Through a series of chemical engineeringbased improvements, it is possible to achieve significant reductions in the carbon footprint from production of materials for the construction industry. The research and development results have been achieved in a collaboration of ROCKWOOL, FLSmidth and CHEC. “FLSmidth provides equipment for the cement industry, and ROCKWOOL produces mineralbased isolation materials. While the two companies are not competing, they both use high temperatures processes in their production technology, which have many similarities,” says Senior Researcher Peter Arendt Jensen, CHEC. The project was entitled “Process technology for sustainable Building materials production” – or ProBu for short. Some of the improvements achieved in ProBu enable use of waste materials as fuel, while others are related to higher energy efficiency during manufacturing. The ProBU project was financed by Innovation Fund Denmark through the Grand Solutions Program.
Enabling increased use of waste as fuel on cement plants
Substituting fossil fuels for waste-based fuels will not only reduce net CO2 emission but may also improve cement plant economy. However, using waste as fuel does involve various challenges, some of which were addressed in the project. Increasing the amounts of waste-derived fuels at a cement factory will typically increase the amount of chlorine (Cl) and sulphur (S) fed into the cement plant. Presently this is dealt with by making a purge stream of gas from the plant which reduces the levels of Cl, but also reduces the thermal efficiency of the plant, and therefore limits the amount of wastederived fuel that can be applied. “Based on fundamental studies on the release of Cl and S from cement raw meal a new concept for removing both species from a cement factory was proposed. The technology still needs some development to be commercially available, but it can potentially remove Cl and S simultaneously from the plants, while keeping a high thermal efficiency,” explains Associate Professor Hao Wu, CHEC.
Moving beyond trial-and-error
The ROCKWOOL melting technology has in the later years been further developed to a shift in the applied fuel from coal coke, to pulverized coal, to gas and further to renewable electricity. All the steps have led to a decreased net CO2 emission per produced isolation material product. The latest development has been to base the melting process on the use of electrical heating. In both ROCKWOOL and cement plants, preheater cyclones are important components, which are needed for a high energy efficiency of the plants is wanted.
“However, the detailed flow behavior in such industrial scale cyclones is difficult to predict and thereby design is often done as a trial-and-error experimental development. To improve the understanding of industrial preheater cyclones a CFD model (Computational Fluid Dynamics, ed.) that can account for the two-phase flow, particle separation, particle agglomeration, heat transfer and erosion was developed. The model has been verified by use of full-scale industrial measurements and is now provided to the companies as a tool that can be used for future design and optimization of cyclone preheater systems,” explains Hao Wu.
Geo-polymers as alternative cement
Traditional Portland cement production is responsible for considerable amounts of CO2 emissions due to both the calcination of limestone and the high temperatures used in rotary kilns (up to 1500°C) required to obtain the cement product. However, alternative cement based on clays, wastes, and strong bases, so called geo-polymers, can obtain similar strength properties while emitting much less CO2 in the production process. A PhD project on geo-polymer properties and process technologies was conducted and provided valuable data on the possible future use of geo-polymers. “The geo-polymer-based concrete production still has some challenges to overcome, such as supply chain, infrastructure, costs, and standardization. Nevertheless, this PhD study has provided data that indicates that if prescriptive standards are provided, and more strict CO2 taxes are enforced, then geo-polymers can be a very relevant alternative to traditional Portland cement,” says Peter Arendt Jensen. With respect to emissions from production processes, most focus has been directed towards minimizing nitrogen oxide (NOx) emissions. Previously developed fundamental knowledge on nitrogen chemistry has been applied in CFD modelling to provide an improved understanding of the interaction of flow, combustion, and nitrogen chemistry in industrial reactors. Two examples of high temperature reactors were simulated using CFD models that also account for combustion and NO formation. Examples included the ROCKWOOL melting cyclone and an FLSmidth calciner. The calciner modeling was evaluated by full detailed scale measuring data. The models have provided new insights with respect to the complex mechanism of formation and destruction of NOx in industrial reactors.
Promising innovations in both companies
As part of the project advanced measurements on industrial reactors were conducted both to obtain an improved understanding of the processes and to support the development of mathematical models. The measurements included both probe based and optical measurements, and included data on gas, gas composition, temperature, local deposit formation and video registration of the reactor chamber. A range of reactors e.g. a preheater cyclone, melting cyclone, calciner and electrical heating equipment were investigated. “A large range of unique data was provided that supported the conducted project activities,” says Peter Arendt Jensen, while noting that besides new insights, the ProBu project has resulted in the education of five PhDs, and strengthened collaboration between the companies and DTU: “Several new collaboration projects have been initiated based on the ProBu collaboration.” Also included in the project were development activities at the companies. At ROCKWOOL, the Integrated Melting Furnace was converted from coal combustion to run on biogas fuel, and furthermore the first cupola mineral technology has been developed to operate on electrical heating. FLSmidth has developed the FUELFLEX Pyrolyzer technology that leads the way for 100 % waste firing on calciners, and furthermore leads to large reductions in the NOx emission from cement plants
