How to get started with alternative fuels

Cement plants are capable of consuming a wide variety of alternative fuels, in line with local availability, plant requirements and emissions limits.

by Hubert Baier, WhiteLabel-TandemProjects UG

When considering wastes from municipal sources, mechanical biological treatment (MBT) plants, are often used to separate different fractions. Unfortunately, there have been numerous MBT investments that have not been properly designed, either from a technical perspective, a financing perspective, or both. Such projects can play into the hands of other sectors and can result in a lack of future investors. For these reasons, all parties need to understand each other’s needs, as well as the process itself. This article outlines ways to increase the chances of success in such projects.
 
The cement industry is increasingly seen as a pillar of sustainable waste management as a user of so-called refuse-derived fuels (RDF). Popular opinion is that this is a cheap energy source that the cement industry will snatch up with the palm of its hand.
However, this simplistic view makes a number of false assumptions. The confusion starts with an imprecise choice of terms. Firstly, ‘RDF’ is a term that is often wrongly used to describe any waste-derived fuel (WDF). This incorrect use of the term RDF fails to define the material in terms of its origin, how it has been prepared (if at all), its quality characteristics or even what purpose it is intended to serve.

 

Below - Figure 1: Converting waste into customised AFs as well as managing its properties requires knowledge from the waste and the recovery process. Only then can and must the processing plant be properly designed.

 
When it was realised by the waste-processing sector that something was missing, five quality classes were hastily invented and became norm EN 15359: 2011, to let the consumer know what properties and pollutant loads the fuel would have. Unscrupulous players have taken advantage of the confusion to market unprocessed wastes as ‘fuels’ or ‘products’ in order to pass on the waste buck to someone else or to circumvent customs regulations. Such practices unfortunately continue in several regions.

Prior planning prevents poor performance

The proper production of solid alternative fuels (AFs) from materials of recycling facilities (MRF) or commingled municipal solid waste (MSW), must take place in several treatment stages. The MSW is separated in Mechanical Biological Treating (MBT) or Biological Mechanical Treating (BMT) plants. These separate organic fractions, impurities and valuables such as recyclable plastic foils and metals, from the combustible high calorific fraction (HCF). This HCF can then be used directly in a pre-combustion chamber in a clinker kiln calciner or as a feedstock for the production of low-grade fuel (RDF) or downstream to high-grade SRF (Figure 2, Overleaf).
The cement industry is the best-known co-processer of AFs. This is great for solving waste issues and provides many cement plants with both cost savings and lower CO2 emissions. However, the fact that so many projects take place in this sector also means that it makes the most AF-related mistakes. Some in the sector are still confused regarding the definitions of different types of fuel, often using the simplified term ‘RDF,’ and too few pay attention to common pitfalls  in AF use. This is because popular opinion still expects treatment plants to cost next to nothing to build and run, that the cement kiln can accept any fuel thrown at it and that cement plants will accept any fuel offered. All false assumptions.
Time and time again, this author has seen examples of hastily-constructed MBT plants that do not take into account the realities of the waste and cement plant in hand. Even in Germany, it is possible to find examples of MBT plants that were not able to produce AFs of sufficient quality to meet the specification of its local customers. In one case of an MBT plant making RDF, none of the six cement kilns in the vicinity operated with a calciner or a chlorine bypass. This, to be polite, was an ‘unfortunate oversight,’ as kilns that only have a preheater, or those that operate using the semi-dry or wet process, cannot use RDF. This meant that the fuel produced by the MBT plant could not be used. It had to be extensively redesigned to produce customised SRF to serve the main burners of the available clients.
In another example, a lot of money was invested in the construction of local MBTs in an Eastern European country. However, each produced useless RDF because the customer requirements had not been properly considered. Some local cement plants continued to import alternative fuels from the UK, while some built their own dryers to improvethe quality of the locally-produced material.


Types of waste-derived fuels (WDF)
 

HCF: Fraction with grain size of 300-800mm and calorific value of 14-16MJ/kg. Can be used in add-on combustion devices such as HotDisk or Step Combustor.

RDF: Low-grade fuel, with long burnout time due to its relatively large size and low calorific value of 16-19MJ/kg. Used in the calciner.

SRF: High-grade fuel, free of 3D particles, with a calorific value of >20MJ/kg. Used in the main burner and / or sintering zone.

Assessment of the clients’ needs

Without proper analysis, it is not possible to assess exactly how tolerant any given kiln will be to the introduction to AF. Even when investors are in good communication with potential users, a wide range of parameters, including gas flow characteristics, moisture levels, thermal energy demand, oxygen content at the point of application, etc., must be taken into account. If the conditions at the feeding points are properly researched, the pre-processing plant can be designed properly so that it provides fuels that fit the bill.
Of course, this also requires preliminary work to assess the available waste in order to identify its quantity, moisture levels, calorific value, types and levels of impurities and so on. This will greatly increase the chances of success and reduce instances when fuels are rejected by the cement plant, to the benefit of all parties. Crucially, the more attention that is paid to both the technical assessment of the cement plant and the waste assessment, the more effective the overall solution will be.

Assessment of the waste composition

An important starting point for co-processing is to determine the thermal potential, quantity of waste and its composition in the areas from which it will be sourced. The waste streams in question should be analysed by sorting with regard to their proportions of usable HCF and their unusable components such as organics, metals, glass and other impurities. This process should take into account the regional characteristics such as whether the source is rural or urban. Seasonal variations and other culturalspecifics may also need to be taken into account.
Plastic materials are more or less comparable around the world. The main components of the HCF include polymers such as LD/HDPE, PA, PET, PVC, as well as paper, composites and textiles. The proportion of each will vary by location.
Unfortunately, organic matter in the same source material will carry high water content, as will saturated paper, cardboard, textiles, etc. These have to be removed or dried, at additional cost. Furthermore, the inputs of non-volatile and product-relevant and emission-relevant volatile heavy metals must also be determined. The sources of alkalis, chlorine and sulphur inputs as well as the grain sizes of individual fractions must also be determined. These data must be validated with the results from a technical assessment conducted by the cement plant or other offtaker.
Remember that these data will also become the basis for the required permits for both the waste processor and the customer. Dubious values, estimations and values sometimes falsely taken from literature or determined from one individual analysis, must be questioned vigerously. Otherwise the effective use of alternative fuels will be hindered or even prevented altogether. If this step is skipped or rushed, it will come back to haunt the cement plant during its discussions with the permitting authorities.
 
 
 

Figure 2: Processing mixed solid waste (MSW) in a pre-processing plant plant is a complex process in which alternative fuel outputs must be carefully defined. The strategy and target(s) must be defined early on in the project so that the technical concept, required equipment and financial aims can be carefully assessed.

The purpose of a pre-processing plant

Once the parameters of the incoming waste and the needs of the cement plant client(s) are known, the MBT plant can be designed. Such facilities are essentially ‘splitting plants’ that increase the recycling potential according to the results of the waste assessment and technical assessment. They separate pollutants and interfering substances, recover recylable materials if necessary, separate organic matter for composting and produce demand-oriented AFs out of the combustible HCF. It is obvious that the processing quality and depth of each step will vary depending on the goals.
With increased focus on plastic pollution and reduction in CO2 emissions, MBTs are increasingly becoming ‘servants of several masters’ and are increasingly viewed as important links in the interplay between governmental waste and climate policy. In fact, the treatment of solid waste in MBT plants has wide-ranging CO2-reduction potentials, derived from several ‘avoidance potentials.’ These include the avoidance of fly tipping, the avoidance of diffuse emission of methane, the avoidance of landfilling untreated waste and a reduction in waste volumes due to rising disposal costs. The separate fermentation of organics and the recovery of recyclable materials such as biomass for composting, paper, plastics, metals, from the upstream are shortened or even eliminated. The use of alternative fuels with ‘CO2-neutral’ portions of biogenetic products made of cotton, cellulose, rubber, etc, is made easier. Finally, producers will avoid extracting and using traditional (read ‘fossil’) fuels.
With the help of a mathematical representation of an MBT, the mass flows and properties can be simulated in advance. Last but not least, there will be a non-negligible quantity of impurities that still need to be disposed of in a sanitary landfill or
be incinerated.

The most popular mistakes

The sustainable production of viable AFs is characterised by the following steps and the avoidance of common mistakes:
 
Bag opener: To enable efficient segregation in the sense of cleaning, all MSW must be broken down. In some cases, ‘bag openers,’ which have been developed for the manual sorting of recycables are misused for comminution of the waste for cleaning. This does not work! A high-performance shredder is recommended for shredding solid waste, including the impurities.
 
Segregation: It has also been shown that popular drum or trommel sieves are best for composting. This is due to the fact that they are designed for the separation of bimodally distributed heaps. Trommel sieves are not suitable for efficient separation of organic material, inertia (stones, ceramics, soil, etc.) and other impurities from MSW with an unclear particle size distribution. This is due to the formation of so-called ‘snakes’ or ‘pigtails.’ These still contain impurities that later turn to ash, thus reducing the throughtput capacity. They have to be shredded again for a sufficient degree of segregation in order to ensure the required HCF purity downstream. Moreover, they actively utilise only ~12% of their tubular screening surface. Flat screens require a higher substructure, but make a better use of their total available screening area for an efficient separation of oversized HCF from impurities.
 
 

Above - Figure 3: Compost sieves do not meet the intended separation effect when processing MSW and may end up as expensive conveyors.

Air separation / wind sifting: Efficient air separation requires requires particles in a narrow particle size distribution range that can be turbulently separated from each other at appropriate wind velocities. It relies on materials that settle according to grain shape or grain size and small differences in density in a sufficiently large settling chamber.
Popular and compact adaptations based on demolition waste sifters do not fulfil these physical requirements due to their design. In many cases they simply end up as costly conveyors. When sifting, a previously determined degree of freedom must necessarily be fixed, i.e. the range of grain size should be kept as narrow as possible by efficient screening in order to be able to separate light HCF from heavy impurities more effectively. The market now offers a range of interesting innovations to do this job. They all have one thing in common: They obey the laws of nature! Any new approach needs to look deeply into the physical principes of the separation. Avoid equipment with combined effects or with additional auxiliary internals, such as separating drums, nozzles, baffles, etc.
 
Metal separation: Magnets are well known for removing ferrous metals and are often employed in large numbers. What always surprises the author is that they are often installed at 90° to the direction of the discharge belt. Magnets are normally more efficient at the point that materials leave the belt.
Eddy current separators should always be mounted after magnets, because iron disturbs this effect. The author has often heard spurious reasons why this order is not obeyed. A popular excuse is that the system had to be assembled in a volume that was approved with the authorities before the specifics of the system were fully defined. Who is the plant for? The authorities?
 
Near Infra-Red (NIR) sorting: The separation of PVC by NIR serves only to remove organically-bound chlorine. Inorganic chlorides, such as table salt cannot be detected or removed by means of NIR during pre-processing. Therefore, the customer’s tolerance for chlorides must be determined before the pre-processing plant can be designed. Does it already have a chlorine bypass or should it really install one?
 
Fine shredder: If any paper, cardboard, textile or other material saturated with moisture has survived the upstream process, it is now reduced to a particle size of <30mm. A deeper look into the combustion kinetics shows that all fuel particles have to pass through the same sequence, but with different time requirements: Drying, pyrolysis, ignition and burnout until the components are used up will define the shape and length of the flame.
As these steps influence the temperature profile and the clinker formation process, large combustion surfaces are mandatory for a proper conversion of SRF. If the moisture content in SRF is still unacceptably high, the alternative fuel must be dried at the conditioning plant or at the side of the kiln using available waste heat. This may require adjustments to the plant that need to be clarified during the technical assessment. In both cases, special attention must be paid to vapour management.

Above - Figure 4: The screened combustible oversize will be sufficiently cleaned by sifters that can separate and settle the material according to small differences in density in a sufficiently large settling chamber.

3D-separation: This separation is mandatory for suitable SRF and can be ensured by the use of a so-called ‘Rocket Mill’ as a robust fuel mill with a drying effect, or by air-classifying fluffy particles that will offer a large reaction surface. Note that size reduction and screening can even be counterproductive here, as the remaining 3D particles still do not get enough time to burn out due to their unfavourable surface/size ratio. They end up unburnt in the clinker bed, leading to well-known and troublesome reducing conditions.
 
Stockpiling: When we feed a cement plant with more SRF, the kiln often loses its tolerance to quality fluctuations in a big way. Homogenisation and quality control are therefore key to safe and stable kiln operation. Docking stations with moving floor trailers, originally installed for ‘experimental purposes,’ are becoming insufficient and should ideally be replaced with proper storage facilities with ‘police filters’ and homogenisation capabilities.
 
Quality assurance: When the thermal substitution rate (TSR) is at a low level, the pyro process can still be fairly tolerant of quality fluctuations, although experienced operators will already see the first signs of destabilisation. The higher the TSR, the greater the potential for quality fluctuations. Quality surveillance is therefore essential for the party preparing the fuel, any supplier and the cement plant user. Only proper control of the contractually-agreed specification, the biogenic portion and the degree of homogenisation in the storage (especially if there are several suppliers) will keep the system running. This will also include the determination of the trajectories of SRF particles when leaving the burner’s tip to burn in the kiln.
After a TSR of around 50% at the latest, quality management becomes more than just a nuisance at the cement plant, it determines the entire kiln process. An apple picker or bucket is no longer enough for representative sampling and analysis. Test routines in the pre-treatment plant, as well as at the cement plant, have to be tuned with regard to standards, acceptance, billing and even penalties.
 
Conclusions
 
There are many potential pitfalls when starting out with alternative fuels in the cement sector, especially when using fuels produced from municipal solid waste. The technical and financial aspects must be carefully considered by all parties in order to work to realise the project’s aims. Be careful to ensure that your project doesn’t join the list
of failures!

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published: Global Cement Magazine AFSI 2021, 12|2021
Keywords: Energy Recovery, Material Recovery, Germany