## 1 Introduction  
Scarcity of fossil fuels and their emissions of fossil fuel pollutants, such as carbon dioxide, into the atmosphere, and the resulting consequences, have led energy policymakers to look for alternative renewable and clean energy sources. In line with this target, biomass, which is a renewable energy generated from biomass, is a promising alternative source for the generation of clean energy and the development of a sustainable society [1]. World trends toward the modern dairy intensification on large production units cause massive animal manure production and accumulation, which can be a source of contamination of land, water, and air. In this sense, there are many efforts to transform these wastes into clean and renewable energy, such as the use of anaerobic digestion to produce biogas. The use of animal wastes is one of the very promising renewable energy alternatives paving the way for a more sustainable energy network. Animal manure is considered very attractive for the production of renewable energy since it is a natural resource that can additionally replace industrial fertilizers and improve soil fertility [2; 3; 4]. Animal wastes as biomass sources do not endanger food security and mitigate environmental impacts and may, therefore, considerably contribute to an appropriate waste management. As a result, converting animal wastes to energy is an important issue that has attracted the attention of academic and industrial researchers [5]. However, the environmental impacts of livestock production are attracting increasing attention, especially the emission of greenhouse gases (GHGs), and improper handling of manure produced by industrial farm operation greatly deteriorates the major environmental media including air, water, and soil [6; 7; 8]. Manure management is becoming an increasing burden for large dairy farms across the world due to land application limitations for nutrients and hauling costs. One solution is to install advanced manure management systems, which remove large portions of water from the manure and treat it to a level sufficient for surface water discharge, livestock consumption, or irrigation [9; 10]. In general, animal manure produced by livestock farms has a moisture content of about 80%, and such livestock manure is dehydrated to a moisture content of 20-25% and solidified to be used as biomass and fuel for thermal power plants. In the treatment process of livestock manure, the manure drying system is the core equipment for agitation, drying, and odor removal. So far, direct dehydration methods using hot air, ultrasonic waves, and microwaves have been most widely used, but such methods cause machine vibration and incur additional facility costs such as decompression devices, and do not ensure smooth agitation of manure, resulting in problems such as drying non-uniformity and delays. In addition, the oil-drying method, which is the method of rapidly evaporating moisture by using oils with a smaller specific heat capacity than water, needs continuous consumption of waste oil during the drying process. If these conventional drying methods for livestock manure are used, manure is not sufficiently dried due to low drying performance, which causes odor, deteriorating the working conditions of workers and causing great inconvenience and annoyance to local residents. [PERSON] developed an innovative design of livestock and poultry manure dryer to improve the transportation efficiency, drying uniformity, and efficiency. The equipment consists of a sloping spiral feeding device to improve the efficiency of material transportation and seepage to preliminarily filter out and separate the moisture in manure [11]. [PERSON] set up a purpose-designed experimental drying apparatus to simulate the commercial manure drying system. Drying experiments were carried out to assess the impacts of hot air temperature, air velocity, and manure layer thickness on the fan's energy consumption, dehydration rate, and nitrogen loss rate using the response surface analysis method.