Alternative fuels in the cement industry

In the following, concepts are introduced based on the mechanical, physical and thermal processing of substitute fuels. With increasing international cost pressures not only the applications of alternative raw materials and fuels are substituted but increasingly also substitute fuels are interchanged by more reasonable offers. However, in order to be able to further increase the applications product and emission neutral, substitute fuel production and pre-engineering must be optimally co-ordinated. Among other things this leads to a retrofitting of the cement rotary kilns with appropriate technical solutions.

1 Initial position

Due to the continuing energy cost fluctuations and the discussions on the sustainable protection of resources the German cement industry was able to continually reduce its entire power requirement as a result of technical optimisation.

After the first oil crisis, for reasons of cost a switch was made first from oil to coal and, due to its high energy content and relatively easy handling, to waste oil and used tyres. Later, this was followed by solvents, bleaching earths and oil sludges. “Lignite-like” solid substitute fuels of production-specific commercial wastes, wood, sewage sludges, bleaching earths and so forth were processed by the main burner, the kiln inlet or – if available - the calciner. Since the use of individually processed wastes provably had no effect on the emission balance of the plant or on process engineering or product quality, the wastes may now also be mixed for processing and use.

Thus, after an initial interference and later urging of German politics (e.g. regarding the ban on the feeding of animal meal in 2000 or the implementation of the TASi (Technical Guideline for Residential Waste) in 2005), permission was granted to increase the use of substitute fuels continuously [1]. In 2010, the substitution rate on the national average of the VDZ member plants increased to 64.2% of the thermal power requirement (Fig. 1).

In the meantime, the substitute fuel production and use have been established to the point that especially countries, extremely dependent on external energy resources increasingly take into account integrated waste and energy concepts. In order to implement such concepts, naturally also cement plants come into focus. Frequently, however, this makes us forget that cement is a standardised mass product of a highly energy-efficient production process.

Generally, “substitute fuel” still implies cheap “disposal” to the extent that even legislation talks about “co-combustion”. But actually, these are highly complex physical-chemical conversion processes, which have not yet been entirely clarified even with regard to the combustion of coal [2,3].

Simplifying the diffusion controlled combustion process (Fig. 2), it can be described with drying, pyrolysis, ignition and burnout as well as the oxygen concentration at the fuel particle. With an increasing thermal substitution rate, this sequence dominates the entire cement-clinker process.

Usually, commercial wastes and particle fractions of high calorific value are separately or jointly pre-reduced and classified, removing metals, harmful constituents or PVC in the process.

Solid substitute fuels with an average lump size of dmax. 250 or 80 mm to be fed via the kiln inlet or a calciner of a rotary kiln, therefore, are subject to a different sample preparation procedure and quality management than those fuels, which must be fed via the main burner (Fig. 3). For a better burnout, these must be comminuted again into finer pieces (dmax. 25 or 3 mm) which are still several powers of ten compared to coal dust. If the processing should be finer yet, the processing reaches increasingly technical and economic limits.

Normally, in the cement plant, ready to be burnt fuels are transported pneumatically to the respective firing point. Especially, if they are fed via the main burner, the inhomogeneous substitute fuel mixtures are differentiated again into individual particle fractions burning at different levels: While thin, large-area particles (e.g. packaging film 2 - 500 µm thick) are consumed by the flame floating, three-dimensional particles (e.g. hard plastic, rubber, wood, etc.), form the tip of the flame or even fly through it ending in the kiln charge material and leading to a reduced clinker burning or the formation of sulphur cycles.

For these reasons, FuelTrack takes the approach of going over processed and quality-monitored fuel mixtures < 80 mm mechanically, while very large-sized substitute fuels > 80 to 250 mm with a known quality can be thermally processed.

2 Mechanical processing for the use in

a clinkering zone burner

Due to the insulating effect, the geometry and other surface effects, the diffusions in fuel particles and the combustion speed proceed at different speeds. Mechanically, the diffusion path can only be shortened or the surface of the fuel particles increased with increasingly finer grain sizes [4].

For this reason, using a certain grinding technology, so far not yet adapted to be used on substitute fuels, the average grain size of dmax. 80 mm can be reduced 100% to < 3 mm. The operating principle of the mill is based on a rotor turning at high peripheral speed (approx. 90 to 100 m/s) and a high air flow rate, whereby the air stream in the grinding zone leads to collisions between grinding element, wall and material being ground and thus to the comminution of the particles. Due to the enormous surface enlargement and the high air flow rates, this principle is basically also suited for drying. This effect can be significantly intensified by using the available waste heat. The former test results show that the operating principle of the vortex mill, originally designed for grinding food or inorganic substances, can also be used successfully for grinding solid substitute fuels (Fig. 4). Depending on the grinding resistance, reduction rates of up to >100:1 are possible.

Surprisingly, tests on different substitute fuel meals performed at the IEVB of the TU Clausthal showed ignition and conversion speeds more known to be associated with hard coal or lignite dusts (Fig. 5). The ignition temperatures (TZ) of the tested substitute fuel meals range between 680 °C and 711 °C and thus, are located in the transition zone between lignite dust at TZ 620 °C and hard coal dust at TZ 760 °C. Thus, the substitute fuel meals show an ignition delay less than the one of hard coal dust and insignificantly more than lignite dust.

This process is perfectly applicable to particles, which are three-dimensional, hard or brittle and so far have led to problems in kiln charge material. They are comminuted quickly and effectively, whereby thin two-dimensional flat particles are barely subjected to a comminution, which is also not required with regard to the burnout behaviour.

While grain sizes reduce 100% to < 3 mm in size and water content, the chemical properties of the substitute fuel components remain unchanged. The physical properties of the substitute fuel meal with regard to its flow and ignition behaviour are now comparable to lignite dust.

3 Optimised clinkering zone firing system

at the rotary kiln

If substitute fuels are to be fed via the so-called clinkering zone burner, they must be further comminuted to dmax. 25 mm with heavy media removed for an improved burnout. In order to react appropriately to different fuel properties, also the burner must be designed according to the requirements.

The clinkering zone burner POLFLAME VN proves to be especially suitable. It can be designed to burn primary fuels as well as refuse-derived fuels with a thermal output of 10 MW to 300 MW and has been especially developed for the use of solid substitute fuels (Fig. 6). Thus, with substitute fuels with an average grain size of dmax. <25 mm it has already been possible to achieve highest thermal substitution rates of 100%.

During operation, the primary air nozzles are selectively adjustable radially and - independently - also tangentially so that any random swirl and divergence are possible and can be adjusted to the flow path of the substitute fuel particle. These flexible nozzles act as injectors and thus, allow for a specific reaction to fluctuating fuel properties, whereby e.g. a formation of rings and undesirable coatings inside the kiln can be counteracted.

An adapted particle size and intense mixing of fuel and combustion air ensures a quick and complete conversion and significantly shortens the reaction zone. This behaviour is especially important for slow-reacting, difficult or slowly ignitable secondary fuels or even anthracite coal [5].

In order to be able to design the burner optimally, the multicomponent mixtures of the solid substitute fuels must first be fractioned in a sizing and chemically analysed [6]. In the process, the fuel particles separate into particle fractions (Fig. 7), in which the same properties result from equivalent solids of revolution, densities or grain shapes, i.e. the particles move on identical flow trajectories.

Now, this method results in statements regarding the flow behaviour or the quality of processing, the former composition of the waste - single components become clearly perceptible - and the suitability of the respective particle fractions for the desired feed point at the rotary kiln. At relevant individual particle fractions, it is now also possible to determine the oxygen requirement or the ignition behaviour [7].

4 Use of substitute fuels at the calciner

In modern plants with calciners only 40% of the thermal output is covered by the clinkering zone burner, while 60% of the thermal power requirement is covered by one or more firing points at the calciner. In the process, the necessary combustion air is fed via the tertiary air duct from the recuperation section of the clinker cooler to the firing points inside the calciner. With regard to reaction, the calciner requires only a temperature of 850 to 900° C to calcinate the limestone fraction. Inside the calciner, the 1000 – 1200 °C hot offgases of the rotary kiln and the 800 – 1000 °C hot tertiary air mix, whereby safe ignition and burning are also ensured by slow-reacting, large-sized substitute fuels.

However, especially the burning of various and slow-reacting fuels takes significantly longer than the preheating and calcining of the raw meal and is therefore the determining factor for the calciner‘s dimensions. For this reason, various suspension flow calciners are available according to fuel properties (Fig. 8). These will allow a control of the combustion temperature and atmosphere so that it is also possible to reduce the NOx emissions.

In order to ensure the burnout of slow-reacting substitute or primary fuels it is not only necessary to support the dwell time inside the calciner with a suitable distance required for calcination and burnout but if necessary, by using an additional burning chamber.

For a long time now, especially for slow-reacting fuels such as biomass, petroleum coke or anthracite, the CC precombustion chamber is used, where in the centre of a vortex flow and with an initial supply of pure tertiary air high temperatures develop starting the combustion. For the use of low calorific, large-sized substitute fuels the dwell time in the short hot zone is insufficient. Therefore, a precombustion chamber, the so-called “step combustor”, has been especially developed for the high feed rate of low calorific, large-sized substitute fuels (Fig. 9).

The typical retention time of 4 – 8 seconds of a previously described suspension flow calciner (cf. Fig. 8) is grossly insufficient for the safe burnout of such slow-reacting substitute fuels. Therefore, in the “step combustor” and as a function of the substitute fuel quality, the transport and discharge rates can be separately controlled, and thus the dwell time extended to a total of up to 15 minutes. An elegant side effect of the PREPOL SC is that due to the transport control by means of air blast nozzles no mechanical internal fittings or moving parts are necessary inside the reaction chamber.

Based on many years of experience in cement plant engineering Polysius AG is now able to offer its customers a future-oriented sustainable overall concept for the use of solid secondary fuels [8].

5 Handling and Storage

Cement processing requires a constant mass flow of equal quality of raw material as well as regular fuels; consequently AFR has to be pre-processed in an appropriate manner to get a constant quality and has to be handled in a constant mass flow, too.

Therefore different types of truck unloading stations had been developed as a compact reception device by Vecoplan (Fig.  10). Walking floor trucks or tipping trucks can unload solid alternative fuels, which will be transported to the following chain of storage and dosage. The receiving station is covered completely to keep the dust emission in the external area as low as possible. The truck driver initiates the truck unloading procedure, automatically also the material conveying to the silo box selected by the control room operator will start. After the rear doors of the truck are closed the device is in progress. The respective silo has to be selected by the control room before the material receiving can be carried out and the correct material is conveyed to the silo is is intended for.

The loading and discharge conveyor consists of a distribution conveyor with the drive that can be moved up and down with steel wire ropes using a lifting device with drive that has two different speeds. The storage usually filled and discharged with AFR material nearly simultaneously. Additionally, during filling or discharge the storage material could be sampled by a subsequent sampling station.

By means of the recent established joint-venture of Polysius and Vecoplan, FuelTrack offers the total chain from identification and sourcing, pre-processing to appropriate alternative fuels and raw materials for the preheater or refining for the main burner to co-process AFR. A tailor-made storage concept, quality monitoring and dosing arrangements up to adaptations of calciner, main burner or clinker production process itself completes the range from one hand for the clients target to save money as a long lasting effect [7].

www.vecoplan-fueltrack.com