The gases coming from the combustion process of an asphalt plant transport part of the fine aggregate that were previously dosed. This generates the need to filter such particulates for their recovery in the mixing process and, also, to avoid harming the environment. For this purpose, the plants feature bag filters. The technology used in the filtering system has an impact on the equipment’s productivity, on the level of maintenance, and on the amount of particulates released into the environment.
The bag filter is made of filtering elements (bags) that have a special fabric that can resist the equipment’s operation temperatures, with a cylindrical shape and smooth, or pleated, surfaces. Image 1 shows the flow of gases generated in the burner going through the dryer, passing through the connection air piping between the dryer and the filter, and getting to the filter. At this point, the dust is retained in the bags’ fabric while the clean gases pass through the filters’ fabric and are sent into the environment.
In images 2 and 3, it is possible to see into detail the filter box with the bags and to have an internal view of the filter. The bags shown in these images are pleated bags and present a filtering area up to nine times higher than smooth bags. These bags have been developed by Ciber with the purpose of obtaining a large filtering area in reduced space.
The filtering capacity of a plant can be measured by dividing the total filtering area by the maximum production rate of the equipment. This capacity may have an impact on the plant’s productivity. As an example, a Ciber iNOVA 2000 plant presents a total filtering area of 1091 m and maximum production of 200 t/h. Therefore, the relation is 5.5 m/(t/h). This number means that for each one tonne per hour of the plant’s productivity, there is a filtering cloth of 5.5 m. The higher this relation, the longer the filter remains clean, because the bags’ lines are clean in a cyclical and guided fashion. In this sense, a cleaner filter allows the air to pass through the bags fabric more easily.
On the other hand, a dirtier filter creates a barrier to clean air passage due to the low relation between the filtering area and maximum productivity, increasing the loss of load or pressure of the plant. This pressure increase reduces the amount of air necessary for combustion, and there is a need to reduce the amount of fuel to be injected in the burner to maintain the constant ideal relation between air and fuel (stoichiometric ratio) that will affect the plant’s production ratio. According to Marcelo Zubaran, Application Engineer and Product Specialist at Ciber Equipamentos Rodoviários, “in those conditions, the plant may start the production at its maximum capacity, but it gets ‘out of breath’ throughout the work journey. For this reason, we say that the filter is the plant’s lungs”.
Regarding maintenance, the technology of the full filtering system has an impact on bags’ wear and, as a consequence, on the maintenance level. The first highlight is the static particulate separator, a component installed in the air piping (between the dryer and the filter), which separates, in a constant flow, the fines suctioned by the exhaust fan in two fractions – the filler, passing in screen number 200 (smaller than 0.075mm) and the larger fines (retained in the 0.075mm mesh). The last and more abrasive fines are forwarded to the mixer without passing through the filter. In this way, only the less abrasive fines are forwarded to the bags, reducing the filter’s wear.
Another feature is the system applied in plants that automatically controls the temperature of the gases that get to the bag filter. It is a closed-loop system in which the drying drum varies its speed according to the ideal temperature of the gases. This temperature is the temperature of water vaporisation (100°C at sea level). Below this temperature, water steam transported along with the gases is condensed in the filter, returning to the liquid state and creating a sludge in the filter fabric when mixed with the dust. On the other hand, the elevated filter temperature may burn the filtering elements, and also waste energy through the emission of hot gases into the atmosphere. This way, the filters will last longer, as they will never be out of the ideal operation temperature.
A third technological highlight is the electronic variation in the closed loop of the turning speed of the exhaust fan according to demand. The exhaust fan is responsible for the suction of the gases into the filter. Its turning speed is proportional to the gas speed. This technology precisely reduces the speed of the gases when the production rate of the plant is below maximum, without the interference of the operator, therefore always ensuring the lowest wear possible.
The filtering area also has an impact on maintenance, as the larger it is, the lower the wear, as the filtering “fabric” will be cleaner for a longer period of time. The size of the filtration area is also inversely proportional to the loss of fines into the environment. The filtration capacity of modern plants exceeds 99.99 per cent. The most rigorous environmental standards do not allow the emission of particulates above 20 mgN/m. Therefore, the larger the filtration area, the lower the emission of pollutants and the easier it will be to operate within environmental limits.
Filtration of combustion gases is necessary to maintain the burner fire in operation. The technology used is decisive for the excellent operation of the filter, regardless of the application. The lungs of the plant should be designed to keep productivity constant and at the maximum level, requiring less maintenance and resulting in less impact on the environment.