Multi-fuel systems are the future

July 17, 2017 Thomas Jennewein

Alternative fuel expert Thomas Jennewein from FLSmidth Pfister® talks about trends in secondary fuels.

Thomas Jennewein shows two examples of alternative fuels. On the left is an alternative fuel, a mixture of wood chips and rubber, which might be used in a calciner. On the right is a very light alternative fuel consisting of RDF mixture of plastic, paper and shredded carpet, which might be used for the main burner. 

The use of oil and gas as a fuel continues to be a heavily debated topic worldwide. These primary sources of energy release carbon dioxide when combusted, which is extremely damaging for the environment with disastrous effects on climate change. Furthermore, it’s quite possible that oil and gas prices will rise again in the not-so-distant future. The use of alternative fuels is therefore perceived as a crucial step forward for the cement production industry, not only to reduce carbon emissions, but also to maximise the return on investment. Thomas Jennewein, expert in alternative fuels at FLSmidth Pfister, discusses his view on the current market.

In comparison to using primary energy sources for heating purposes, what challenges does the use of alternative fuels pose?
Considering that coal and lignite as well as oil and gas have been used as an energy source since the industrial age, their processing has been optimised so that these fossil fuels are available in standardised forms. Additionally, they are currently very low priced, they have beneficial material properties, such as their handling ability, and the calorific value is fairly steady all year round.

Alternative fuels on the other hand, are considerably different. Prior to processing, alternative fuels are an undefined material mixture, often collected on a landfill. That requires one or more processing steps in order to turn it into a useful energy source. In the cement industry, alternative fuels are often received from a number of different suppliers and it is no surprise that their behaviour changes from charge to charge. Humidity and density also vary substantially depending on the origin of the fuel, so foreign bodies can never be fully excluded.

In this installation, rotor  weighfeeder Pfister® TRW-S is utilised for the feeding of fluffy and dusty alternative fuels with a feed rate of 8 TPH. A homogenisation bin is installed above the red rotor weighfeeder. 

How can this challenge be addressed?
Dosing systems must be able to handle different levels of humidity and density as well as varying levels of quality, even for the same material. Additionally, one should keep in mind that recycling companies inevitably produce different quality output from one day to the next. For example when shredding, the blades wear out over time. This influences the results of the cut.

From small to large: Pfister® rotor weighfeeders for alternative fuels come in different sizes depending on the requirements of the specific application.

The combination of these factors means that we cannot simply use customised weighfeeders, which are only able to process one type of material. Instead, we need “multi-fuel” weighfeeders, capable of processing the extreme variance and inhomogeneity of alternative fuels. The rotor weighfeeder Pfister® TRW-S (the ‘S’ refers to the secondary fuels) is exactly such a device. Its pre-hopper is mounted directly at the inlet of the rotor weighfeeder and is equipped with a stirrer to prevent adhesion of the secondary fuel, while simultaneously loosening and homogenising it. Its rotor resembles a circular conveyor, consisting of an ideal number of big rotor chambers, and its robust construction is made especially for the dosing of various types of alternative fuels. All Pfister rotor weighfeeders are fitted with the control strategy ProsCon®, which allows the system to react to variations in flow density or changes in flow behaviour, even before the material is discharged, resulting in material continuously being fed into the firing process.

What can the cement industry do to improve the quality of alternative fuels?
During onboarding, fuel suppliers undergo several checks and audits regarding particle size, chlorine content and calorific results as well as humidity and granularity.

But often, this is not enough. In addition to these inspections, most cement producers also employ an oversized magnetic separator to guarantee the quality of the material, although foreign bodies still cannot be fully excluded. Receiving, feeding and weighfeeder systems should therefore be employed as they do not have narrow gaps and their engines are equipped with an overload protection.

There are several factors to consider. The goal of any cement manufacturer is to produce clinker of a satisfactory quality, meaning not only is the calorific value and the price ratio of interest, but also the chemical properties of the fuels related to the respective raw material. In order to reach a high substitution rate, several different alternative fuels are used within a cement plant. Many cement plants are able to apply ten or more alternative fuels. The choice of alternative fuel depends either on the respective availability or the cost/performance ratio as well as the chemical composition of the fuel.

How would you describe the current market for dosing systems for alternative fuels?
The market for alternative fuels is currently difficult because of the low oil prices. In order to start firing with alternative fuels, a plant first needs to invest in alterations such as the building of storage, transport and dosing and modifications of the combustion process. In the past, these investments would have to be amortised within less than ten years – and not long ago, less than five. The pressure has clearly increased.

On the other hand, the whole industry has committed to a considerable reduction of carbon emissions and an important step towards this goal is the use of alternative fuels. Here, I would point out that the process starting with the initial decision of using alternative fuels to the final implementation can easily take several years due to necessary approvals, investment planning and implementation of the fuel supply.

Can you identify any trends in employing alternative fuels in the cement industry?
There appear to be two opposing trends. First, some cement plants are using an increasing amount of fluff, a fuel with very short burning time and therefore suitable for the main burner. Here, the substitution rate could be increased even further at a number of plants. As this dried fluff shows a high proportion of dust, ATEX conform and pressure-proof dosing systems should be employed. Because of the high calorific value and the low bulk density, I can only advise using highly precise dosing systems to guarantee stable fuel processing.

The contrary trend shows that some cement plants try to apply extremely coarse fuels in the calciner. Although this reduces the effort of preparation and makes it more lucrative, it clearly proves a bigger challenge for fuel storage, transport and the dosing process. Unless significant building extensions are undertaken, this trend will remain limited. This is because the retention time – the time between feeding the material into a modern calciner until its complete burn out – is only between eight and ten seconds. In the case of only one pre-shredding device being employed, “burning chambers” are necessary, which would require an extensive re-build of the calciner.

If only one shredding step is applied, more and larger particles remain. In order to guarantee smooth and trouble-free operation, conveying and weighing systems are needed to allow big chunks to pass through without blocking.

What would you recommend to cement plant operators regarding the use of alternative fuels?
The installation of multi-fuel systems is clearly the tendency. With small alternative fuel substitution rates, a simple dosing system might be enough as opposed to a costlier rotor weighfeeder. But the question is why not invest in something decent, capable and future-oriented right from the start? Especially considering the cement industry is aiming for higher alternative fuel substitution rates. These will never be handled to a satisfactory level by volumetric dosing devices, so if an investment has to be made in a few years, would it not be wise to invest in the right system from the beginning, such as the gravimetric rotor weighfeeder Pfister® TRW-S? These devices are multi-fuel per se and able to handle a larger range of materials with different flow behaviour.

Considering that the quality, availability and the prices of alternative fuels are constantly changing, this is an important aspect for cement plant operators to consider.

A selection of alternative fuels: Chunky, fowderous, fluffy, light, heavy - The variety is large.

Thomas Jennewein

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