Thermovision analysis of heat and power plant chimneys technical condition

This paper describes the thermovision analysis conducted on two types of chimneys: structural steel one and reinforced concrete one located in Kielce Heat and Power Plant, Poland. As a result of the research temperature distribution obtained from the thermograms were shown on the diagrams and analysed along their horizontal direction. It proves, both of the chimneys that were examined are in proper technical condition. The article also discusses the importance of the analysis as a diagnostic tool that allows early-stage detection of structural technical conditions and thermal insulation degradation. A factory’s chimney should work constantly to maintain its operation, since modifications in the production line caused by construction works are high cost and usually difficult to implement. However, analyses are critical to ensure chimneys are well maintained. The literature presents many examples of destructive influence of high temperature on the reinforced concrete chimneys. When failure of chimneys occurs, even the whole plant unit may be closed down. Thermography is then an ideal, non-destructive and cost-effective instrument for structure testing and can therefore be a useful, initial step to locate potential damage.


Introduction
Thermal imaging used to be reserved for technical applications in areas of defence, medicine, scientific research and industry. Its wide usage was described by Meola [1]. Nowadays it is a technology that contributes to everyday innovations. Its applications are expanding and some of the technology's consumers can use on daily basis. For example, infrared cameras were used during the Covid-19 pandemic to screen people for fever at airports or hospitals to check if they have higher temperature thus potentially carry the disease [2]. What once was used only by military and defence groups, an intelligent image recognition system, is nowadays widely used as a security system in buildings, parking lots, public areas etc. The benefits of the image recognition system are substantial: detection of potential threat in any weather conditions without the installation of additional light source.
Building thermography is one of the most demanding applications for infrared camera systems. Increased energy efficiency of buildings has highest priority and infrared thermography greatly contributes to reaching this goal. Inspections with infrared cameras quickly and easily indicate insulation problems of buildings. Podawca et.al. [3] discusses the low thermal insulation of partitions in large-panel prefabricated buildings, constructed in 70's and 80's. Ever since the first years of their exploitation defects related to freezing of vertical and horizontal joints began to appear. This caused lack of thermal comfort for its users. The measurements that were carried out were based on thermovisual analysis before and after thermal modernization of an example of a 4-story building. The taken tests indicated spots with reduced temperature, especially in joints between prefabricated panels: walls, ceiling and floors. The implementation of research results led to improvement of thermal perceptions by residents as well as reduction of heat loss indicators demand by 1/3 [3].
Thermomodernization of historical buildings, with aim to improve energy performance, is particularly difficult. Those buildings have many existing problematic structural areas and non-invasive method is an indispensable. Among much research, detailed thermovision analyses were carried out for the historic wooden building in the region of Podhale, Poland [4]. Based on the thermograms the temperature field disturbances in the building's envelope were analysed precisely. Authors stated that the presented testing methodology, which is to precede project execution should be applied to vernacular timber architecture [4]. The In-situ method is not only recommended by Fedorczak-Cisak et.al. [4] but also by Albaticia et.al. [5]. In 2012 a "novel" method was discussed for an evaluation of the emissivity of common building materials such as: fiber-reinforced-concrete, unpainted plasterboard, brick, chipboard, aluminium and steel. One of the key points for the correct and reliable application of the method is the determination of the actual surface temperature of opaque building elements. Measuring thermal emissivity of an outer surface of a building element under real working conditions is necessary to obtain actual surface temperature. This parameter as well as many others led to assess a building energy sustainability and efficiency [5]. Historical buildings conservation guidelines are very demanding in terms of renovation of the facades or thermal retrofitting of walls. The undertaken analysis show that internal insulation improves the thermal parameters of walls but unfavorably affects conditions within thermal bridges [6]. Thermal imaging testing verified the undertaken numerical analysis and was suggested as a part of the pattern that should be taken in any historical buildings for the assessment of the extant state and modernization of Lviv's historical architecture in this particular article [6] but also as reference for similar buildings.
Building thermography is also useful in terms of larger scale perspective that was observed by Struha et.al. [7] where the thermovision monitoring was implemented to show the heat islands that occur in most cities over time. The implications of such analyses include recommendations that rest areas should be designed more carefully and designers should take into consideration heat radiation of the proposed construction materials, particularly as temperature rises and climate change is unstoppable and progressive. Thus, such analyses are important in city planning and therefore cities and towns should get prepared and propose solutions so that the impact on urban environment was minimized [7].
An experimental method of calculating the coefficient of thermal homogeneity was shown by Vasilyev et.al. [8] and based on that method a metric of temperature was obtained using thermovision controller and applied to conduct thermal testing of climate chamber [8]. It turns out the described method reduced the time and cost of testing compared to methods described in Russian standards [8].
Infrared cameras are more likely to be used for measuring temperature or heat flux but there are also other possibilities to apply them for example to determine the velocity profiles in the convective boundary layer of an isothermal vertical surface [9]. The results were obtained from temperature distributions recorded with a thermal imaging camera. Authors of the article encouraged with the obtained results of their study are working on additional research to prove whether having velocity field available will allow to determine the temperature field especially in hard to reach places.
Infrared cameras are also used in wide range of research on liquids. Thermography is an optical method in heat transfer and fluid flow visualization. On the basis of thermogram research, substantial thermal diversity of drops was found [10]. What is more three complex fluid flow configurations were analysed and research shows that flow conditions are two dimensional [11]. In another work laser spot heating locked in infrared thermography was used to find the thermal conductivity of fluid [12]. In conclusion the proposed method gives fast, non-contact method with fine accuracy (uncertainty of about 5%) for measuring thermal conductivity in liquids and gasses as well. It gives wide range of advantages comparing to conventional techniques for thermal characterization [12].
A common problem in industrial sector are different types of leakage that appear during complex mechanical processes. Based on a compressed air loss in steel pipe, the approach shown in Dudic et.al. [13] can also be applied for leakage of other fluids in steel pipes. Perhaps it can also be used in reinforced steel chimney for flue gases leakage detection.
Even the on line level of the liquid can be measured using infrared camera especially in circumstances when toxic or difficult to access environment is a part of a study. Dinca et. al [14] proposed an algorithm based on the thermal interia property of the liquid and solids presented with thermovision camera. It can also be used in other applications where liquid level is impossible to obtain with classic measurements.
In this paper two active chimneys, a part of Kielce Heat and Power Plant in Poland, were examined with an infrared camera. The scope of the study includes early-stage detection of the possible outer surface and insulation degradation and shows that routine inspections shall be implemented as a diagnostic element in case of such structures.

Measurement methodology
The Kielce Heat and Power Plant was established in 1987 and is the largest heat energy supplier in Kielce. It covers an area of 106 hectares with three chimneys. Two of them are the subject of this research. Both chimneys are still in operation, but have different characteristic including shape, height, diameter and construction material. Location of the chimney is shown in Figure 1 and 2. Structural steel chimney (Fig. 1) is 2.8m in diameter measured at the base of the chimney and 80m in height. This chimney is thermally insulated with 100mm of mineral wool and its double wall. It exhausts fumes from biomass combustion. Their temperature at the height of the inlet to the chimney reaches 140˚C. The reinforced concrete chimney (Fig. 2) has a diameter of 7.8m again at its base and height of 120m. It does not have an insulation but its wall width is 0.3m. This chimney is designed to emit exhaust gases, which are a product of combustion of fine coal. At the height of the inlet to the chimney, the flue gas reaches a temperature of 140˚C.
The difference in chimneys overall dimensions is visible in Figure 1 and 2 which are taken at the same scale.
The research of the chimneys was carried out on 8th of September 2020 between 12 and 1 p.m. using thermal imaging cameras: Flir E30 and Flir S61. Only the thermal images taken with the first camera were taken into account during the study as they are with better resolution. Parameters like: air temperature, relative humidity and distance between the device and the object were examined using a smartphone, in which the second of the mentioned thermal imaging cameras was built into. "Distance" and "Air" applications were used for the above. Infrared camera Flir E30 can therefore register the thermal radiation emitted by physical bodies at temperatures of about -20˚C to about +350˚C with its sensitivity of 100mK or less than 0.1°C. It offers a 160 x 120 IR pixel resolution which gives 25.600 temperature measurements on each image. The environmental parameters operating temperature range from -15˚C to about +50˚C.
The steel chimney was measured from four different positions North, South, East and West and the reinforced concrete chimney was measured from three sides excluding West as nearby construction works made it impossible.

Research analysis
As a result, several thermograms were taken for both of the chimneys. Sample thermographic photos are shown in Figure 3 and Figure 4 for reinforced concrete chimney and in Figure 5 and Figure 6 for structural steel chimney. Thermograms of chimney No. 1 (structural steel chimney) were taken in two parts: upper and lower.  Thermographic studies are very helpful in monitoring temperature changes and also the reinforcement corrosion that is more likely to occur due to acid substances and condensation of the flue gases that penetrate thermal insulation as well as the concrete from outside [15]. Every industrial chimney is constantly exposed to the natural environmental anomalies on one side and to the flue gases degradation on the other.
Equipment like an infrared camera helps estimating conditions of any industrial chimney that should be done regularly to avoid heavy construction works. On the basis of external observations and detailed macroscopic examinations of construction elements and  [16].
Problematic measurements of displacement and deformations of multi-flue chimneys of inner structural surface of reinforced concrete industrial chimneys were also discussed in other article and shown by authors as a diagnostic element that sould be checked as chimneys rquire regular inspecions of their conditions [17]. Among several others diagnostic elements mentioned in the article, thermographic examination was also a part of this complex process presented [17].  Thermovision studies of the outer surface of chimneys can show several dependencies and indicate the places of cracks occurrence in masonry wall chimneys and places with damaged insulation in steel chimneys. Thermogram in Figure 3 showed spots indicating the places of higher temperature. This can lead to the conclusions that there might be a probable spot of reinforced concrete cracks. When discovered at early stage it may support the detailed recognition and further investigation. The greater the temperature difference shown on the thermogram the greater likelihood of reinforced concrete degradation occurring. Structural steel chimney shown in Figure 5 has more unified thermogram therefore it is more difficult to easily find losses in the insulation.  Changes in temperature of the structural steel chimney undergo less variation on north side compering to the reinforced concrete chimney, where the temperature difference is even 4 o C, while in case of structural steel one it is around 2 o C. The temperature difference is the result of its technical condition with visible concrete cracks and location of 3 steel brackets around it, which also affects the results. It is worth mentioning that the reinforced concrete chimney is painted in two colors (visible on Figure 2 as a darker grey): white and red alternately so the disparity in thermal transmittance should be included in thermograms' analysis and may have impact on temperature difference. Surface with a different colors of paint generates lower heat gains which translates into lower radiation [18]. This in turn is the basis to determination of absorption and emission coefficients.  On the other hand, using technologies available in the industry, heat transfer on the surface can be limited or intensified. An example is the different arrangement of composite layers [19] or microstructural coating [20].
Despite temperature variations on reinforced concrete chimney temperature distribution, temperature's decrease along the flue outlet is more visible on this chimney (Fig. 8).
On east elevation reinforced concrete chimney's surface temperature is higher compering to north side, as the photos were taken at noon, this side got heated up by sun radiation from early in the morning. Weather conditions like moisture, sun operation and wind should be taken into account as higher temperature shown on the thermogram not always indicate heat losses.
Moreover, Figures 9-10 show changes of temperature along chimneys height on south and west side. As regards structural steel chimney, temperature increases with elevation which is shown in Figure 9 and Figure 10 and after around 70% of its overall height temperature gets lower which is caused by cooling down process of the fumes with the height of the chimney. Visible temperature drops shown in Figure 7, Figure 9 and Figure 10 -north, south and west orientation respectively appear in the same location. They are the results of chimney support brackets location around its diameter within 2/3 of its height and also the joints connection of steel structure. Places where temperature drops are shown on both of the thermograms shall be carefully examined. The detailed investigation should be applied to see whether chimneys need any conservation works. This may also be affected by defects in isolation as regards structural steel chimney.

Conclusion
Polish law requires buildings to be kept in good and safe conditions [17,21]. This also includes plant room and chimneys. The thermovision method should be used to examine the technical condition of different types of factory chimneys in first place and can be employed as a cost effective preventive measurement. Its application is simple, fast and safe method of measurement, although many variables should be taken into consideration, such as: weather conditions, infrared camera accuracy or thermal transmittance. When considering high structures, such as chimneys, measuring points should be selected carefully to assure accurate coverage of the entire structure. Emissivity factor should be considered foremost in order to obtain an accurate thermogram.
The analyses of the two chimneys of the Heat and Power Plant in Kielce shown that the structures are in good condition and do not require major repairs. This type of thermographic research should be taken regularly to avoid serious conservation works. It will contribute to receiving early alert system as well as to saving money.