Matthias Mersmann, Chief Technology Officer of KHD Humboldt Wedag International AG Sven Schmitgen, Product Manager of Humboldt Wedag GmbH
Development of co-processing in the cement industry
Burning waste derived fuels has a tradition spanning over seven decades in central Europe and has developed into a specific field of competency in the cement industry with massively increased knowledge and experience. Using a variety of different wastes to process them to a wide range of waste-derived fuels (WDF) has turned out to rush over markets in waves, triggered by the coincidence of several enabling and forcing factors within the local ecosystems of the market regions. The most basic enabling factor for co-processing in cement plants is the actual availability of sufficient waste material in the region/country where the cement plant is located.
If sourced from municipal solid waste(MSW), there gion or municipality needs to have a respective collection system implemented to allow centralized pick-up for pre-processing, which is a basic but mandatory prerequisite to be able to use MSW in cement production. This requires the socio-economic system to have developed to a certain stage.
The next forcing factor is then facilitating the use of MSW in bigger amounts when dumping of MSW gets expensive as a consequence of ever more stringent environmental rules in further developed countries. It is being recognized all over the world that raising the substitution of fossil fuels by alternative fuels (AF) in cement kilns goes along with the development of the societies’ awareness to reusing waste and technologies for its pre- and co-processing.
Properties and burn-out behavior of AF
As much as AF differ in their chemical and physical properties, as much do they vary in their denominations: WDF, MSW, RDF (refuse- derived fuel) and fluss are only a few of the name tags which are being attached to alternative fuels. Meanwhile some attempts for standardization have been undertaken, but so far none of those was adopted on a wider, global scale.
From a technical point of view, the chemical and physical properties of AF are much more relevant than those denominations. Looking at the ability of certain WDF to replace fossil fuels, we must accept one very basic understanding: None of the WDF components which are burnt in a cement kiln were created with the intention of being used as fuel. This means that their chemical, physical and particulate properties are far away from matching any specifications of materials that were created or treated to be fuel.
What is generally accepted for fossil fuels, namely the necessity to pre-process them to reach the required properties, varies heavily for fuel alternatives. While coal is being ground to very fine powder, WDF are sought to be burned in lumpy particles. While coals are being classified into internationally accepted categories as to their main combustion properties like ash-content, moisture, content of volatile matter and heat content, there (currently) is no way to generally classify these properties for the heterogeneous mixtures of material which make up WDF. Material (e.g. wood, plastics, textile, rubber, paper, glass, mineral etc.), size (small, medium or large), shape (flat, round, cylindrical, cubical etc.) and chemical properties all vary across their categories and form a unique combustion behavior, leading to very different ignition and burn- out values every time. The only constant is, that AF is always more challenging to burn because solid fuels come in bigger particle sizes than fossil fuels, liquid WDF always come with either lower heat or higher contaminant content.
Investigations of the ignition and combustion behavior of certain typical constituents of WDF mixes have shown that, depending on size, shape and material, individual pieces of AF can take up to several minutes to burn out. (ZKG Magazine 7/8, 2019)
Interaction of pre-processing and co-processing
It is important to look at the complete processing routine from the source of waste material all the way down to its final use as fuel in a cement kiln. The entirety of this chain has been found to be described best by the terms pre-processing and co-processing. Pre-processing defines all preparational steps until the injection of material into the kiln. Co-processing on the other hand cover the energy- and mass-integration of the material within the clinker production process.
The interaction between pre- and co-processing is vital to successfully use WDF in cement production, because both influence costs and applicability. A commonly used path in pre-processing is, for instance, sorting and size reduction to produce high-quality RDF. High-quality RDF may consist of mainly light weight, dry and small particles with high calorific value. This RDF can be injected into either the main burner or the calciner without much technological adaptation to the co-processing technology (kiln burner or calciner). This path requires a high effort in pre-processing, yet a low effort in co-processing.
The other extreme would be to invest only little effort into pre-processing by just sorting out the inert matter and then use this low-quality WDF in a special reactor which is able to efficiently co-process even low-quality WDF.
The optimal pre-processing depth, resp. the ideal point to switch from pre-processing to co-processing depends on a number of case-specific influences: available co-processing technology at the cement plant, available RDF supply, RDF pricing, to name just the most important ones.
Many cement producers have experienced that the markets for WDF do change over time when looking at availability, price and quality. There is a trend perceivable for more versatile and robust special equipment in cement plants, which allows a multitude of different WDF of changing qualities to be co-processed repeatedly with predictable, good results. All major OEM equipment-suppliers are nowadays offering products and technologies that are intended to especially use WDF with low pre-processing levels. Good technologies must offer a robust design that prevents operational problems and good combustion conditions and long fuel retention time to ensure complete or at least sufficient burn-out and clean combustion of AF.
Pyrorotor® – the most versatile AF-reactor
KHD’s Pyrorotor® is a rotary combustion reactor that can use waste materials with inferior burning properties sustainably and efficiently as fuel in the cement production process.
Picture1: Overview of Pyrorotor®
With in the range of solutions for co-processing of AF in the cement production process, the Pyrorotor® covers the demands for highest thermal substitution rates (TSR), even for lowest-quality AF. Picture 2 compares KHD’s product line-up for AF usage. The Pyrorotor® offers highest flexibility in regards of the size and types of alternative fuel that can be processed. Whole tires, tire chips, coarse and lumpy materials, hard-to-ignite materials, or even contaminated and hazardous materials can be combusted sustainably. Complex and expensive pre-processing of the sourced AF can be significantly minimized or even avoided entirely.
Picture 2: Simplified classification and requirements of typical AFR to be used in a kiln burner and different Pyroclon® calciner add-ons
The Pyrorotor® can be installed in a new production line or easily retrofitted to an existing one because it neither requires space in the existing preheater tower, nor does it add additional load to it. The mechanical concept of the Pyrorotor® has been based on the most robust equipment known and trusted in the cement industry for centuries: the rotary kiln.
The rotating drum allows intense mixing of the AF with hot tertiary air, as well as long retention time to ensure complete burn-out of the used waste materials. At the same time, the constant movement avoids build-ups and clogging. All mechanical parts have been tried and trued in hundreds of kiln installations. The rotary tube is safely supported and balanced on two roller stations. The required torque for the rotation of the drum is induced by two friction-driven rollers. The installed drives offer a large reserve of torque and were engineered to repeatedly handle a wide and varying range of loads and rotation requirements.
The rotational speed of the drum can be adjusted in the range of 0.3 rpm to 3.0 rpm to adapt the retention time to the needs of the specific material. AF is fed into the Pyrorotor® via sluice systems like rotary airlocks or double pendulum flaps to prevent false air from entering the system.
Picture 3: Pyrorotor® components
Picture 4 is a gas flow sheet that depicts the Pyrorotor® positioned between the rotary kiln and the calciner. A controlled portion of tertiary air is branched off to provide the required flow of hot air to the Pyrorotor. Depending on type of fuel and any desirable combustion, the combustion process inside Pyrorotor® can be operated with varying lambda- levels, so that also a pronounced pyrolysis can be achieved for energy-efficient and reduced emission-optimized combustion. After sufficient retention time inside the rotating drum, the ashes are dropped directly into the kiln riser duct, where they form part of the kiln inlet feed. Combustion off- gases and entrained small fuel particles leave the rotating drum to create a second stage of combustion in the regular calciner. This staged concept, which can be operated within a wide range of lambda numbers, greatly facilitates energy efficiency and NO× reduction.
Picture 4: Pyrorotor® gas flow scheme
References
Since its first installation in 2017, to this date a total of eleven Pyrorotor® have been sold. Picture 5 shows an installed Pyrorotor® and a table of achieved performance values in a realized upgrade project in Europe. The total TSR of the calciner is 98% with only a little portion of fossil fuel remaining as backup. Coarse WDF with up to 300mm edge length in 3D is burned in this Pyrorotor®.
Picture 6 shows a typical retrofit arrangement of a Pyrorotor® in an existing preheater tower structure. As the Pyrorotor® is placed directly over and parallel to the rotary kiln, there is almost no limitation for retrofitting a Pyrorotor® in an existing plant. There is no need to have space in the existing preheater tower or to add additional load to it. All connections for the tertiary air duct and the material feed can be placed freely. To this date, Pyrorotor® has been retrofitted to different types of calciners from various suppliers already and can produce an even bigger benefit, if combined with a calciner modification at the same time. Production increase and emissions reduction can then be achieved alongside TSR levels above 85%.
Picture 5: Pyrorotor® installation and operating values
Picture 6: implementation of a Pyrorotor® into an existing preheater tower structure
Summary
The versatility and robustness of the Pyrorotor® are unique in the cement industry and outperform similar AF utilization systems. Highest TSR can be achieved thanks to the combination of long retention time and constant mixture of AF and hot tertiary air. Combined, these conditions ensure a complete burn-out of any fuel and allow the use of almost any type of unprepared waste material. Due to its wide range of lambda-value operability, the gasification can be intensified for a highly-efficient thermal conversion and NO× reduction at the same time.
The Pyrorotor® is a one-time investment into a sustainable future, both economically as well as ecologically. It enables the use of any waste derived fuel, available today or maybe in the future, as alternative fuel option without the need to repeatedly modify a cement plant. Moreover, a Pyrorotor® reduces the ecological footprint of cement and clinker production because it transforms waste into cement, while also reducing NO× emissions at the same time. Pyrorotor® represents best what KHD stands for: technologies that enable both, a cost-efficient cement production, and a cleaner environment.