Boiler ash: the case of a computational model which aims to become a benchmark for regional industry

Professor and researcher of the School of Mechanical Engineering, Albio D. Gutiérrez, one of the authors of the scientific article. Credit: Faculty of Engineering Communications Office.

Excessive ash accumulation on the surfaces of solid fuel boilers such as coal or biomass often causes problems when excessive deposits form, resulting in what is known as "slagging". These accumulations affect energy efficiency and create problems associated with maintenance, operation and personnel safety. The development and implementation of a computational model based on the concept of digital twins of the 4.0 technology allows attacking this problem, by offering estimates and analysis for decision making in shorter times, without the difficulties derived from invasive processes, which in turn translates into an opportunity to consider and reduce future problems.

Coal-fired boilers and the problems associated with the waste inside them

Coal-fired boilers, such as those used by the paper industry in the region, constantly face a problem derived from their operation, since the high temperatures at which they work turn the ashes resulting from such processes into the cause of operational problems. In view of this, the researcher and Professor of the School of Mechanical Engineering Albio D. Gutiérrez, in co-authorship with Stefany Saenz Prado, a Master's student in Chemical Engineering, and Professor Jaime Jose Acuña Polanco, from the School of Natural Resources and Environmental Engineering (Eidenar), developed and implemented a model based on digital twins using computational fluid mechanics (CFD) to address the problem of slagging in a 95 MW power boiler located in a paper company in the industrial sector of Yumbo, Valle del Cauca. The boiler produces steam used for power generation and pulp drying in the paper mills. It is a large piece of equipment, close to a six-story building.

"The boilers have ash removal systems, but if the temperature is too high the ash melts and sticks to the surfaces, in this case it sticks to the walls and affects heat transfer," explains Professor Albio D. Gutiérrez, adding that the purpose behind these boilers is to maintain optimum heat transfer to generate enough steam to meet the cellulosic pulp manufacturing process for paper and cardboard. The process of ash accumulation on the walls in the form of hard deposits (slagging) prevents heat transfer to the water and produces an increase in the combustion temperature inside the boiler, which can compromise the integrity of the equipment, causing, as has already happened, the melting of boiler components. This is detrimental in terms of economics, safety and productivity for the companies.

Faced with this problem, the research had three objectives. 1) training of operators for a better use of coal, considering the variability of the deposits, amount of ash produced according to the type of coal used and its tendency to adhere to the walls of the boilers. 2) construction of a digital twin of the boiler using 4.0 technology, based on CFD. And 3) construction of a mathematical model for the optimization of coal blends, to determine their tendency to form slagging.

The results of the work showed solid evidence of the usefulness of digital twins, which opens the door to their use in other sectors of regional industry.

Digital twins based on computational fluid mechanics, a model with great potential for the industrial sector in the region

The model used to analyze the boiler includes its boundary conditions and the complexity of flow conditions and associated transport phenomena. This model is made possible by its discretization into smaller geometries. It consists of thousands to millions of small volume elements relative to the size of the equipment. These elements are linked by means of algorithmic technology and high-performance computing. The elements exchange information resulting from solving the equations describing the physical phenomena of the problem, such as the equations of conservation of mass, momentum, energy and chemical reactions. As a result, multiple information related to velocity fields, temperature, chemical species, heat transfer quantities, etc., is obtained. This makes it possible to determine, for example, how well the fuel burns in the boiler and how efficient the energy transformation is.

A particular aspect of the work on the boiler slagging issue is that a more practical methodology was chosen than the one usually used in the research area. Instead, the creation of the digital twin was approached from a more practical approach for the industry. "When analyzing the ash sticking phenomenon in a boiler, the state of the art shows methods that are quite complex and computationally expensive," says Professor Albio D. Gutiérrez. Instead of modeling the complex mechanisms of the trajectories of millions of particles and the solution of the physics associated with them and that determines the adherence of the molten ash to the boiler walls, it was preferred to carry out an information survey in the boiler, taking advantage of maintenance shutdowns. This information survey specifically considered the areas affected by slagging. This information was then incorporated directly into the model. This methodology allowed to reduce considerably the computational cost and the time associated with the solution of the problem.

As a result of the work, the following findings were obtained: in terms of the effect on energy efficiency caused by ash incrustation, an approximate loss of 16% in heat transfer and a temperature increase of around 50°C was estimated. "It may not seem much, but when we reach the temperature limits, and when we consider a high amount of gases, this translates into a large amount of energy and a possible impact on the structural integrity of the equipment," explains Professor Gutiérrez.

The importance of CFD-based digital twins for industry

Advances in the development of 4.0 technology have allowed models such as these to enable customized measurement, focused on the needs to be solved. "With the information that the industry has, such as blueprints and operating conditions, you can create a model that allows you to describe the behavior of the equipment. You can virtually indicate what composition is inside, what speed, at what temperature, at what level of radiation, how much energy is being lost, etc.," says Professor Albio D. Gutiérrez, adding that the CFD-based digital twin can be used in other fields of industry, in addition to thermal processes. Its application in regional industry brings benefits in chemical and food processes, water purification, risk analysis such as floods, hurricanes, oil spills, renewable energies, dispersion of pollutants in the atmosphere, and even in the medical field, where it was used during the pandemic to analyze the spread of aerobic diseases in dental environments and operating rooms. The purpose of this technology is to serve as a guide, prevent risks and reduce time and costs in procedures aimed at solving specific problems faced by different industries, both regionally and nationally.

"I know this technology from my doctoral training and the experience I received in the U.S. industry. When I worked in Tulsa, Oklahoma, for John Zink Hamworthy Combustion, a subsidiary of Koch Industries, we applied these valuable technologies to the world's largest companies in the oil, gas, chemical, petrochemical, pharmaceutical, food and more. For these companies, we provided valuable answers, verified operating conditions and prevented equipment damage in as little as three weeks per project. My intention with this technology since I returned to Colombia to join the Universidad del Valle is to continue expanding this technology in the industries of the region, to solve short, medium and long-term problems. The case of the boiler work is a typical medium and even long-term problem. We are working for three years. The goal is to strengthen the approach of the industry with Universidad del Valle and benefit the industry with valuable answers, not only in the medium term as in this case, but for something that needs to be solved in a matter of days", are the words of Professor Albio D. Gutiérrez, who last May 12 made a trip as a speaker representing Universidad del Valle at the Spring Technical Meeting of the Central States Section of The Combustion Institute, which took place in Cleveland, Ohio, United States. During the event, the professor showed the results of the work done on the boiler. The work aroused the interest of the attendees due to the practicality with which the problem was solved.

In addition to making the presentation at the event, the professor took the opportunity to learn about the research being carried out in terms of energy transition related mainly to the use of hydrogen in sectors such as energy, transportation and defense. Also, in the area of forest fire prevention. One of the most interesting impressions of Professor Albio D. Gutierrez during his participation in the event was that most of the current research in the United States is focused on experimentation, and there is currently a need for many professionals who can model phenomena, equipment and processes using digital twins. A representative of the National Science Foundation (NSF) of the United States even called during a closing event for more professionals in the area of computational modeling.

Regarding his participation in the conference, the Professor will present an outreach event to the university community in the coming days.

If interested in being in touch with the researchers or any further information about the scientific project, please write the Faculty of Engineering Communications Office: comunicaingenieria@correounivalle.edu.co.