Drying Characteristics of Chicken Manure Under a Variable Temperature Process

Abstract

In recent years, employing an auxiliary heat source, combined with residual heat from chicken house ventilation, to dry chicken manure has emerged as a novel manure treatment method. This method uses residual heat from chicken house ventilation in the early drying stage and an auxiliary heat source in the late drying stage, resulting in a variable temperature process. Nevertheless, limited research has been conducted on the variable drying process of chicken manure, and no standard currently exists for setting the drying parameters. To investigate the variable temperature drying process of chicken manure, this study set up a simulated drying system. First, the effects of drying temperature (45–65 °C), air velocity (0.6–1.8 m/s), and moisture content nodes (35–55%) on drying time, energy consumption, and total nitrogen loss were investigated using single-factor experiments. Then, an orthogonal experiment was performed to analyze the comprehensive impact of different influencing factors. Finally, the drying effect was evaluated using a multi-indicator comprehensive scoring method, and the drying parameters were optimized through a combination of orthogonal and single-factor experiments. The results showed that the drying temperature during the second stage had a significant effect on chicken manure drying performance under the variable temperature process (p < 0.05), while the moisture content nodes and air velocity yielded no significant effect (p > 0.05). In actual production, the following optimal parameters are recommended—drying temperature in the second stage: 57.5 °C, air velocity: 1.2–1.5 m/s, and moisture content nodes: 45–50%.

1. Introduction

Chicken manure is rich in organic matter and nutrients necessary for plant growth and is an excellent raw material for making organic fertilizer. Chicken manure drying, as a critical step for manufacturing organic fertilizer, can effectively remove the water in chicken manure and block and reduce its fermentation. Additionally, the drying process can eliminate harmful microorganisms, minimize the proliferation of pathogens, and mitigate potential risks to both the environment and human health. After drying, the water content of chicken manure is significantly reduced, and its volume is greatly decreased, allowing for convenient storage and transportation. Negative characteristics such as odor, pathogens, and stickiness are also remarkably improved. However, chicken manure drying, similar to the drying process of other materials, requires significant energy consumption to remove moisture. In recent years, the technology involving utilizing residual heat from chicken house ventilation for drying chicken manure has been widely promoted worldwide. This method removes moisture from chicken manure by utilizing the heat from air exhausted from chicken houses, demonstrating a positive effect on reducing energy consumption [1,2]. However, several challenges have emerged during the application of this technology in China, particularly during extended periods of rainfall or when the relative humidity of the exhaust air exceeds 85%. These conditions can significantly reduce the dehydration rate of chicken manure [3]. In winter, low ventilation rates and low inlet air temperatures further complicate the drying process, resulting in an increase in time for the moisture content to drop to 30%. This, in turn, can lead to manure accumulation and increased ammonia emissions [4,5,6].

To address the issue of low drying rates caused by prolonged rainy weather or high relative humidity in the external environment, a method for drying chicken manure combining residual heat from chicken house ventilation with auxiliary heat sources has been developed in China. A schematic diagram of the manure drying system is shown in Figure 1. Two stages make up the drying process. In the early drying stage (first stage), residual heat from chicken house ventilation is used to pre-dry the chicken manure. In the late drying stage (second stage), auxiliary heat sources such as biomass-burning furnaces, hot blast stoves, or heat pumps are employed to dry the chicken manure at higher temperatures, accelerating the dehydration rate during the final drying phase [7,8].

Currently, this technology has been widely used, both domestically and overseas, due to its favorable treatment effects. However, there is a lack of theoretical research on the drying characteristics of chicken manure under variable temperature conditions. It remains unclear how drying temperatures can be set to optimize energy savings and how drying parameters can be adjusted to minimize nitrogen loss during the drying process. Previous studies regarding the drying characteristics of chicken manure have primarily focused on environmental conditions to elucidate their effects on drying rate and drying time. Liska et al. [9] investigated the relationship between temperature, wind speed, and dehydration rate within a temperature range of 70 °C to 110 °C, developed a drying model for chicken manure, and determined the diffusion coefficient during the drying process. Similarly, Ghaly et al. [10] examined the effects of temperature, wind speed, and sample thickness on the drying process within a temperature range of 40 °C to 60 °C, resulting in a water loss model for chicken manure. However, these studies were conducted under constant-temperature drying conditions, and their findings are mainly applicable to traditional drying methods, such as electric heating drying and coal-fired hot air drying. These traditional methods are associated with high energy consumption and significant nitrogen loss [11,12,13,14]. Utilizing residual heat from chicken house ventilation for variable-temperature drying offers a promising approach to address these issues. Therefore, this study will conduct theoretical research on the variable-temperature drying process. Using drying time, energy consumption, and total nitrogen loss as key performance indicators, the study will investigate the effects of drying temperature, air velocity, and moisture content levels on these performance indicators. Based on single-factor experiments, an orthogonal experiment will be performed to optimize the drying parameters, aiming to reduce energy consumption and nitrogen loss during drying. The ultimate goal is to identify the optimal drying parameters to achieve efficient energy usage and minimize nitrogen loss in the variable-temperature drying process of chicken manure.

The ultimate objective of chicken manure drying is to remove the moisture from chicken manure in order to produce organic fertilizer granules. Thus, one goal of this study is to allow for more nitrogen to be retained in organic fertilizer particles.

2. Materials and Methods

2.1. Experimental Material

We collected chicken manure samples from a commercial laying hen farm. The samples were collected in the laying hen house without any prior treatment. The manure samples was placed in a portable insulated box and transported to the laboratory on ice. Upon arrival, the samples were stored in a −20 °C refrigerator. Before each experiment, the samples were thawed at room temperature (22 °C) for 4 h and then placed on drying trays. The properties of the manure samples employed in this paper are listed in

Table 1. Properties of chicken manure samples.

Samples	Moisture (%)	Ash (%)	pH (%)	TKN (g/kg)
Chicken manure	78.5 ± 2.0	26.4 ± 1.5	6.5 ± 0.1	55.42 ± 1.98

Notes: TKN (total Kjeldahl nitrogen) is the sum of ammonia nitrogen and organic nitrogen; ash refers to the proportion of inorganic material remaining at the conclusion of the chicken manure drying process after water removal.

.

2.2. Experimental Apparatus

The drying experiments were executed by means of an experimental drying system, as illustrated in Figure 2. This system was meticulously engineered to replicate actual production conditions, aiming to ensure that the experimental results closely approximate the scenarios encountered in industrial applications. The experimental drying system predominantly comprised the following components (as shown by the numbers in Figure 2): ① a centrifugal fan for force convection (G1323A3, Hengshui Yongdong Scientific Inc., Hengshui, China), ② a humidifier (HTJ-2027B, Jiangmen Honetian Technology Co., Ltd., Jiangmen, China), ③ an electric heater (BXCP101, Shenzhen FHS Scientific Inc., Shenzhen, China), ④ a dry and wet temperature thermometer and a proportional controller (JWSK-5ACWD, Beijing Kunlunhai Technology Co., Ltd., Beijing, China), and ⑥ a tension sensor (ZNLBM-IIX, Bengbu Zhongnuo Sensor System Co., Ltd., Bengbu, China). The perforated tray (⑤) was used to hold the chicken manure samples, facilitating the passage of the heated air from the centrifugal fan at the bottom to dry the samples. Samples weight was monitored using the tension sensor, with the measured values displayed on an external monitor outside the experimental drying system. The manure samples from different groups were placed in a perforated tray (Φ = 60 mm) with a layer thickness of 40 mm, and the initial weight of the manure samples was 125 g. Total nitrogen concentrations in the manure samples were determined using Kjeldahl digestion (KDY 9810, Kaihong Weiye Science, Beijing, China). Energy consumption was measured with an electric energy reading socket (DL333502, DeLi Group Co., Ltd., Beijing, China).

2.3. Experimental Uncertainty

Errors and uncertainties are inherent in both the instruments and the drying process during measurement [15]. Temperature, air velocity, weight, energy consumption, and total nitrogen concentration were measured using appropriate instruments, as described in the previous section. The associated uncertainties are summarized in

Table 2. Uncertainties of the parameters during the chicken manure drying process.

Parameter	Uncertainty
Temperature of temperature sensor	±0.1 (°C)
Total nitrogen concentration	±0.2 (g/kg)
Air velocity	±0.1 (m/s)
Manure weight	±0.1 (g)
Energy consumption	±0.1 (kWh)

.

2.4. Experimental Procedure

In the practical application of utilizing residual heat from chicken house ventilation, the average temperature of the exhaust air in autumn and winter was approximately 20 °C. In the second stage, the drying temperature could reach 45–65 °C, with the aid of auxiliary heating sources. Therefore, the drying temperature for the early stage was set at 20 °C, while the second-stage drying temperature was set within the range of 45–65 °C. At the same time, the air velocity was maintained between 0.6–1.8 m/s, and when the moisture content dropped to nodes between 35 and 55%, the auxiliary heat source was activated to increase the drying temperature, based on the environmental parameters at the application site. To optimize the drying parameters, a single-factor experiment was performed to examine the effects of five second-stage drying temperatures (45, 50, 55, 60, and 65 °C), five air velocities (0.6, 0.9, 1.2, 1.5, and 1.8 m/s), and five moisture content nodes (35, 40, 45, 50, and 55%) on the variable drying process of chicken manure. The experiments were designed following the principle of single-factor test. A total of 13 groups of drying experiments were conducted (

Table 3. Drying conditions of different experimental groups.

Group	Drying Temperature in the Second Stage (°C)	Air Velocity (m/s)	Moisture Content Nodes (%)
1	45	1.2	45
2	50	1.2	45
3	55	1.2	45
4	60	1.2	45
5	65	1.2	45
6	55	0.6	45
7	55	0.9	45
8	55	1.5	45
9	55	1.8	45
10	55	1.2	35
11	55	1.2	40
12	55	1.2	50
13	55	1.2	55

).

Next, orthogonal experiments were conducted using the drying temperature in the second stage, air velocity, and moisture content nodes as influencing factors, with energy consumption, total nitrogen loss, and drying time as response targets. The factor levels were determined according to the outcomes of the single-factor experiments (refer to Section 3 for details). The experimental results were evaluated using a multi-indicator comprehensive scoring method, with the goal of minimizing energy consumption and nitrogen loss while optimizing drying efficiency. The weights assigned to each response factor were as follows: energy consumption (35 points), total nitrogen loss (35 points), and drying time (30 points), reflecting their relative importance. The total score was calculated using the following equation: total score = minimum energy consumption/energy consumption × 35 + minimum total nitrogen loss/total nitrogen loss × 35 + minimum drying time/drying time × 30.

2.5. Calculation of Drying Parameters

The weight of the manure samples was determined with a tension sensor, and the weight data were logged at 0.5 h intervals. Simultaneously, the initial moisture content of the samples was ascertained via the straight-forward drying approach at 105 °C. The real-time moisture content of the chicken manure was computed using the following formula (Equation (1)):

$${\mathrm{M}}_{\mathrm{t}}={\displaystyle \frac{{\mathit{W}}_{\mathit{t}\mathit{i}\mathit{m}\mathit{e}\mathit{l}\mathit{y}}-{\mathit{W}}_0(\mathbf{1}-{\mathit{M}}_0)}{{\mathit{W}}_{\mathit{t}\mathit{i}\mathit{m}\mathit{e}\mathit{l}\mathit{y}}}}\mathbf{\times }\mathbf{100}\mathbf{\%}$$

(1)

where ${\mathit{W}}_{\mathit{t}\mathit{i}\mathit{m}\mathit{e}\mathit{l}\mathit{y}}$ is the real-time weight (g) of the manure samples, ${\mathit{W}}_{\mathbf{0}}$ is the initial weight (g) of the manure samples, ${\mathit{M}}_{\mathbf{0}}$ is the initial moisture ratio of the samples, and ${\mathit{M}}_{\mathit{t}}$ is the real-time moisture ratio of the samples.

The total nitrogen loss refers to the proportion of total Kjeldahl nitrogen content lost during the chicken manure drying process. It is determined and calculated using the following method:

$$y_1=1-M_2/M_1$$

(2)

where $M_1$ and $M_2$ are the total Kjeldahl nitrogen concentration (g/kg dry weight) before and after drying, respectively.

During a single drying cycle, the electricity consumption (kWh) of the fan and electric heater was measured using an electric energy reading socket. In the actual drying process, the primary heat for the first stage of drying was derived from residual heat contained in the exhaust air from the chicken house ventilation. As a result, only the fan’s energy consumption was measured during the first drying phase. In contrast, during the second drying stage, when the auxiliary heat source was activated, both the auxiliary heat source and the fan contributed to energy consumption. Therefore, in the second stage, total energy consumption was determined by measuring the energy usage of both the fan and the electric heater.

3. Results and Discussion

3.1. Drying Characteristics at Different Drying Temperatures

The drying of chicken manure was carried out in two stages. The drying temperature in the first stage was maintained at 20 °C. When the moisture content of the chicken manure dropped to 45%, the drying temperature increased to 45, 50, 55, 60, and 65 °C in the different experimental groups, with an air velocity of 1.2 m/s throughout the drying process. As shown in Figure 3, the drying time gradually decreased with increasing drying temperature during the second stage. Zheng et al. found that temperature was the primary factor influencing the hot-air drying rate of porous materials such as sludge, and increasing temperature can improve the dehydration rate [16]. The results of the experiments demonstrated that as the drying temperature increased by 5 °C each time, the duration taken to reduce the moisture content of the manure to 10% was reduced by 3.2, 4.3, 2.5, and 1.3 h. When the drying temperature in the second stage reached 55 °C, the effect on reducing drying time was significantly greater than at other temperatures (p < 0.05). This can be explained by the evaporation dynamics of water in chicken manure. At the early stage of drying, the primary water removed is free water on the surface, which evaporates easily. In the middle and later stages, the primary water removed includes capillary water, adsorbed water, and bound water, which require higher heat energy to evaporate [17,18,19]. Although increasing the drying temperature enhances the drying rate, once the moisture content drops below 30%, the remaining water is primarily bound water, and the effect of temperature on increasing the drying rate diminishes. Therefore, the experimental data indicated that the drying times for the drying temperatures of 55 °C, 60 °C, and 65 °C were not significantly different (p > 0.05). Consequently, to effectively shorten the drying time at a lower temperature, the optimal drying temperature for the second stage was determined to be 55 °C.

Since the experiment aimed to simulate the real drying conditions, the energy consumption in the first drying stage was calculated based only on the fan’s energy consumption. In contrast, in the second drying stage, the total energy consumption was determined by measuring the combined energy usage of both the fan and the electric heater. According to the experimental results shown in Figure 4, when the drying temperatures in the second stage were set at 45, 50, 55, 60, and 65 °C, the corresponding energy consumption values were 2.79, 2.52, 2.36, 2.48, and 2.87 kWh, respectively. The analysis of the results revealed that energy consumption was significantly higher at drying temperatures of 45 °C and 65 °C (p < 0.05). At 45 °C, the energy consumption peaked because the drying time was the longest, leading to the increase in fan energy usage. The longer the drying time, the higher the fan’s energy consumption. On the other hand, as the temperature increases, the power of the electric heater rises. The experimental results further indicated that when the drying temperature exceeded 60 °C, energy consumption again began to rise. Therefore, to achieve optimal energy efficiency, the most suitable drying temperature for the second stage was determined to be 50–60 °C. Using traditional drying methods, we could see that previous studies had concluded that drying temperature was generally positively correlated with energy consumption. For example, Zeng et al. concluded that drying time was reduced and the energy consumption was decreased by increasing the temperature in the sludge drying process using hot air [20]. The difference between the method in this study and the traditional drying method was that the energy consumption in the first drying stage was calculated based only on the fan’s energy consumption. Therefore, the conclusions described above were reached.

As shown in Figure 5, the total nitrogen loss of chicken manure in different experimental groups showed no significant difference (p > 0.05). The primary reasons for this phenomenon may include the following: Nitrogen primarily exists in the form of NH₄-N and Org-N. Among these, NH₄-N (including NH₃ and NH₄⁺) mainly originates from the decomposition of uric acid and undigested proteins. In the early drying stage, the water content of chicken manure is relatively high, and a large amount of NH₄-N is dissolved in the water [21,22,23]. As the moisture content decreases to approximately 45%, most NH₄-N is released due to its volatile nature. At this point, the remaining nitrogen in chicken manure primarily exists as organic nitrogen. Despite the fact that an increase in the drying temperature can reduce the drying time, the degradation of uric acid and the generation of NH₄-N remain very limited in the second drying stage [6]. Therefore, higher drying temperatures cannot effectively reduce nitrogen loss. As a result, the drying temperature in the second stage had no significant effect on nitrogen loss during the chicken manure drying process (p > 0.05).

The above results are consistent with those of Nahm [21]. As the temperature continued to increase, a higher drying temperature was able to rapidly reduce the water content in the laying-hen manure to lower than 30%, resulting in the quick decrease in the activities of the microorganism, thus protecting the laying-hen manure against further aerobic fermentation and leading to the fixation of more organic nitrogen in the laying-hen manure.

Considering the effects of drying temperature in the second stage on both the drying rate and energy consumption, the optimal drying temperature for the second stage was determined to be 55 °C.

3.2. Drying Characteristics at Different Air Velocities

The drying time at different air velocities of 0.6 m/s, 0.9 m/s, 1.2 m/s, 1.5 m/s, and 1.8 m/s is shown in Figure 6. In this group, the drying temperature in the second stage was fixed at 55 °C, and the moisture content node was set at 45%. The results showed that drying time decreased as air velocity increased. During the early drying stage, when the drying temperature was relatively low, the time required to reduce the moisture content of chicken manure to below 45% was 26.7 h, 22.8 h, 20.7 h, 19.5 h, and 19 h, respectively. This trend occurs because an increase in airflow over the surface of the chicken manure reduces the thickness of the boundary layer, thereby decreasing the water vapor impedance transfer from the manure surface to the air. This process facilitates water evaporation from the chicken manure, effectively increasing the surface evaporation rate. However, as shown in Figure 6, when the air velocity increased to 1.2 m/s, the rate of dehydration improvement decreased significantly. This can be attributed to the fact that once the air velocity reaches a certain threshold, the airflow becomes sufficient to remove the evaporating water from the manure surface. At this point, the rate of water diffusion within the manure becomes the primary factor influencing dehydration during the low-temperature drying stage. In the second drying stage, as the remaining water in the chicken manure primarily exists as bound water, the drying temperature becomes the dominant factor affecting dehydration. Therefore, the influence of air velocity on drying time weakens beyond 1.2 m/s. The results indicated no significant difference in drying time among the groups with air velocities of 1.2 m/s, 1.5 m/s, and 1.8 m/s (p > 0.05). The findings regarding the effect of air velocity on the dehydration rate of chicken manure in this study are consistent with the results reported by Ghaly et al. [10]. Similar conclusions were also drawn by Aboltins [24] and Wu et al. [25].

Figure 7 shows the energy consumption of each group under different air velocities. The results indicate that higher air velocities correspond to increased fan energy consumption. When the air velocity reached 1.8 m/s, energy consumption was at its highest. This outcome can be attributed to the fact that the fan used in the experiment is a variable frequency fan, where fan power consumption is directly proportional to air velocity. Additionally, since there is no significant difference in drying time during the second drying stage across different air velocities, the energy consumption of the resistance wire remains essentially constant. As a result, higher air velocities led to higher total energy consumption. This conclusion was consistent with Zeng’s study on the relationship between wind speed and energy consumption in a sludge drying process [20]. Although chicken manure and sludge are different, they are both porous materials. Consequently, their drying characteristics are similar.

As shown in Figure 8, the total nitrogen loss decreased with increasing air velocity. During the early stage of drying, the temperature was relatively low, and the drying time was prolonged. Under these conditions, an increase in air velocity accelerated the dehydration rate, which in turn inhibited the degradation of uric acid and the generation of NH₄-N. In the second drying stage, the drying time is relatively short, and the degradation of uric acid and generation of NH₄-N is negligible [26]. Nitrogen loss increased with the increase in the thickness of the manure layer and the decrease in the air velocity. As a result, higher air velocities led to reduced nitrogen loss throughout the entire drying process. Similarly, it was also found that the total nitrogen loss increased with the increase in the thickness of the manure layer and the decrease in the air velocity within the test range [3].

Considering the combined effects of air velocity on drying rate, energy consumption, and total nitrogen loss, the optimal air velocity for the second drying stage was determined to be in the range of 1.2–1.5 m/s.

3.3. Drying Characteristics at Different Moisture Content Nodes

In this set of experiments, an air velocity of 1.2 m/s was sustained, and the temperature was increased to 55 °C when the moisture content dropped to 35%, 40%, 45%, 50%, and 55%, respectively. The experimental results (Figure 9) show that the drying time gradually decreased with increasing moisture content nodes, with a successive reduction of 0.5 h, 1.2 h, 2.5 h, and 5 h as the moisture content nodes increased. This trend can be explained by the fact that as the moisture content nodes increased, the transition to the second drying stage occurred earlier, and the drying rate remained consistently high during the second drying stage. Furthermore, it was observed that when the moisture content node was below 45%, the influence of the moisture content nodes on drying time diminished. The drying times at the moisture content levels of 35% and 40% did not show a significant difference (p > 0.05). A possible explanation for this phenomenon is that when the moisture content approaches 30%, the remaining water in the chicken manure primarily exists as bound water. This observation aligns with the conclusions presented in Section 3.1.

The energy consumption of each group is shown in Figure 10, with values of 2.69, 2.41, 2.30, 2.24, and 2.39 kWh, respectively. The experimental results reveal that when the moisture content node was set at 35%, the energy consumption was the highest. This occurred because a lower moisture content node extended the drying time in the first stage, leading to increased fan energy consumption. Additionally, when the moisture content node was set at 55%, the energy consumption was higher than that observed in experimental groups with moisture nodes of 40–50%. This result can be attributed to the larger amount of water that needed to be removed during the second drying stage, and the higher the moisture content nodes, the longer the second drying stage. At the same time, it can be seen from the changing trend of the bar chart that the influence of water content nodes on energy consumption is similar to that of drying temperature in the second stage. This result also indirectly shows that the energy consumption in the variable-temperature drying process is related to the drying time required for the two drying stages.

Analyses of Figure 11 showed that the total nitrogen loss gradually decreased as the moisture content node increased. For every 5% increase in the moisture content node, the total nitrogen loss decreased by 2.5%, 2.8%, 3.2%, and 3.5%, respectively. This trend was directly related to the drying time of the first stage. When the moisture content node was set higher, the drying time of the first stage became shorter. During the early drying stage, the water content of chicken manure is high, and the drying temperature is relatively low, which creates favorable conditions for microbial activity and the degradation of organic matter [4]. A shorter first-stage drying time results in lower uric acid degradation and reduced production of NH₄-N. In contrast, during the second drying stage, the temperature is high, which accelerates the dehydration rate of chicken manure and shortens the drying time. These conditions are not conducive to microbial activity or further hydrolysis of organic nitrogen. Consequently, the degradation of organic nitrogen and the generation of NH₄-N during this stage are negligible [27]. Therefore, a higher moisture content node results in less NH₄-N generation and lower NH₄-N volatilization throughout the drying process [6]. In the actual production process, reducing the fermentation time and retention time of chicken manure can reduce nitrogen loss in the chicken manure drying process.

Considering the effects of the moisture content nodes on the drying rate, energy consumption, and total nitrogen loss, the optimal moisture content node was determined to be 45–50%.

3.4. Orthogonal Experiment

To further determine the optimal technological parameters for the variable temperature drying of chicken manure, a three-factor, three-level orthogonal experiment was designed [28]. On the basis of the results obtained from the previous single-factor experiments, three levels were selected for each factor within the optimal range. The factor levels are presented in

Table 4. Orthogonal design of the experiment.

	Factor
	Drying Temperature in the Second Stage (°C)	Air Velocity (m·s−1)	Moisture Content Nodes (%)
1	52.5	1.2	45
2	55	1.35	47.5
3	57.5	1.5	50

. The orthogonal experiment was conducted according to the selected factor levels, with three replicates per group. The results of the orthogonal experiment are shown in

Table 5. Results and analysis of the orthogonal tests.

Number	Drying Temperature in the Second Stage (°C)	Air Velocity (m·s−1)	Moisture Content Nodes (%)	Drying Time (h)	Energy Consumption (kWh)	Total Nitrogen Loss (%)	Evaluation Score
1	1 (52.5)	1 (1.2)	1 (45)	27.9 ± 0.44	2.36 ± 0.08	29.7 ± 0.6	79.71
2	1 (52.5)	2 (1.35)	2 (47.5)	27.0 ± 0.17	2.23 ± 0.12	28.1 ± 0.2	83.79
3	1 (52.5)	3 (1.5)	3 (50)	25.5 ± 0.29	2.57 ± 0.09	25.4 ± 0.8	83.31
4	2 (55)	1 (1.2)	2 (47.5)	24.8 ± 0.17	2.48 ± 0.05	27.3 ± 0.4	82.99
5	2 (55)	2 (1.35)	3 (50)	23.2 ± 0.17	2.58 ± 0.08	24.4 ± 0.4	86.71
6	2 (55)	3 (1.5)	1 (45)	24.2 ± 0.17	2.64 ± 0.12	25.0 ± 0.8	84.23
7	3 (57.5)	1 (1.2)	3 (50)	19.8 ± 0.17	2.67 ± 0.08	21.6 ± 0.6	94.08
8	3 (57.5)	2 (1.35)	1 (45)	21.0 ± 0.29	2.49 ± 0.06	22.8 ± 0.8	92.65
9	3 (57.5)	3 (1.5)	2 (47.5)	19.7 ± 0.44	2.82 ± 0.05	23.9 ± 0.5	89.31
K1	246.81	256.78	256.59	Primary and secondary factors: the drying temperature in the second stage > water content nodes > air velocity.
K2	253.93	264.58	256.09
K3	276.04	256.85	264.10
Range	29.23	7.80	8.01

Notes: K₁, K₂, and K₃ are the sum of score values at each level of the three factors.

, and the results of the variance analysis are presented in

Table 6. Variance analysis of the orthogonal tests.

Source	Sum of Squares	df	Mean Square	F-Value	p-Value
A	154.88	2	77.44	67.05	*
B	13.40	2	6.70	5.80
C	13.42	2	6.71	5.81
Pure error	2.31	2	1.16

Notes: F_0.05(2,2) = 19.0; F_0.01(2,2) = 99.0; * indicates significant differences (0.01 < p < 0.05).

.

Using the multi-index comprehensive scoring method (refer to Section 2.4 for details), the drying time, energy consumption, and total nitrogen loss from each test group were evaluated, and the scoring results are presented in Table 5. Based on the scoring results, a range analysis was conducted to assess the impact of each factor on the drying process. The analysis revealed that the drying temperature in the second stage exerted the greatest influence on the drying process, followed by the moisture content node, with air velocity having the least impact. By comparing the sum of the score values across different levels of the three factors, it was determined that the optimal drying parameters are as follows—drying temperature in the second stage: 57.5 °C, moisture content node: 47.5%, and air velocity: 1.35 m/s. Under these conditions, the drying effect reached its optimal performance.

Additionally, Table 6 presents the variance analysis of the orthogonal test results. According to the analysis, the drying temperature in the second stage had a significant effect on the drying performance of chicken manure under variable temperature conditions (p < 0.05). In contrast, the moisture content nodes and air velocity showed no significant effects on the drying performance (p > 0.05). Thus, the air velocity and moisture content nodes can be set more flexibly within the experimental temperature range.

Based on the above results of the analysis of variance and the results of the single-factor experiment, in actual production, the optimal drying parameters could be set as follows: drying temperature in the second stage: 57.5 °C, air velocity: 1.2–1.5 m/s, and moisture content node: 45–50%. These conditions are expected to minimize drying energy consumption and reduce nitrogen loss during the variable drying process.

4. Conclusions

(1)

In this research, single-factor experiments were employed to explore the impacts of drying temperature, air velocity, and moisture content nodes on drying time, energy consumption, and total nitrogen loss, and an orthogonal experiment was performed to analyze the comprehensive impact of different influencing factors. Additionally, the drying parameters were optimized through a combination of orthogonal and single-factor experiments.

(2)

By analyzing the influence of drying temperature of the second stage, air velocity, and water content nodes on drying time, energy consumption, and total nitrogen loss, we found that the optimal value of the drying temperature of the second stage was around 55 °C, the optimal value range of air velocity was 1.2–1.5 m/s, and the best value range of the water content node was 45–50%.

(3)

In the range of experimental parameters, the drying temperature in the second stage had no significant effect on total nitrogen loss during the chicken manure drying process (p > 0.05). In contrast, air velocity and moisture content nodes exerted significant effects on total nitrogen loss (p < 0.05), and the air velocity had significant effects on energy consumption (p < 0.05).

(4)

From the results of the orthogonal experiment and variance analysis, the drying temperature in the second stage exhibited a significant effect on the drying performance of chicken manure under variable temperature conditions (p < 0.05). However, the moisture content nodes and air velocity showed no significant effects on drying performance (p > 0.05). Thus, the air velocity and moisture content nodes can be set more flexibly within the experimental temperature range. In actual production, the following optimal parameters are recommended—drying temperature in the second stage: 57.5 °C, air velocity: 1.2–1.5 m/s, and moisture content nodes: 45–50%.

(5)

The findings of this study provide a theoretical foundation for utilizing residual air exchange heat and auxiliary heat sources in chicken manure drying systems. Additionally, these results provide a basis for setting the parameters for the optimal usage phase of the auxiliary heat source.