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部分工艺的能源效率，例如热电联产（Combined Heat and Power, CHP）机组工艺。干燥同时也是多种能源转化工艺（例如生物质气化（biomass gasification））中不可或缺的单元操作。当采用低温空气（≤100–110 ℃）干燥时，通常选用带式干燥机（belt dryer）。本文旨在探究主要设计参数对低温带式干燥机干燥成本的影响规律，实验对象包含三种不同物料：森林剩余物、树皮，以及木屑与烟尘污泥混合物。本研究通过调整料层高度、空气温度、空气流速以及物料的初始/最终含水率这四类参数开展影响分析，旨在评估上述参数中哪些对干燥成本存在实际影响。研究结果显示，若不同空气温度对应的热价保持一致，则提升空气温度可实现最低干燥成本；但最优料层高度因物料种类而异，且提升空气流速未必能降低干燥成本。在敏感性分析中，为纳入温度对热价的影响，本研究针对每一档干燥空气温度（1、5、10和15 €/MWh）调整了热价水平。该分析结果表明，所有工况下采用最低空气温度均可实现最低干燥成本，这说明热价对干燥经济性存在显著影响。此外，研究结果证实，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；但当高低温空气对应的热价处于同一量级时，则优先选用更高的空气温度。总体而言，本文表明，若要优化干燥总成本，需重点关注主要设计参数。例如，在木屑或树皮干燥工艺中，若料层高度过低，干燥总成本可能比最优工况高出数十个百分点。