Dozens of variations of insulation and heat resistant materials were modelled

“Conversion of the coal-fuelled, coal-fired power plant to biomass is not only a challenge for technology, but also for the life cycle, in the strict sense of the heating plant itself, as well as in the wider sense of society and the state,” says Ing. Jan Ečer, Chairman of the Board of Directors of DITHERM - the general contractor of the project for increasing the lifetime of the FK2 boiler liners.

The preparation of such projects begins with the selection procedure, where it is necessary to get acquainted with the size of the aggregate, its condition and the target performance, as well as the combustion technology and, of course, the composition of the fuel. If the aggregate has been run for a long time, the initial study is also influenced by this fact. A team of experts first calculates volumes, heating curves and simulates the flue gas and heat process by analyzing the shape of the aggregate, especially its detailed curvature. For proper identification of lining in individual parts of the aggregate, chemical analysis of fuel and knowledge of combustion technology must also be available. “The chemical components of the flue gas are involved in high-temperature reactions with the lining material,” says Ing. Mojmir Nesvačil, DITHERM’s Chief Designer.

Based on the initial study, which experts spend weeks preparing, the design phase begins: choosing the right material composition for individual parts of the boiler, and how to apply lining materials resistant to heat, abrasion and alkali. There is a large amount of input data and factors influencing such proposals, so more variations are usually created. Once the team determines one of the variants, a long process of selecting materials commences, to match the chemical composition and consistency to the given operating conditions.

The vast majority of manufacturers do not have the necessary materials in their current product portfolio, so they have to produce them directly for the job. For them, however, they do not have the technology and raw materials that will allow them. “We have long negotiations with potential material suppliers to explain what we want. They have to convince us that we will receive material of guaranteed value from them in time. We choose from the best, in terms of the quality of the material (guaranteed parameters) and the raw materials from which the material is made. Then the price is decided,” says J. Ečer. The materials that eventually come into the boiler come from the Czech Republic and other European countries.

“Already long before the planned boiler shutdown, we have all the information to specify the requirements and list the tender for the supply of materials. Tenderers have a precise assignment from us and adapt the recipe for making materials. Most of the material is selected from several manufacturers. We had two suppliers for the insulation, three for the refractory concrete,” adds Ing. Vojtěch Kavan, Chief Technician and Project Manager at the project in Hodonín.

J. Ečer continues: “Selection procedures for projects such as increasing boiler life are very demanding. The investor thoroughly examines the quality of the materials and the technology used, the organizational and professional capabilities of the main contractor and, of course, the price. Sometimes we get to other technologies and materials than we originally designed, which requires maximum expertise, flexibility, and a team that works well together.”

Once the contract is signed, the time-consuming preparation of the project begins. A project team is determined to atomize the project into its smallest detail. Practically repeating the analysis from the phase when the offer was made. All input data is verified, combined and the technology and material designs are given a detailed form that is transferred to the manufacturer. Following are the laboratory tests, most often at TZÚS in Pilsen, selection of assembly companies and other services. The design team is in full use, the entire lining, including detailed information about application technologies and material properties, must be redrawn by the start of construction work.

“We performed hundreds of thermal calculations before we started implementation, we modelled dozens of variants of insulations and masses in different layers and structures so that the lining would withstand temperatures up to 890°C, and on the other hand, did not reach the surface temperature of 65°C,”  adds M. Nesvačil.

From coal to biomass

“The original lining in the boiler which switched to biomass (biomass boiler) was not sufficient for this transition. The boiler had other operating conditions, the lining must withstand more negative effects than for the combustion of coal, be it thermal shock, abrasion, or even alkaline corrosion. This is alpha and omega lining life,“ says M. Nesvačil. Then V. Kavan added: “The most disturbed are the linings in the cyclones, and the walls and bottom in the combustion chamber. Alkaline vapour forms and penetrates into the lining. When they reach the appropriate temperature, they react with the lining material. Compounds having a chemical composition different to the lining, a different bulk density and other thermal expansion are formed, which causes a gradual disruption of the lining integrity.”

The demolition of the original lining was commenced by DITHERM in April 2013. The boiler will have a new lining at the end of June. Altogether, 300 tons of refractories are being used and 90 % of the lining is being changed. “The rest is the original lining, but it is located in less exposed places and is still in good quality,” explains M. Nesvačil. The speed of implementation is a thorough pre-project preparation and the company’s own know-how. “As one of the first, we began to apply modelling in 3D. We have a sophisticated solution system for expansion and working joints. It is very fast, efficient and high quality, “says J. Ečer.

Rules for work and quality control = priorities

The project is accompanied by very strict safety and control measures, both in the field of work safety and workplaces, as well as in environmental protection and quality control. Safety of work and waste disposal is negotiated with the investor in advance, when the construction starts, only details such as access roads, material transport and processing, waste disposal and disposal, chemical storage facilities are fine tuned. Less than 300 tons of rubble was destroyed on the Hodonin project.

Checking the quality of the new lining has several stages. The first is laboratory testing of the material prior to its application. However, the implementation company struggles with several influences on the linings. This is the working environment and the construction conditions. Furthermore, fluctuating temperatures, humidity, dustiness and, last but not least, the method of preparing the material for its application. All of these influence the final parameters of the lining materials, especially the concrete.

The fitters (assembly workers) are therefore thoroughly trained to handle the materials and prepare them for assembly. They assemble according to DITHERM technology standards. Often, they decide on small amounts of water or temperature differences of the units to ensure that the concrete remains resistant to abrasion and chemical influences.

Samples are taken from the materials in the building, which are processed according to the relevant ČSN and subsequently stored for possible complaint procedures.