Middle-low temperature drying dynamic characteristics for poultry manure and its parameter optimization

Abstract: The poultry industry in China is developing. The estimated flock in 2017 was over 1.4 billion hens with a yearly manure output of 56 million tons. Such a large amount of poultry manure has a tremendous pressure on environment because of the associated air, water and soil pollution. Environmental and health problems such as odor and pathogens can be eliminated by decreasing the moisture content of poultry manure. Thermal drying has been proven to be an effective method to remove moisture. Drying at middle-low temperature can not only save energy but also reduce the emission of ammonia compared with traditional drying method at high temperature. Middle-low temperature heat sources, such as solar energy and warm weather in-house, can be used to dry poultry manure. Furthermore, dried manure can be utilized as a raw material to produce organic fertilizer. In order to study the drying characteristics of poultry manure at middle-low temperature, drying experiments of poultry manure were carried out in a temperature-controlled drying oven. The effects of drying temperature, manure thickness and air speed on manure moisture ratio and drying rate were examined. After that, common drying models were applied to simulate the drying process. The optimization of the middle-low temperature drying parameters was also performed by orthogonal test. The result showed that the middle-low temperature drying process of poultry manure could be divided into 2 falling rate periods: The first falling rate period and the second falling rate period. The removed water in the first stage was mainly free water, the removed water in the second stage was mainly the capillary water, adsorption water and bound water, and the drying rate in the second stage was lower than that in the first stage. With the increasing of temperature, the decreasing of thickness, and the increasing of air speed, the appearance of inflection point became earlier and the drying time became shorter. Exponential model was more suitable for describing the poultry manure drying process at middle-low temperature and the effective diffusion coefficients were 2.25×10-7-2.35×10-6 m2/h. In terms of drying effectiveness, the most efficient technological parameters were presented as follows: Drying temperature of 55 ℃, manure thickness of 6 cm, and air speed of 1.2 m/s, and the drying effectiveness under this condition was 0.47 h/g. The order of the factors for increasing drying effectiveness was drying temperature > thickness > air speed.