Temperature gradient in wood during grinding

Grinding is a commonly used method for producing pulps for papermaking, but its rather poor energy efficiency is a drawback. This paper focuses on developing a model dealing with temperature rise in wood during grinding. The model paves the way for the development of theoretical methods which can be used for reducing the energy consumption of the process. In grinding, wood is loaded by grits, which cause stress waves in the wood matrix. The stress waves fatigue the wood and ultimately separate fibres from the matrix, but because of wood’s viscoelasticity, part of the mechanical energy of waves is converted into heat. In order to understand the wood temperature increase in this process, a mechanistic model is developed here. The model is based on three hypotheses: a flux of mechanical energy occurs through the wood, the magnitude of the flux can be derived from the contact mechanics of the grits, and the rise in wood temperature can be determined from the dissipation of the flux. A temperature distribution in the actual grinding process was simulated with the model. The simulated temperatures were compared with a measured temperature profile obtained from the literature. The modelled and measured temperatures matched quite well. The simulations show that an increase in grit size results higher temperatures, whereas an increase in the distance between grits gives lower temperatures. The main result of the study is that the Hertz theory of contact mechanics can be considered an adequate method for analysing the effect of grits in the grinding process. The result shows that the Hertz theory is applicable when fatigue models are developed; these models can then be used to reduce the energy consumption of the process.