LIVING WITH ASH
June 26, 2014 § 1 Comment
Fly ash has been in the news since the Dan River, NC contamination incident earlier this year. Much of the attention has been on remedying the current situation: ash in unlined pits, especially proximal to surface water. This is appropriate because future contamination events from existing disposal sites need to be prevented. Current proposals place the timing of resolution out in the fifteen year timeframe.
So, what happens to all the fly ash produced in the interim? It could go to lined pits. In this regard there is similarity with the measures for temporary storage of liquid drilling wastes. Neither has been classified as a hazardous waste by the EPA. But in current draft NC legislation, liquid drilling wastes will be required to be stored in double lined pits with sensors between the layers. The same could do the trick for fly ash in solid or liquid form. The former is vastly preferable. This is because fly ash is light and fluffy and comprises spherical particles. This material will stay suspended and not easily settle to the bottom as sludge for removal. Bottom ash, the other type of ash in a coal combustion unit, is more amenable for pond settling.
Beneficial Use of Ash:
All coal deposits contain a certain proportion of minerals associated with the coal. These are oxides of elements such as Silicon, Potassium, Iron and Calcium. Many of these are in the clay family. When the coal is combusted these oxides remain inert. They end up in the bottom of the retort (bottom ash) or fly out of the top (fly ash). Fly ash constituents have a unique character: they comprise small spheres. As a result the material is light and fluffy. Transportation could result in dusting.
Fly ash falls into two classifications: Class F and Class C. Both classes have oxides of the same elements noted above, but Type C will have substantially more lime (CaO). When blended with water the mixture of oxides will form a substance not unlike cement. To get the same cementing consistency with Type F one needs to add lime when completely replacing portland cement. But in either case the material can very usefully replace cement in part or whole in the preparation of concrete. Concrete is essentially sand and/or gravel bound together with cement. Displacing cement is good for the environment because the manufacture of cement is a major producer of carbon dioxide.
Concrete prepared with fly ash based cement is believed to have improved properties over the conventional material. In particular the cement flows better than portland cement in part because of the spherical character of the particles. Improved performance is also claimed for bricks made with fly ash, lime and gypsum. Over 10% of fly ash in India (20 million tons per year) is converted to this product with the strong backing of the World Bank.
Hurdles to Use
Why, then, does not all fly ash get used in this fashion as opposed to being placed in landfills or ponds? One reason is that the producer may not be located close to the potential user and the cost of transport of this low density material could be high. There is also the risk of dusting. Fly ash can have trace amounts of Arsenic, Vanadium and other heavy metals. The original coal is the source. While representing possible hazards in transport, in the use in concrete they are benign. These elements, more than likely in the form of oxides, can be expected to be trapped in the concrete. They will not be subject to leaching because they are in the concrete body. But even when the concrete is converted to rubble at the end of life, they ought to remain in a form not subject to leaching in large measure because they will not be water soluble.
The second, and more important, reason for limited use of fly ash is the carbon content in some ashes. When the EPA introduced laws to reduce oxides of nitrogen (NOx) in the flue gases, the result was more unburnt carbon in the fly ash. When this material exceeds about 6%, it is not acceptable as a cement substitute. Two distinct types of technologies exist to overcome this problem. One is to physically separate the unburnt carbon using for example froth floatation or electrostatic methods. In most such cases the carbon is recycled for use in the combustion process. The other is to perform a controlled burn of the carbon usually utilizing the heat in some way. One elegant technique is to use microwaves. The energy is absorbed only be the carbon and not the ash constituents. This is similar to the fact that in a microwave oven the food absorbs the energy (gets heated) while the ceramic container stays substantially cool. In fact the industrial process can use the same frequency as household ovens, thus making it inexpensive because those components are mass produced.
What North Carolina Ought to Do
According to published reports very little if any of the fly ash produced in North Carolina is being used in concrete manufacture. One manufacturer testified in a state senate hearing that he was importing fly ash from other states for his use. A cursory examination of the fly ash from Duke Energy plants indicates that the majority is Class F. It will need the addition of lime (unless blended with regular cement), but concrete manufacturers actually prefer Class F because Class C ash can harden up spontaneously when wet, whereas Class F can be controlled. The carbon removal will require expense and the resulting sale of the fly ash may not always prove profitable. Of note is that Wisconsin recycles on average 85% of the ash and the national average is over 40%. So the economics are likely not prohibitive in all instances. Policy support from Raleigh could help. Any fly ash not going to land fill is a good thing.