Reliable feeding of alternative fuels

In order to burn cement with a persistently high quality, a steady kiln temperature is a rerequisite. This kiln temperature is mainly influenced by a regular, well controlled inflow of fuels. Since cement manufacturers worldwide are thriving for a replacement of primary fuels like gas or coal by alternative fuels, fuel dosing has become more challenging. Alternative fuels (AF) are cheaper but at the same time more demanding in dosing. The Pfister rotor weighfeeders are capable of handling the large variety of alternative fuels with great precision. Grupo Cementos Molins shares their experience with dosing alternative fuels in this article.

 

 

Cementos Molins is an experienced player in the cement manufacturing market. At their plant in Sant Vicenç dels Horts near Barcelona/Spain it is momentarily operating one kiln which is fired by a main burner and a precalciner. The plant is operating 24/7 and all year round except about 3 weeks for maintenance shutdown of the kiln. Both, main burner and precalciner are fed with petcoke as well as about 40 % AF. Due to general availability in the vicinity Cementos Molins decided on refused derived fuels (RDF), biomass, dried sewage sludge (DSS) and tyre chips.

This rotor weighfeeder with the name Pfister TRW-S and, for the explosion-proof version Pfister TRW-D, consists of a rotor wheel with chambers with a specific size and arranged in a proven way. The material is extracted out of a pre-hopper and is transported in the rotor chambers from the inlet of the rotor weighfeeder to its outlet. The rotor body is mounted on bearings which form a weighing axis. This axis is eccentric to the rotor shaft and goes through the middle of inlet and outlet. The third point is suspended at a load cell which weighs the content in the rotor wheel gravimetrically. The measured gravimetric force provides information on the mass of the respective alternative fuel in the rotor weighfeeder before material discharge. The rotor speed is controlled invers to the measured force which results in a highly accurate material mass stream at the outlet. 

“The challenge in dosing alternative fuels is certainly their wide variety in materials, humidity, forms and sizes”, explains Juan José Riesgo who contributed his expertise with alternative fuels to the dosing project at Cementos Molins. “AF certainly are more demanding to handle as, for example, petcoke. They can be sticky, they can be of odd sizes, they show all these material differences. These aspects can all be solved, but we have to have a very close look at finding the optimal solution.”

With the Pfister TRW-S and TRW-D two gravimetric feeders with an advanced technology were designed: Both models are able to handle even different AF. So, between 2011 and 2014 three AF installations were realized: The first step comprised an AF installation for the main burner TRW-D with a dosing capacity of 7 t/h. The second step followed suit and comprised a Pfister TRW-S installation for alternatives fuel to the precalciner with a maximum feed rate of 15 t/h. The third Pfister rotor weighfeeder TRW-D for AF was then installed in 2014 which was another dosing system to feed the main burner. Additionally, an increase in the feed rate at the pre-calciner is under planning: “We’ll use an existing Pfister rotor weighfeeder at the main burner installation to feed the precalciner. We’ll modify only the feeding shoe to increase the output at 7”, explains Gonzalo Lopez.

Bottleneck: temporary shortage of AF

Of course, fuel cost is always dependent on the momentary market price of the respective fuel. Oil and gas were cheap buys in the past few years but are getting more expensive again which makes working with alternative fuels more interesting again from the economic viewpoint. “We know that in a midterm aspect replacing primary fuels by AF is saving us money”, says Gonzalo Lopez, Head of Engineering Department at Cementos Molins. Besides, AF have the “green aspect” as they help saving our natural resources or even prevent CO2 emissions. 

Virtually any cement manufacturer worldwide nowadays thrives to increase their AF rate as much as possible. Alternative fuels are wide in their range and come in a large variety of forms, sizes and material properties. As stated above, it makes sense for cement manufacturers to use those kinds of AF which are locally available. As dried sewage sludge (DSS) turned out as an ideal fuel for the main burner, Cementos Molins is using this fuel plus biomass there. Additionally, the pre-calciner is fired with refused derived fuels (RDF), biomass and tyres. A momentary shortage in DSS due to the closing of one of the three treatment plants in the Barcelona region unfortunately prohibits Cementos Molins to use as much DSS as they would like. At the same time, the fact that usually one supplier cannot supply the complete amount of AF needed by a cement plant another aspect appears: Every supplier or treatment plant respectively is producing AF with sometimes slightly or even markedly different material properties, even in the same kind of AF. Cementos Molins, for example, is dealing with different batches of granulated DSS humidity ranging from 10 – 15 %.

Future-oriented dosing technology

The challenge of odd and much differing sizes even of one kind of AF was already mentioned before. As alternative fuels also tend to be highly abrasive the plant managers have additionally installed anti-wearing pipes at the curves and about 500 – 700 mm after. “We are a modern, future-oriented company”, points out Gonzalo Lopez, “this is why we are planning to increase the amount of alternative fuels in our production process even further.” Pfister dosing systems are able to handle all these alternative fuels with one and the same systems. ”Altogether we have a very good performance of the Pfister rotor weighfeeders for alternative fuels which delivered sustainable productivity to our business and cuts cost.”

Grupo Cementos Molins

The activity of Cementos Molins Group focuses on manufacturing, distributing and selling cement, concrete, mortars, aggregates and concrete prefabricates, and running activities and production. Molins operates plants in Spain, Argentina, Uruguay, Mexico, Bolivia, Columbia, Bangladesh, India and Tunisia. With an annual output of 1,300,000 tons of clinker the plant near Barcelona/Spain is one of the largest of Grupo Cementos Molins. 
cemolins.es

Author

Juan José Riesgo, Master of Mining Engineering, Regional Sales Manager 
Pfister® Weighing, Feeding and Dosing