Techno-economic evaluation of biomass drying in moving beds: The effect of drying kinetics on drying costs

Drying woody biomass holds the potential to improve the energy efficiency of certain processes, such as in CHP plants. Drying can also be a necessary unit process in several energy conversion processes (e.g. in biomass gasification). Belt dryers are typically used for drying when low temperature air (≤100–110 °C) is used. This article aims to produce new knowledge about the influence of the main design parameters on the drying costs of a low temperature belt dryer when three different materials (forest residue, bark, as well as sawdust and soot sludge mixture) are dried using it. The influence is analyzed by changing the following parameters: bed height, air temperature, air velocity and initial/final moisture contents of the material. The study aims to evaluate which of these parameters has an actual effect on drying costs. Results indicate that the lowest costs are achieved with the highest air temperature if the heat price is the same for every air temperature level. However, an optimal bed height depends on the material. Increasing the air velocity does not necessarily decrease the costs. In the sensitivity analysis, to factor in the influence of the temperature on the heat price, the price was changed for every drying air temperature (1, 5, 10 and 15 €/MWh). This analysis showed that the lowest drying costs are achieved by the lowest air temperature in all cases, thus indicating that the price of the heat has a remarkable influence on the economics of drying. Furthermore, the results support the use of low temperature heat sources in drying if they are clearly less expensive than higher temperature heat sources. However, if the prices for lower and higher air temperatures are of the same magnitude, the higher air temperatures are preferable. In general, this paper shows that it is important to pay attention to the main design parameters to optimize total drying costs. For example, if an overly low bed height is used in woodchips or bark drying, the total drying costs might be dozens of per cent higher than in the most economic case.木质生物质干燥可提升热电联产（CHP, Combined Heat and Power）电厂等特定工艺的能源利用效率，同时亦是诸多能量转化过程中不可或缺的单元工序，例如生物质气化工艺。当采用低温空气（≤100–110 ℃）时，带式干燥机通常为生物质干燥的首选设备。 本研究旨在针对森林残余物、树皮以及锯末与烟尘污泥混合物这三种不同物料的低温带式干燥工艺，探究核心设计参数对干燥成本的影响规律，以期获取全新的专业认知。本次分析将调整以下四类参数：料层高度、空气温度、空气流速以及物料的初始/最终含水率，进而明确上述参数中哪些会对干燥成本产生实质性影响。 研究结果显示，当不同空气温度对应的热价保持一致时，采用最高空气温度可实现最低干燥成本。不过，最优料层高度会因物料种类的不同而发生变化。提升空气流速未必能够降低干燥成本。在敏感性分析环节，为充分考量温度对热价的影响，本研究针对四种干燥空气温度工况（1、5、10及15 €/MWh）分别设置了对应的热价。分析结果表明，在所有工况下，采用最低空气温度均可获得最低的干燥成本，这充分说明热价对干燥经济性具有显著影响。此外，若低温热源的成本显著低于高温热源，则干燥工艺可优先选用低温热源；但如果高低温空气对应的热价处于同一量级，则更高的空气温度更为经济。 总体而言，本研究证实，优化干燥总成本需重点关注核心设计参数。例如，若在木片或树皮干燥工艺中采用过低的料层高度，其总干燥成本可能较最优工况高出数十个百分点。