An Integrated Study of Flue Gas Flow and Superheating Process in Recovery Boiler using Computational Fluid Dynamics and 1D-Process Modelling Kunal Kumar Andritz Oy, Viljami Maakala Andritz Oy and Ville Vuorinen Aalto University ABSTRACT Superheaters are the last heat exchangers on the steam side in recovery boilers. They are typically made of expensive materials due to the high steam temperature and risks associated with ash-induced corrosion. Therefore, detailed knowledge about the steam properties and material temperature distribution is essential for improving the energy efficiency, cost efficiency and safety of recovery boilers. In this work, for the first time, a comprehensive 1D-process model (1D-PM) for superheated steam cycle is developed and linked with a full-scale 3D CFD (computational fluid dynamics) model of the superheater region flue gas flow. The results indicate that first; the geometries of headers and superheater platens affect platen-wise steam distribution. Second, the CFD solution of the 3D flue gas flow field and platen heat flux distribution coupled with the 1D-PM affect the generated superheated steam properties and material temperature distribution. These new observations clearly demonstrate the value of the present integrated CFD/1D-PM modelling approach. It is advantageous for trouble shooting, optimizing the performance of superheaters and selecting their design margins for the future. The developed integrated modelling approach could also be relevant for other large-scale energy production units such as biomass-fired boilers. 1 INTRODUCTION Recovery boilers are used to combust black liquor for chemical recovery and producing high-pressure superheated steam. The generated steam is utilized for self-sustainable pulp mill operations and electricity generation. For instance, in Finland in 2017, 8.1% of total electricity was generated with black liquor combustion in recovery boilers [1]. Global production of chemical wood pulp has been forecasted to increase annually by 1% [2]. Tran et al. [3] noted that approximately 1.5 kg of black liquor dry solids (BLDS) are produced per 1.0 kg of chemical wood pulp production and around 3.5 kg of superheated steam is generated per 1.0 kg of BLDS combustion in recovery boilers. Hence, black liquor is a vital biomass based renewable energy source from future perspective. Over recent years, the global interest for carbon neutral energy production has been continuously increasing to mitigate the climate change. Simultaneously, the conventional role of recovery boilers as chemical recovery units is shifting towards renewable energy production [4]. In addition, the average capacity of recovery boilers has been increasing. The current largest capacity of a recovery boiler is 12000 tds/d and even larger recovery boilers have been planned. Therefore, it is essential to develop new computational models for such large boilers to understand their heat transfer phenomena in detail, and to improve their contribution for renewable energy production. The superheater region in a recovery boiler is the focus of this work. The superheaters are single-phase heat exchangers. They are used to convert saturated steam into superheated steam by capturing heat from hot flue gas (≈ 30% of total). They are the last and one of the largest heat transfer surfaces in a recovery boiler before the steam turbine. Thence, the optimal performance of superheaters, including higher quality superheated steam production and reduction in material issues such as corrosion, is essential for efficient and safe recovery boiler power plant operation. Previously, detailed studies have been performed for coal-fired boilers and bubbling fluidized bed (BFB) boilers to analyze the heat transfer sections including superheaters, and to study the steam generation process using integrated CFD/1D-PM modelling approaches. In integrated CFD/1D-PM simulations, a flue gas side 3D CFD (computational fluid dynamics) model is coupled with a 1D-PM (1D-process model) of water-steam side. It is beneficial to utilize the 1D-PM for large and complex flows in the steam cycle, which are not feasible to solve with standalone CFD modelling. The reasons are large computational cost, time and availability of computational resources. 1 Edge et al. [5] studied the steam generation process in a 500 MWe natural circulating coal-fired boiler using integrated CFD/1D-PM simulations. Schuhbauer et al. [6], Chen et al. [7] and Park et al. [8] performed integrated CFD/1D-PM simulations, and studied the heat transfer between hot flue gas and water-steam cycle in coal-fired boilers. Yang et al. [9] also carried out integrated simulations to analyze the temperature distribution on furnace walls and the heating process of supercritical- carbon dioxide (S-CO) in a conceptual higher efficiency (≥50%) coal-fired boiler. Moreover, Hovi et al. [10] carried out transient integrated simulations, and investigated the effects of rapid load change situations on flue gas temperature, heat transfer and pollutant formation in a BFB boiler. Hence, it is seen that integrated modelling approach is state-of-the-art for coal-fired boilers and BFB boilers. However, in these previous studies the 1D-process models have mainly focused on the water circulation process, and the models utilized for the heat transfer sections have been simplified and based on porous media method. The porous media method does not provide an accurate solution for the flow field and material temperature distribution for the tube bundles in the heat transfer section. Therefore, in the present work, each superheater platen is modeled separately and comprehensively on both the CFD side and the 1D-PM side, which has not been done previously to the authors’ knowledge. In context of recovery boilers, the flue gas flow field and heat transfer in superheater region have been studied previously using standalone CFD simulations. Saviharju et al. [11] analyzed the flow field and temperature distribution in the upper furnace for two recovery boilers. Leppänen et al. [12, 13, 14, 15] studied deposit formation in recovery boilers and compared the results with experimental data. Maakala et al. [16] used surrogate-based analysis with CFD to optimize the heat transfer in superheater region. Maakala et al. [17] developed a detailed 3D CFD model for superheater region, and obtained a detailed 3D solution for flue gas flow field and heat flux distribution to superheater platens. However, the effects of flue gas side on the steam cycle and vice versa have not been well explored in the superheater region of recovery boilers, even though recovery boilers contribute around 25% of global industrial biomass based energy production [18]. In addition, there are few previous studies available where full-scale 3D CFD modelling approach has been adopted for recovery boiler simulations. Therefore, the main objective of this paper is to improve the understanding about heat transfer between hot flue gas and superheated steam cycle. The study includes the effects of 3D flue gas flow field in superheater region on heat flux distribution, steam distribution and material temperature distribution among the superheater platens. For this purpose, a full-scale 3D CFD model of superheater region is coupled with a detailed 1D-PM and integrated CFD/1D- PM simulations are performed. The developed 1D-PM is validated with reference data. This integrated CFD/1D-PM modelling approach is the novelty of this work. 2 METHODS AND MODELS 2.1 Description of the Case Figure 1-a shows the domain of the recovery boiler CFD model. The superheater region is marked in the figure by a rectangular box. The capacity of the recovery boiler is 1000 tds/d. The combustion of black liquor is assumed to be completed before the flue gas reaches the superheater region. Therefore, the furnace is not considered in this work. Table 1 shows the main operating parameters of the boiler. The reference data for recovery boiler was obtained at approximately 80% of its total capacity. It comprises of mass and energy balance calculations, and data from a measurement campaign. The model inlet is located between the tertiary air supply level and nose arch. This is done to assure that the tertiary air supply has minimum effect on the flue gas flow and the flow field is steady when flue gas reaches the superheater region. Similarly, the outlet is located far away from the superheater region to prevent the impact of outlet boundary conditions to the numerical solution of superheater region. The boiler walls, rear wall screen and boiler bank are evaporating surfaces. They are used to convert saturated water into saturated steam at almost a constant saturation temperature. The chosen recovery boiler has four stages of superheating (SH) including SH1A, SH1B, SH2, SH3 and SH4. The first stage superheaters (SH1A and SH1B) are counter-current superheaters according to the flue gas flow direction. The other superheaters are co-current heat 2 exchangers. Each superheater is made of 21 platens that are equally spaced across the width of the boiler. In reality, each platen has in-line, thin, seamless and tightly spaced tubes that carry steam inside. In this work, the superheater platens are considered as flat plates and linked with the 1D-PM on the steam side. Similarly, the boiler walls are modelled as flat surfaces instead of tightly fitted heated riser tubes. The boiler bank is modelled as a porous medium with calculated porosity, inertial loss coefficients and heat sink values. These simplifications are used to reduce the calculation time and complexity of the integrated CFD/1D-PM simulations. a b Figure 1: a) A two dimensional view of the recovery boiler geometry. 1) furnace (not considered), 2) inlet, 3-7) boiler walls, 8) nose level, 9) rear wall screen, 10) boiler bank, 11) outlet, 12) steam inlet and 13) superheated steam to the steam turbine. The boiler walls 3 and 4 are the side walls. The superheater region is represented with a rectangular box.