Abstract This work presents an investigation of an indirect firing scheme using biomass (cardoon) as supporting fuel for a pulverized-lignite boiler during its operation at a thermal load of 35%, lower than the current minimum one. Results are compared with an alternative indirect firing scheme which employs pre-dried lignite (PDL) as supporting fuel. The numerical investigation of the boiler has been conducted using the commercial software ANSYS Fluent® v15.0, supported by in-house built functions for the combustion rate of the fuels and the drag force exerted on biomass particles. The simulation and the comparison of the combustion of two different fuel blends in a co-firing strategy is important in order to evaluate technically the available possibilities and select among them the optimal firing concept (number and position of the operating injection ports) and the ideal fuel blend for the operation of the unit at lower than technical minimum thermal loads, in order to attain reduction of emissions and improvement of the boiler flexibility. Due to the implementation of a two-stage over-fire air (OFA) system and the consideration of sub-stoichiometric conditions exhibited at the nominal load, this work also takes into consideration the modeling of the boiler convective section, following the porous media analysis provided by the available Macro Heat Exchanger Model, ANSYS Fluent®. The validation of the applied models has been performed in previous works of the same group of authors, while the combustion results regarding crucial combustion parameters have been compared against corresponding values derived by a suitably-developed thermodynamic model. The agreement between these two models is good, since the maximum percentage deviation is calculated to be in the range of approximately 10%. Based on the numerical results, it can be concluded, that the utilization of both types of supporting fuel can ensure the stable operation of the boiler at thermal loads, lower than the technical minimum of the unit, promoting its flexibility. Between the two different supporting fuels, it is observed, that biomass ensures higher combustion efficiency compared to pre-dried lignite. However, it is also indicated, that the reduction of NO x emissions and the intensity of the induced thermal loading on the membrane walls with the utilization of biomass is more dependent on the firing strategy compared to lignite, proving the key role that this parameter plays to the operation of a boiler using this specific fuel blend.

中文翻译：

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使用预干燥褐煤或生物质作为辅助燃料的褐煤锅炉在极低热负荷运行中混烧方案的比较研究

摘要 这项工作提出了一项间接燃烧方案的研究，该方案​​使用生物质（苁蓉）作为褐煤粉锅炉在 35% 热负荷下运行期间的辅助燃料，低于目前的最低负荷。结果与采用预干燥褐煤 (PDL) 作为辅助燃料的替代间接燃烧方案进行了比较。锅炉的数值研究是使用商业软件 ANSYS Fluent® v15.0 进行的，并由内部构建的燃料燃烧率和生物质颗粒上的拖曳力函数支持。混合燃烧策略中两种不同燃料混合物的燃烧模拟和比较非常重要，以便从技术上评估可用的可能性并从中选择最佳点火概念（操作喷射口的数量和位置）和理想的机组在低于技术最低热负荷的情况下运行的燃料混合物，以减少排放并提高锅炉灵活性。由于实施了两级过火空气 (OFA) 系统并考虑了在标称负载下出现的亚化学计量条件，因此在多孔介质分析之后，这项工作还考虑了锅炉对流部分的建模由可用的宏热交换器模型 ANSYS Fluent® 提供。应用模型的验证已在同一组作者的先前工作中进行，而关于关键燃烧参数的燃烧结果已与适当开发的热力学模型得出的相应值进行了比较。这两个模型之间的一致性很好，因为计算出的最大百分比偏差在大约 10% 的范围内。根据数值结果可以得出结论，两种辅助燃料的使用都可以保证锅炉在热负荷下稳定运行，低于机组的技术最小值，提高了其灵活性。据观察，在两种不同的支持燃料之间，与预干燥的褐煤相比，生物质能确保更高的燃烧效率。但同时也表明，