fine powder-like material involved.

It has been known to the power generating authorities that cenospheres or floaters (light weight particles) found at the top of ash ponds have commercial value. They have been harvested from a few selected ash ponds around the world for the last 30 years. The return to the few power stations from the sale of their floaters to industry has been in the order of around $10,000 per year for an extraction of generally less than 100 tonnes per year. These returns and quantities are irrelevant in the context of the total ash involved. Some power stations have also sold their bottom ash (residuals in the boiler itself) as coarse aggregates for a return of less than $5 per tonne. Again these returns and quantities are irrelevant in the context of the total ash involved. It has been known to the flyash industry that certain flyashes contain magnetite. Technically it is not a major problem to separate out the magnetite from the flyash using wet drum separation techniques. This can be done away from the power station sites if it is found to be commercially viable. In Poland in particular, it has been known at least since the 1970's that it is technically feasible, if not economically feasible, to separate the aluminium from the flyash.

ENVIRONMENTAL ASPECT:

In N.S.W. alone every year some 6 million tonnes of ash are sluiced with some 50 million tonnes of water and deposited into various ash ponds around the power station sites. At the proposed new power station at Mount Piper, near Lithgow in N.S.W., it is proposed to first condition the ash with water and then semi dry dispose of the total ash into a nearby unused mine site. It is claimed that this is the trend in ash disposal overseas and environmentally a better option than the current wet disposal method.

At each power station, hundreds of millions of dollars are spent to install either electrostatic precipitators or fabric filters in a vain attempt to stop polluting the air and water. But currently what is in fact happening is that the ash ponds are acting as concentrators of air borne particles, which accumulate at the top of the ash ponds as floaters and pollute the air in a large scale. The ash ponds also act as concentrators of dangerous trace elements which pollute the water streams and ground water systems around the power stations. Hundreds of millions of dollars per year per station are spent in capital and running costs with respect to ash ponds and water treatment plants associated with the current unsatisfactory handling and disposal of ash. The environmental protection authorities the world over have permitted this rather unsatisfactory method of ash disposal because to date there has been no practical alternative. The water conditioning and semi-dry disposal option as proposed for Mount Piper Power Station is faced with the same long term environmental problems as the current wet disposal option with little if any measureable cost savings.

Any innovation (wanting to be taken seriously by the environmental protection authorities and the electricity generating authorities) to improve the current universal practice of flyash handling and disposal must address the major environmental problems involved in the context of the total quantity and cost involved, and the practicality of incorporating the innovation within the context of flyash handling as part of the overall power generating operation. This innovation does that.

OUTLINE OF THE INVENTION:

This invention is the result of ten years of studying flyash, its chemical properties, its sizes and its specific gravities. Studying where a particular type of flyash particle occurs in the power station and in what manner the particles should be extracted so that it is within the constraints of the overall power station design and particularly the ash handling part. Studying what relevance particles extracted from flyash have to various niche markets.

Since to date the main use of flyash has been in the cement substitution market, flyash is almost always described with respect to cement i.e. as a fine particle product with about 70% by weight having a diameter of minus 45 micron, an average specific gravity of around 2 and a loss on ignition of around 1% from modem power stations. This invention discloses that flyash consists of a spectrum of particles with specific gravities in the range between 0.3 to 8.0 and sizes ranging from 0.3 micron to 1000, micron in diameter and that these properties rather than flyash's relationship to cement are the critical properties from an environmental and commercial point of view.

Power stations are currently equipped to handle flyash in a dry as well as in a wet state but not equipped or designed to separate the flyash into air polluting portion, water polluting portion and non air nor water polluting portion. This innovation is an improvement in the flyash handling part of the power station. Incorporation of this innovation will not only give the power station a net return (rather than net cost currently) of some millions of dollars per year handling about a million tonnes of flyash per year, but more importantly from an environmental stance, the flyash would be separated into air polluting portion, water polluting portion and non air nor water polluting portion at the power station itself and thus avoid the current uncontrolled separation of the flyash in an environmentally dangerous fashion at the ash disposal area.

AIR POLLUTING PARTICLES:

In modem power stations the early release ash particles accumulate at the higher levels of the power stations into containers known generally as economiser bins - generally four bins. The flyash from the economiser bins is currently piped downwards and remixed with the main flyash (collected in hundreds of bins generally at ground level) and/or bottom ash and disposed of into ash ponds or old mine sites, from which it becomes air bome and causes the major air pollution. It is not currently known that most of the air polluting (light weight) particles accumulate in the economiser bins. Nor is it currently known that the particles in the economiser bins are predominantly very light weight particles with specific gravities in the range between 0.3 and 1.2 and have great commercial value. Approximately 5% to 15% of the total flyash produced accumulates in the economiser bins, from an environmental and commercial point of view these particles should be separated out and harvested (not remixed as is currently done) in the following fashions.

a) Option 1.

Install two or more bagging machines just downstream of the surge bins associated with economiser bins and bag these particles for sale as ungraded lightweight particles. The said operation can and should be primarily gravity based.

b) Option 2.

Between the bagging machines and the surge bins, install self cleaning dry mechanical sieving machines to separate the particles into two grades. The particular sieve size will vary from power station to power station, but would generally be around the 100 micron range. Thus two grades of light weight particles are available for sale - a plus 100 micron and a minus 100 micron grade. This would give a much greater commercial return to the power stations, and again the said operation can and should be primarily gravity based.

c) Option 3.

It would be more profitable if the minus 100 micron particles are again dry sieved into plus 30 micron and minus 30 micron. But this would be technically difficult. Large modem power stations each produce around 100,000 tonnes per year of these air polluting light weight particles. The commercial value of these particles would vary from $Aus 100 to $Ausl000 per tonne depending on whether they are graded and the degree of grading. Incorporation of this innovation will give the power station a return of somewhere from ten million to 100 million dollars from the sale of these particles. More important than this, most of the air bome particles are removed from the residual flyash, thus making them much safer for wet or semi dry disposal insofar as air bome pollution is concerned.

2. WATER POLLUTING PARTICLES.

The chemical composition of flyash varies depending on the type of coal used at the various power stations. Generally about 80% - 97% of flyash consists of alumino silicate particles, and between 3% - 20% consists of various oxides of iron, calcium, magnesium, titanium, sulphur etc. But flyash also contains (this is known) a large number of dangerous trace elements like barium, strontium, chromium, vanadium, nickel, copper, lead, zinc, zirconium, yitrium, molibdenum, lanthanum, arsenic, tin, niobium, tantalum, tungsten, cobalt etc. Each of these dangerous heavy trace elements varies in weight from less than 0.002% to .2% of the total flyash. When flyash is sluiced with water and disposed of into ash dams, or conditioned with water and semi dry disposed of into old mine sites, these trace elements concentrate to dangerous proportions, due to the millions of tonnes of flyash disposed every year, and they contaminate the ground water system as well as water way system. It is extremely difficult if not impossible to remove these trace elements once the flyash is disposed of either in the wet form or semi dry form. This innovation shows how power generating authorities can remove these environmentally dangerous trace elements from their flyash at the power station itself and can do that sometimes at a considerable profit. These dangerous trace elements occur as contaminates of the magnetic and paramagnetic particles of the flyash and not the non magnetic particles. If the flyash is dry magnetically treated to separate the magnetic portion, most of these trace elements are also removed from the flyash. The residual flyash is significantly free of water polluting trace elements.

In modem large power stations some 700,000 tonnes of flyash are collected in the main flyash collecting bins currently situated at ground level. These bins should be positioned at the highest level feasible, around 5 to 10 metres high. Flyash from these bins should then be gravity fed to run over a number of dry magnetic drum separators which will separate the flyash into a magnetic portion containing the environmentally dangerous trace elements (bagged for sale) and a non-magnetic portion containing very little environmentally dangerous trace elements, for wet or semi dry disposal or conversion into aerated concrete products as per this innovation. The actual dry magnetic separation technology adopted will have to be fine tuned for the particular fineness of flyash encountered at the various power stations.

The flyashes from Liddell and Bayswater power stations in the Hunter Valley in N.S.W., contain around 10% magnetic portions. This magnetic portion has a market value of around $A100 per tonne. By magnetically treating the flyash at the power stations to remove the environmentally dangerous water polluting trace elements, as per this innovation, the NSW Electricity Commission will net around 15 million dollars per year. 3. NON AIR POLLUTING NOR WATER POLLUTING PARTICLES NON MAGNETIC PORTION

In NSW approximately 6 million tonnes per year of flyash is wet sluiced into ash ponds in an environmentally unsound fashion at an annual running cost (ignoring all the capital costs) of around $18 million. This part of this innovation, utilising the properties of fresh dry flyash, will show that power stations all over the world should be converting their flyash into aerated concrete products from an environmental and cost point of view.

It has been known in Europe for at least 60 years that fine sand or flyash can be converted via aluminum powder aeration into aerated autoclaved concrete, having specific gravity of around 0.5 and a compressive strength of around 3 MPA. It has been claimed that this technology has numerous advantages over aeration of concrete using foaming agents. Nevertheless, this technology has not been embraced by the power generating authorities around the world (with the exception of Poland, perhaps) for very good reasons. Using the aluminium powder aeration technology, it would cost the NSW Electricity Commission some 1.8 billion dollars in capital cost and about 1.8 billion dollars in operating cost per year to convert its 6 million tonnes of flyash per year into aerated autoclaved concrete, with a labour force of some 10,000 men for just this part of the operation. It would be difficult to incorporate this operation within the context of power generation, of which the ash handling and disposal is only one part. Furthermore, aerated autoclaved concrete products have been slow in getting market acceptance the world over due among other things to their cost disadvantage with conventional masonery products. The power generating authorities would be left holding mountains of very expensive masonery products year after year. Dry fresh flyash has air encapsulated not only inside the particles but also between the particles. This air between the particles is currently destroyed when the flyash is water conditioned for wet or semi dry disposal. This part of this innovation will show how this aeration between the particles can be beneficially utilised. Dry fresh flyash should be conditioned with just the right amount of water and a bonding agent like cement such that the aeration between the particles is not destroyed and the conditioned flyash has the plasticity similar to clay. The amount of water to be added will vary greatly from flyash to flyash, but it can be seen with the naked eye when excess water has been introduced and the aeration between the particles of the flyash has been destroyed without being able to be restored again. As a rough approximation (since it varies greatly from flyash to flyash) around 200 kilos of cement and 80 kilos of water would be required to condition per tonne of fresh dry flyash to aerated concrete, ready to be pressed or extruded into standard masonery products, using standard clay masonery products making equipment introduced downstream of the current water conditioning pans installed in semi dry disposal set up similar to that proposed for Mount Piper Power Station in NSW. The resultant aerated concrete products will have a specific gravity of around 1.2 and a compressive strength of around 5 to 10 MPA, and will get ready market acceptance because of its cost advantage over conventional masonery products. The whole operation will be environmentally sound, be less cumbersome than the current semi dry disposal set up and give to the NSW Electricity Commission a net positive cash return in the order of $5 per tonne of flyash handled (total of $30 million per year) rather than a net negative cash return in the order of $3 per tonne of flyash handled (total of $18 million) in the wet or semi dry disposal set up currently installed.

Claims (1)

Claims defining the invention are as follows:-
1. Claim 1.

   A new improved process of handling effluents of industrial coal burning operations, consisting of light weight air polluting flyash and main flyash which consist of a magnetic portion with water polluting trace elements as contaminants and a non magnetic portion containing aeration between as well as within the particles of the flyash; by first keeping the light weight air polluting flyash separate rather than combining the light weight flyash with the main flyash, secondly dry magnetically treating the main flyash to remove the water polluting trace elements and finally utilising the aeration between as well as within the particles of the residual non magnetic portion of the main flyash by conditioning it with a bonding agent to form aerated concrete.