CC BY
15
© Ion V. Ion, Razvan Mahu, Florin Popescu, Gabriel Mocanu, Robert Chivu УДК 621.18
PREDICTION OF TAR FORMATION IN A BIOMASS HEATING BOILER
Ion V. Ion, Razvan Mahu, Florin Popescu, Gabriel Mocanu, Robert Chivu «Dunarea de Jos» University of Galati, Romania
Abstract: THE RELEVANCE of this study is the issue of predicting the formation of resin in a biomass heating boiler. THE PURPOSE. Biomass is an important source of energy, especially in the field of heating. The thermochemical decomposition of biomass is accompanied by the formation of tar, a product present in the combustion gases, which condenses upon cooling the gases forming, together with the entrained ash, deposits on the convective surfaces of the boiler. These deposits reduce the overall heat transfer coefficient and lead to corrosion of the metal surfaces. This work aimed to analyze the influence of the operating regime of a boiler on the generation of tar. Tar formation depends, in addition to the type of biomass, on the operating conditions of the boiler. METHODS. In order to analyze the formation of tar, modeling and numerical simulation of biomass combustion (briquettes from a mixture of sawdust and agricultural residues) in a downdraft combustion boiler was used. RESULTS. The model developed for biomass combustion was tested and validated by using the experimentally obtained results. The measurements for temperatures and carbon monoxide concentrations in the two combustion chambers of the boiler were used to calibrate the model. CONCLUSION. The results of the simulation showed that with the reduction of the excess air in the boiler, the tar concentration also decreased. This result is consistent with the results obtained by other authors at biomass gasification. The developed model was designed to be used for any thermochemical decomposition process of biomass.
Keyword: prediction; tar formation; biomass; boiler; calibrate the model.
For citation: Ion V. Ion, Razvan Mahu, Florin Popescu, Gabriel Mocanu, Robert Chivu. Prediction of tar formation in a biomass heating boiler. KAZAN STATE POWER ENGINEERING UNIVERSITY BULLETIN. 2023; 15; 1(57):117-124.
ПРОГНОЗИРОВАНИЕ ОБРАЗОВАНИЯ СМОЛЫ В КОТЛЕ ДЛЯ ОТОПЛЕНИЯ
БИОМАССЫ
Ион В. Ион, Рэзван Маху, Флорин Попеску, Габриэль Мокану, Роберт Чиву Университет Галати, Румыния
Резюме: АКТУАЛЬНОСТЬ настоящего исследования заключается в прогнозировании образования смолы в котле для отопления биомассы. ЦЕЛЬ. Биомасса является важным источником энергии, особенно в области отопления. Термохимическое разложение биомассы сопровождается образованием дегтя - продукта, присутствующего в газах сгорания, который конденсируется при охлаждении газов, образующих, вместе с осаждениями осаждения на конвективных поверхностях котла. Эти отложения снижают общий коэффициент теплопередачи и приводят к коррозии металлических поверхностей. Целью данной работы является анализ влияния режима работы котла на образование смолы. Образование смолы зависит, помимо типа биомассы, от условий эксплуатации котла. МЕТОДЫ Для анализа образования смолы использовалось моделирование и численное моделирование сжигания биомассы (брикеты из смеси опилок и сельскохозяйственных отходов) в котле для сжмгания с нисходящим уровнем. РЕЗУЛЬТАТЫ Модель, разработанная для сжмгания биомассы, была испытана и проверена с использованием результатов, полученных в экспериментальном порядке. Для калибровки модели использовались измерения температуры и концентраций окиси углерода в двух камерах сгорания котла. ЗАКЛЮЧЕНИЕ. Результаты моделирования показали, что с уменьшением избыточного воздуха в котле концентрация дегтя также снизилась. Этот результат согласуется с результатами, полученными другими авторами при газификации биомассы. Разработанная модель предназначена для использования в процессе термохимического разложения биомассы.
Ключевые слова: прогнозирование; образование смолы; биомасса; котел; калибровка модели.
Для цитирования: Ион В. Ион, Рэзван Маху, Флорин По песку, Габриэль Мокану, Роберт Чиву. Прогнозирование образования смолы в котле для отопления биомассы // Вестник Казанского государственного энергетического университета. 2023. Т. 15. №1 (57). С. 117-124.
Introduction and literure review
Biomass continues to be the main source of renewable energy in the EU, with a share of almost 60%. The largest application of biomass is the heating and cooling sector, which uses approximately 75% of all biomass. Forestry is the main source of biomass used for energy (forest residues, wood processing residues, firewood, etc.), representing more than 60% of all EU domestic biomass supplied for energy purposes. Almost 27% of biomass comes from agricultural biomass (agricultural crops and agricultural by-products), the remaining 13% being covered by waste (municipal, industrial, etc.) [i|. In the European Commission's long-term vision for a prosperous, modem competitive and climate neutral economy is estimated that, by 2050, the gross domestic biomass consumption will amount to (170 - 252) Mtep. There are significant opportunities for increasing the use of agricultural residues and by-products as well as waste [2].
Compared to coal, biomass lias a lower calorific value and a higher content of moisture, volatile matter and ash. The high content of moisture and ash creates problems in the thermochemical conversion processes (combustion gasification pyrolysis) of biomass, such as the high production of tar and ash that have the effect of harmful gas emissions, slagging and fouling with decrease of boiler performance [3, 4, 5].
Slagging deposits are formed through ash melting and sintering in the area with the highest temperature of the combustion chamber. Fouling deposits are formed in the convective part of the boiler by condensation of volatile species, especially tar [6, 7].
Direct burning of biomass is considered the most complex thermochemical decomposition process, involving drying, pyrolysis and gasification. During pyrolysis, the biomass components cellulose, hemicellulose and lignin are broken down into char, combustible gas and tar. Tar is a by-product of thermal conversion and is a very viscous liquid, consisting of hydrocarbons, oxygen-containing compounds, derivatives of phenol, free fatty acids, and esters of fatty acids. The formation (quantity and composition) of tar depends on the type of biomass, boiler design and operation. Because its dew point is high it condenses and forms deposits on the cooler surfaces of the boiler, worsening heat exchange and damaging the boiler surfaces. Studies conducted in [4] shows that a tar deposition of 1 mm decreases the thermal efficiency of a wood pellet boiler by 7,26%.
The reduction of tar concentration in a boiler can be obtained by pre-treatment of the biomass (modification of the physico-chemical properties and addition of additives), addition of additives during combustion or by modifications to the boiler [5]. The use of dolomite or lime during biomass combustion leads to increased deposits because the additive also forms deposits. On the other hand, the addition of 1% dolomite and 1% lime to wood pellets leads to a reduction of tar deposits by approximately 82% of the layer thickness and 76% of mass reprted to the surface. This reduction is explained by improving the properties of the pellets: particle density, bulk density, mechanical durability, and Low Heating Value and by increasing the temperature in different points in the combustion chamber of the boiler. The addition of 2% lime to pellets increases their production price by 14% [5].
The main purpose of this research is to better understand the tar formation mechanism and to find the operating regime that leads to the minimization of tar production during biomass combustion in a boiler. As operating conditions greatly influence the boiler performance it appears that optimization could provide a means of significantly improving existing systems at a lower initial outlay. When used carefully in conjunction with experiments. Computational Fluid Dynamics (CFD) modeling is a powerful tool in the optimization process, enabling the modeling of fluid flows, heat transfer and emissions in a boiler. It provides cost-effective help in optimizing existing boilers as well as new ones. However, tliis is not easy to do, and successful modeling of a low power boiler requires adequate submodels to simulate all the important combustion processes, flow and heat/mass transfer processes and their interactions. The complexity of these models affects the computation time and accuracy of the modeling results differently.
Factors that influence the thermal decomposition of biomass
The transformations that take place during the thermal decomposition of biomass are influenced both by the physical and chemical properties (initial moisture, biomass composition, inorganic matter content, density, thermal conductivity, specific heat, particle size and shape), and by the operating conditions (temperature, heating rate, pressure) [8-14].
Materials and Methods. Moisture content
The percentage of water initially present in the biomass affects the temperature evolution in the mass of the particle primarily due to the energy absorbed through evaporation. Disregarding this aspect in the modeling and simulation of biomass combustion leads to the overestimation of the temperature in the numerical solution, with all the negative consequences it entails (wrong estimation of the decomposition rate, conversion time, composition of volatiles, fractions of decomposition products, etc.). In the case of the dominant conductive heat transfer regime (larger particles) and at initial moisture levels of (15-20)%, or even higher, corresponding to naturally dry biomass, water evaporation occurs slowly, from the outside to the inside, continuously influencing the thermal decomposition process. Moreover, the heat transport in the mass of the particle also takes place through the water vapor that diffuses from the outer, warmer layers to the inner, colder ones, where it condenses. It should also be mentioned that water vapor has a reducing effect on char (gasification), but the reaction rate is much lower than in the case of oxidation (approx. an order of magnitude).
Composition
Obviously, the composition of biomass determines the fractions and compositions of the thermal decomposition products. Cellulose, hemicellulose and lignin break down differently, as found in experimental research, and the simple fact that there is a non-negligible variability between the proportions of the three main components of biomass, related to its origin, leads to the possibility of changing the proportions of the possible products following thermal degradation. Adding the fact that the very compositions of hemicellulose and lignin (cellulose practically does not depend on the biomass source) are not very clearly established, being able to differ more or less between the various types of biomass, the problem becomes even more complicated. However, some generally valid characteristics can be highlighted [9]. Regarding the char, the thermal decomposition of cellulose under medium-fast pyrolysis conditions (typical conditions for combustion installations) generates a small amount of char, at temperatures of (400-500)°C, the mass fraction of char being approx. (5-10) %, and at high temperatures (above 650°C) practically no solid residue remains. At the opposite pole is lignin, for which low temperatures fully favor the formation of char, with fractions of more than 60% solid residue, and even raising the temperature above 700°C, the amount of residue can reach 20%. Hemicellulose exhibits intermediate properties, with a percentage of fixed carbon at 600°C of approx. 7.5%.
The composition of the tar varies quite a lot between the biomass components, cellulose and hemicellulose producing condensable volatile fractions that mostly contain simple depolymerization products (levoglucosan, xylose) but also products resulting from the dehydration of the former (loss of one or more water molecules from the chemical structure initial, such as: dianhydroxylopyranose, furfural, etc.). Tar derived from lignin contain large amounts of phenol derivatives and aromatic compounds. It should be mentioned, however, that, depending on the thermal conditions, there are certain common pyrolysis products with low molecular weight, which appear in appreciable quantities especially at higher temperatures, being the result of the successive decomposition of the monomers. Representative examples would be: acetol, acetic acid, formic acid, acetaldehyde, furaldehyde, etc.
The gases resulting from the pyrolysis of the three main components of biomass do not differ essentially, the dominant chemical species being carbon dioxide (C02), water (H20) and, in smaller quantities, carbon monoxide (CO).
Inorganic fraction
A large part of the experimental studies carried out for the characterization of the thermal decomposition of the biomass components (cellulose - CL, hemicellulose - HCL and lignin - LG) used fractions isolated and extracted by specific methods from various plant sources, and subsequently purified. However, there have also been studies on the effects that the inclusion of inorganic salts can have on the way decomposition occurs, and the conclusions of the studies indicate that, on the one hand, lignin seems to be practically insensitive to their presence, but on the other the other side, both cellulose and hemicellulose, are significantly affected [9].
Although the exact mechanism by which mineral salts intervene in the decomposition of biomass is not fully clarified, their catalytic effect has been well highlighted. Thus, in the case of cellulose [9], an increase in the char fraction was observed from approx. 5% to 18% by using potassium salts (KC1, 1%), under similar conditions, and the slight reduction of the tar fraction and the increase of the gas fraction respectively, but the most obvious effect was the reduction of the
levoglucosan percentage (from 60 to 20% in the case of initially pure cellulose), the main component of cellulosic tar, respectively the corresponding increase in its decomposition products (acetic acid, glycolic aldehyde, etc.). For hemicellulose, all inorganic salts had the effect of increasing the fractions of C02 and char, with the reduction of species with lower molecular weight in the tar composition. In conclusion, inorganic salts present a catalyzing effect of decomposition reactions, generally favoring the production of char and/or gas at the expense of tar production. The intensity of this effect, however, depends on the origin and composition of the biomass [10], as can be seen from Figure 1.
Fig. 1. The effect of inorganic salts on the Рис. 1. Влияние неорганических солей на thermal decomposition of biomass [10] (U = термическое разложение биомассы [10] (U = untreated biomass, D = deminemlized biomass) необработанная биомасса, D = деминерализованная
биомасса)
*Sotirce: compiled by the author
Physical properties, temperature and heating rate
Among the physical properties of biomass, the most important, respectively those that have been proven to have the greatest influence on the way in which the thermal decomposition takes place, are the apparent density and the effective thermal conductivity [11, 12]. Concretely, for example, it is known that the thermal decomposition of hardwood is different from that of softwood, the latter being characterized by an easy ignition and a much higher combustion rate, a reduced amount of char, etc., which limits its use as such in applications involving combustion. However, by chopping and subsequently pelletizing or briquetting the soft essence wood (respectively by reducing the porosity and, implicitly, increasing the apparent density) it is possible to obtain thermal performance close to those encountered with hard essences. Regarding the thermal conductivity, this parameter particularly affects the decomposition of the char. Temperature and heating rate are largely interdependent. For the case of biomass combustion, the thermal regime is usually severe, i.e. the temperatures reached are quite high (over 1000°C), and the heating rate can reach values of the order of 1000 degrees/min for particles of small size (< 1 cm). The main effect of increasing the temperature and heating rate under pyrolysis conditions is the reduction of the char fraction and the biomass conversion time. Tar fractions generally show (for all biomass components. CL, HCL and LG) increasing trends with temperature, but only up to certain values (450-550°C), bey ond which thermal cracking of the various tar components occurs and respectively the reduction of the total fraction. Emissions of non-condensable volatile fractions increase with temperature, especially due to the decomposition of tar. However, these effects also depend on the residence time of the volatiles in the high temperature area, of course referring strictly to pyrolysis. In general, it was found that in the combustion process of most types of biomass of technical interest (composition, shape and dimensions), the drying and volatilization stages of the biomass are controlled by the thermal regime, while the oxidation of the carbonaceous residue is dependent on mass transfer from biomass to the outside [13].
Shape and dimensions of the biomass
The effect of biomass particle sizes on the characteristics of the thermal decomposition process lias been investigated in numerous experimental studies. Their results demonstrated the existence of two distinct thermal regimes, differentiated by the Biot number (Bi), which represents the ratio between the intensity of the convective heat transfer on the outside of the particle and the conductive on the inside. At numbers Bi « 1, the thermal resistance inside the particle is reduced compared to the resistance to heat transfer on the outer surface, so the particle is in a convective thermal regime (thermally-thin particle), and for numbers Bi > 1. the situation reverses, the particle
being in a thermally conductive regime (thermally-thick particle). In the convective thermal regime, the internal temperature of the biomass particle is practically uniform, a much simpler case to model mathematically. Practically, only small-sized particles (< 1 mm) subjected to a severe thermal load (combustion, fluidized bed regime) fulfill this condition. In the rest, the existence of the thermal gradient inside the particle, i.e. the thermally conductive regime, reduces the decomposition rate due to the reduction of the thermal load of the inner layers, to which is added the effect of the reduction of the thermal conductivity in the outer layers with the formation of the char.
For large particles, the residence time of volatiles is another parameter that plays an important role. The longer the residence time, the more the secondary reactions of the volatiles change their composition upon leaving the particle. These secondary reactions are, on the one hand, thermal cracking (the breaking of large molecules into molecules of smaller mass), and on the other hand, decompositions catalyzed by both the presence of inorganic salts and the char, favoring formation of char. However, this effect is partially counterbalanced by the increase in the efficiency of char gasification, especially due to the water vapor.
Most theoretical studies on the thermal decomposition of biomass and its components have used regular geometric shapes, such as the sphere, cylinder or rectangular parallelepiped. In the case of irregular shapes, most of the time the approximation of the particle with a sphere of equivalent diameter was resorted to, primarily due to the simplicity of the mathematical formulations resulting from the use of this shape. Experimentally, it was found that for a series of particles with different shapes but with the same volume, the minimum decomposition rate corresponds to the shape with the minimum external surface, respectively the spherical shape [14]. In the convective thermal regime, however, the shape of the particle plays a reduced role, the conversion time being almost the same [15].
Prediction of tar formation in a downdraft biomass boiler
For the study of tar generation during biomass combustion, the numerical combustion model developed in [16, 17] was used. The numerical model is made of a static component, namely the chemical kinetic model and a dynamic component, the implementation function assembly (UDF) plus numerical CFD model. The chemical kinetic-model aims to generate some functions for the numerical representation of all quantities needed in the dynamic model, whose only parameter is temperature. The independent character of the representation of the quantities of the mathematical model conferred by these functions leads to the great simplification of their implementation in the external function. Due to the differentiation by biomass components (cellulose, hemicellulose, lignin) based on the hypothesis of superposition of effects, the determined functions have a unique form, invariable when changing the type of biomass. This aspect attributes to the numerical model a particularly valuable property, namely the possibility to represent through a combination of all individual functions any type of biomass for which the mass fractions of the initial components (main components + ash and moisture) and the density are known.
The study was carried out on a 40 kW downdraft (or two-stage) combustion boiler, fed with briquettes made from a mixture of hardwood sawdust and ground agricultural residues. The characteristics of the briquettes, the characteristics of the boiler and the CAD model of the boiler are presented in [18-22].
The discretization grid is finer for the air intake channels and for the briquettes. The briquettes in number of 7 were placed transversely in the upper chamber and were shaped as cylinders with a diameter of 70 mm and a length of 320 mm. The total weight of the briquettes was 6,44 kg.
The CFD program was checked and validated using the experimental measurements made on the boiler, more precisely the measured values of the temperature in the two combustion chambers and the concentration of carbon monoxide in the two combustion chambers. Figures 2 and 3 show the evolution over time of temperature and CO concentration. The simulation was done for the first 20 minutes after the cold start of the boiler. This is indicated by the fact that the temperature in the combustion chambers at the time of fuel ignition is equal to that of the air introduced into the boiler. Experimental measurements were performed only after the first 10 minutes after the start-up. One can observe close proximity between the estimated and measured values, but also some overlapping of the variation trend.
1600 1400 1200 X 1000
I
I 800
о.
Р 600 400 200
Д Д
-Upper combustion chamber Lower combustion chamber
u ы vtpww
K—11T Л —-
0 Я? CÎ5 „1° #
V V "S3 W5 В3 Л' V «р л6
experimental
Time [s]
Fig. 2. Comparison between estimated and Рис. 2. Сравнение расчетных и измеренных
measured temperature values in the boiler значений температуры в камерах сгорания
combustion chambers. котла.
*Source: compiled by the author
0.05
0,045
0.04
О 0,035 U
с 0,03
"I 0,025 ■
я 0,02
1 1 1
— Upper combustion chamber
t V —-
V ""b
J •
«Р ^ ^
Time [5]
Fig. 3. Comparison between estimate experimental г. 3. Сравнение расчетных и измеренных values of СО concentration значений концентрации СО
*Sonrce: compiled by the authors
Results and disscussions
In order to fiind out the influence of the boiler operating regime on the combustion process and tar formation, two combustion regimes were simulated: combustion with normal air flow (0.04 kg/s) and combustion with reduced air flow (0.02 kg/s). Part of the simulation results (turbulence intensity, flow velocity, distribution of CO, H20, 02 concentrations) are presented in [18, 22]. The temperature distribution in the boiler and the tar concentration for the two combustion regimes are presented in Figures 4 and 5, respectively. With their help, the characteristics of the combustion process in the two cases can be compared.
a) b)
Fig. 4. Temperature distribution in the median Рис. 4. Распределение температуры в среднем
longitudinal section, at air flow = 100% (a) and at продольном сечении при расходе воздуха = 100%
airflow = 50% (b). (а) и при расходе воздуха = 50% (б).
*Source: compiled by the author
0.0000
a)
Fig. 5. Distribution of tar in the median longitudinal section, at air flow = 100% (a) and at air flow = 50% (b).
b)
Рис. 5. Распределение смолы в среднем продольном сечении при расходе воздуха = 100% (а) и при расходе воздуха = 50% (б).
*Source: compiled by the author
It is noticeable the incomplete decomposition of the tar for the normal air flow, the tar produced on the lower part of the briquettes passing into the secondary chamber, where, due to lower temperatures, it does not burn and is evacuated to the chimney. At low flow, the tar fraction at the exit is almost zero. However, it must be remembered that, in the absence of heat transfer to the outside, the temperature of the combustion gases in the convective system is much higher than in reality, which favors the decomposition of the tar.
The results obtained from the combustion simulation are in agreement with those obtained in the work [23], in which the influence of the type of biomass and the air flow on the performance of a gasifier was studied and it was found that, for all types of biomass studied, the reduction of the flow of air leads to the reduction of the formation of tar species.
Conclusions
The ability of CFD to encompass combustion parameters makes it an indispensable and accessible tool in the optimization of combustion processes. The results of the calculations obtained in the present study are particularly valuable, given the fact that under the given conditions, this is the only method at hand for carrying out detailed research, with a level of information detail inaccessible to experimental methods. The comparison of some calculated quantities, for example carbon monoxide or carbon dioxide emissions, the oxygen concentration in the combustion gases, or the temperature values in the combustion chambers, with the experimental measurements, demonstrates the ability of the developed model to estimate the phenomenon as a whole. A feature of the developed model consists in the ability to calculate, in addition to the chemical composition of the primary gas mixture resulting from the decomposition of biomass, and the composition of the secondary gas mixture resulting from the thermal decomposition of tar.
The application of the model to biomass combustion in a downdraft boiler, with a power of 40 kW, allowed highlighting the influence of the boiler's operating regime on tar formation. Reducing the combustion air flow leads to reduced tar formation. This is in agreement with the results obtained by other authors.
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Authors of the publication Ion V. Ion - professor, «Dunarea de Jos» University of Galati, Romania. Rdzvan Mahu - professor, «Dunarea de Jos» University of Galati, Romania. Florin Popescu - professor, «Dunarea de Jos» University of Galati, Romania. Gabriel Mocanu - professor, «Dunarea de Jos» University of Galati, Romania. Robert Chivu - professor, «Dunarea de Jos» University of Galati, Romania.
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