Abstract
According to the problem of large amount of carbon emissions during the cold chain distribution process, a cold chain distribution route optimization method for fresh agricultural products under the carbon tax mechanism was proposed. Firstly, with the goal of minimizing carbon emission cost and comprehensive cost, quantitative analysis of carbon tax mechanism is introduced, considering the demand quantity, demand time and unloading time constraints, a mathematical model of the problem is established. In addition, an improved quantum bacterial foraging optimization algorithm is put forward, which uses the bacterial optimization algorithm information update strategy to maintain group memory, and uses the carbon tax cost as the decision variable of the improved algorithm. Through experimental simulation, comparative analysis of the shortest distribution path, uninitialized pheromone bacterial foraging optimization algorithm and quantum bacterial foraging optimization algorithm on the last selected study model, the method proposed in this thesis can effectively optimize the distribution path, reduce carbon tax cost and comprehensive cost.
Keywords
Introduction
Nowadays, reducing carbon emissions and lowering integrated costs have become hot research topics in cold chain logistics distribution [1]. In global carbon emissions statistics, transportation carbon emissions account for 14%, and road carbon emissions account for 70% of transportation carbon emissions [2]. In the delivery of refrigerated trucks will consume a lot of energy and a huge carbon emission, therefore, reasonable planning of cold chain logistics distribution paths to reduce carbon emissions and comprehensive costs is urgently needed to be resolved. VRP (Vehicle routing problem, VRP) initially planned the route based on the delivery distance [3]. On this basis, considering the impact of the delivery time on the route [4], the TVRP (Time vehicle routing problem, TVRP) model was established [5]. Due to the characteristics of transportation products, it is necessary to establish a JIT model [6]. JIT (Just in Time, JIT) delivery model needs to consider the customer point of working time to affect the cost of, proposed a two-time work aims model [7]. Uncertainty of demand and delivery time at the distribution point will affect the results of experiment, and the performance of the model and algorithm solution needs to be comprehensively considered [8–10]. Secondly, the path length uncertainty will demand and delivery time and cost of the distribution points of impact [11]. In the actual transportation process, traffic congestion will increase the distribution cost and path length. For this, a multi-channel network distribution model is proposed [12]. At the same time, it is believed that disturbance events such as vehicle failures will affect the distribution plan [13, 14]. However, the refrigeration time of refrigerated trucks and energy consumption during transportation will generate a lot of carbon emissions [15], and a multi-objective optimization model for carbon emissions has been proposed [16]. Finally, by analyzing different situations of traffic congestion, a cold chain delivery model combined with time-varying networks is proposed [17]. Meanwhile, Rao Weizhen build low-carbon collaboration around the vehicle routing problem model, a two-stage algorithm for solving [18]. Ma Qizhuo proposed an optimal VRP model for urban distribution [19]. Rao Weizhen analyzed the impact of road gradient changes on carbon emissions and established a low-carbon distribution model [20]. Chen Zhi proposed a problem model considering the impact of different delivery times and driving sections on carbon emissions, and verified the effectiveness of the model [21].
Conclusively, many scholars have conducted in-depth research on different aspects of low-carbon distribution, but there are limitations. The main manifestations are as follows: (1) Existing models only consider the impact of distribution distance, capacity and time cost on carbon emissions, but carbon tax mechanisms are rarely introduced [22]. (2) Existing models have little consideration on the impact of the unloading time of refrigeration equipment and the transportation cost of different demand times on carbon emissions [23]. According to the above analysis, this article considers the impact of unloading time and different demand times on carbon emissions, introduces a carbon tax mechanism, and establishes a mathematical model of minimum comprehensive cost. Then, using the carbon tax cost as the decision variable of the algorithm, an improved quantum bacterial foraging optimization algorithm (QBFO) is put forward to solve the model. Finally, the effectiveness of the proposed model and algorithm is verified by local optimization and comparison of several algorithms.
The full text is divided into six parts, this part is the introduction, the second part is to describe the research problems and hypotheses. The third part establishes the distribution logistics model under the carbon tax mechanism. The fourth part designs an improved quantum bacterial foraging optimization algorithm for solving the model. The fifth part is to verify the validity of the algorithm for solving the model (simulation experiment of solving the model). Finally, the conclusion and prospect of the research problem are summarized.
Problem hypothesis and description
To issue model the following assumptions: (1) Only one distribution center. (2) The same K refrigerated trucks load capacity, and the maximum load capacity of the distribution process is not available. (3) The location of the delivery point, demand, demand time, and unloading time are known. (4) K vehicles are driving at a constant speed. (5) Each delivery point only by a delivery of refrigerated trucks, and each refrigerated truck can serve multiple delivery points.
Therefore, the cold chain logistics distribution problem refers to the analysis of the carbon emission and the degree of cargo loss of refrigerated trucks in the conventional VRP problem, and the establishment of corresponding solution strategies. The problems studied are described as follows: One delivery center transport fresh agricultural product to N delivery points. The delivery center has K refrigerated trucks, and the K vehicle models have the same fuel type and traveling speed. Each refrigerated truck serves customers which need to meet demand time, vehicle capacity and customer demand. During the transportation of these vehicles, the path selection should be rationally planned to achieve the lowest cost.
Mathematical model of low carbon path optimization problem
Symbol and parameter description
N: Number of delivery points;
K: Number of refrigerated vehicles;
qi: Customer demand;
C1: Fixed cost coefficient of refrigerated truck;
C2: Refrigerated trucks transport cost factor;
d ij : Customer point (i, j) transport distances;
Q ij : Customer capacity (i, j) Vehicle capacity;
Q: The maximum capacity of the refrigerated truck;
Q m : Product quality remaining after vehicle leaves customer point;
p1: Unit fuel price during transportation;
p2: Unit price of fuel oil;
P: Unit price of fresh produce;
Ti: Unloading time;
ω: Carbon tax factor;
Symbol and parameter description
This paper analyzes the impact of comprehensive vehicle transportation costs, carbon tax costs, and time penalty costs on the optimization of cold chain logistics distribution [24]. A cold chain logistics distribution optimization problem model with the minimum comprehensive cost as the goal is established as follows [25]:
Equation (2) express transportation vehicles in transit cannot exceed the maximum capacity. Equation (3) indicates that every customer point served by only one transportation vehicle. Equations (4) and (5) indicate that the vehicle is allowed to depart and arrive only once for any customer. Equation (6) represents the elimination of the sub-loop conditions. Equation (7) indicates that vehicle transportation is continuous during the distribution process.
In Equation (1),
The problem studied in this article is fresh produce. Considering the impact of product consumption on transportation refrigeration and unloading refrigeration costs, a carbon tax coefficient ω was introduced to quantitatively analyze carbon emissions. In Equation (1), the carbon tax cost Z2 is as follow:
This article introduces the method of calculating carbon emission costs [26]. ρ0 is the oil consumption per unit distance with zero load capacity, and ρ* is the oil consumption per unit with Maximum capacity. In Equation (8), when the vehicle capacity is Q
ij
, the carbon emission cost FC
ij
of the customer node (i, j) is as follows:
This paper introduces the freshness decay function of fresh produce [27]. ∂1 is the fresh attenuation coefficient of the product during transportation, ∂2 is the fresh attenuation coefficient of the product during unloading, and ∂1 < ∂2 [28].
This paper considers the time demand window (E
j
, L
j
), which indicates the time range of the earliest and latest time points when the customer can accept the transportation vehicle. ɛ1 is the time penalty coefficient for customers who arrive early, and ɛ2 is the time penalty coefficient for customers who arrive late. In Equation (1), the time penalty cost Z3 is as follows:
Common BFO
The common bacteria foraging optimization algorithm (BFO) [16] is a kind of algorithm based on global random search, and its main operations include chemotaxis, Reproduction and Elimination. The chemotaxis is the core of BFO, including Tumbling and Swimming. When the bacterial individual is to search nutrients, it will turn to search better fitness value until it reaches the maximum number of chemotaxis or meets worse solutions. Then a half of individual with weak ability of foraging will be eliminated according to fitness value of all individuals in a cycle, meanwhile, the other half ones will be replicated to keep the number of bacteria. Elimination is to create new bacteria individuals to keep the total number of bacteria, due to some individual may die in the searching process, which is conductive for chemotax is to jump out of the local optimization. If the elimination reaches enough iteration, the algorithm will end.
Improved QBFO
It is known that the short step results in the lack of effective information sharing among individuals during the evolution process, which is not conductive to the diversity of the population. If the step is too short, the possibility of premature convergence will be highly, otherwise, the convergence will reduce too greatly to obtain the optimal solution [19]. Considering the quantum theory can make the individual appear in any position of the whole feasible search space with strong global search performance and population randomness [30], the QBFO is proposed in this paper.
The state and the position of the individual bacteria are uncertain in the quantum space, and they are determined by wave function of ψ (Y, t). The probability density function of the individual position is represented as |ψ|2. The attractive potential based on the Delta potential well model is established on each dimension of the attractor. The potential energy function can be expressed as follow:
Where, Y = x
id
- P
d
is the distance between the individual position of x
id
and the attractor. It is introduced into Schrodinger (Equation 11) to obtain ψ (Y, t) (Equation 12) and Q (Y) (Equation 13) in each dimension, in which L represents the feature length of Delta.
The motion of the individual in the potential field follows the above |ψ|2, and its position is uncertain. But in the practical application, the individual bacteria must have a certain position in any time, so the individual motion equation will be got through the Monte Carlo simulation in this paper. Take the random number of u in (0, 1), and u = e-2|x
id
-P
d
|/-L, then the position update equation of the d dimension variable of individual i is shown in Equation 14:
In Equation 15, β is contraction expansion coefficient, and β <1.782 is to ensure the convergence of the algorithm [31]. The method is to change β1 into β2 linearly following the evolutionary algebra (Equation 16), and mbest is the average value of the best position vector in the population (shown in Equation 17).
Considering the disadvantages of the fixed step, a dynamic indentation control strategy is proposed here to expand the search space under the premise of the convergence.
The implementation steps of QBFO are as follows: Initialize the parameters. Including the number of individual bacteria of s, migration times of N
ed
, reproductive times of N
re
, chemotaxis times of N
c
, swimming times of N
s
and migration probability of P
ed
. Initialize population. Generate s individual bacterial vector of x
i
randomly in the solution space. Calculate the fitness function J of each individual bacteria. Start to cycle. Transfer cycle l = 1:N
ed
; reproductive cycle k = 1:N
re
; chemotaxis cycle j = 1:N
c
. Start to chemotaxis. The following operation is performed for each bacteria i: Turning: generate a random vector of Δ ∈ R
n
to adjust the direction, and each element in vector Δ is a random number in [–1, 1]. Update the position of the individual bacterial x
id
(d = 1, 2, …, D), and the rest D-1 variables remain unchanged.
Swimming: evaluate the fitness of x
i
(j + 1, k, l), if the fitness is superior to x
i
(j, k, l), x
i
(j, k, l) will be replaced with x
i
(j + 1, k, l) and swim in accordance with the direction of the move until the fitness value is no longer improve or reach the maximum number of steps. Make d = d+1, if d = D, then go to step 5(d), otherwise go to step 5(a) to continue to operate the next variable. Make j = j+1, and change the swimming step of individual bacteria according to the dynamic indentation strategy in Equation 21, where A is dynamic indentation coefficient.
Reproduction based on quantum behavior. After a complete chemotaxis cycle, the current individual best position and the global optimal position are to update. Calculate the average best position mbest of the population according to Equation 17 and update its position according to Equation 14. Migration. Sort all the bacteria according to the energy, and migrate the bacteria (s, Ped) whose fitness is low according to random initialization. Judge whether the cycle is complete.
The flow chart of the Improved Quantum Bacterial Optimization Algorithm (QBFO) is shown in Fig. 1.

Flow chart of QBFO.
Experimental parameters
This paper selects literature data and 13 supermarket stores with centralized logistics distribution as service points [15]. The refrigerated transport truck has a maximum load of 10 tons, and the unit price of fresh vegetables is 3,500 yuan/ton. Transportation vehicles are delivered from the transportation center at 5 am. The fixed transportation cost of each refrigerated truck is 80 yuan, the fuel type is diesel, and the uniform speed is 45 km/h. The fuel consumption per unit of refrigerated transport truck is 17.5 l/km, and the fuel consumption per unit is 22.5 l/100 km. The unit cooling cost per unit of time for vehicle transportation is 2 yuan/h, the unit cooling cost per unit of unloading is 6 yuan/h, the carbon emission is 2.5 kg/l, and the carbon tax cost is 142.63 yuan/ton. The number of the transportation center is 0, and the number of each supermarket customer point is 1,2,3...12,13. The detailed demand information of the supermarket customer points is shown in Table 1.
Supermarket customer point detailed demand information
Supermarket customer point detailed demand information
According to the algorithm and case data, seeking answers to the cold chain logistics distribution model. The number of population is 40, the coefficient of contraction-expansion=1,=0.5, the migration probability is 0.25, the number of migration = 3, the number of reproduction = 5, the number of chemotaxis = 20, the number of swimming = 4 and the coefficient of dynamic indentation A = 0.7. Then the total number of chemotaxis iteration MAXIER = 300 and the maximum number of iterations QBFO is 150 generations. The test running environment of this article is: MATLAB R2010a was used to run the computer with 2.8 GHz Intel Core I5 CPU and 4GB RAM to generate the initial schedule. Considering the lowest carbon tax cost and the shortest distribution route, the optimal distribution route for 10 transportation runs is shown in Fig. 2:

The lowest carbon tax and the shortest distribution route.
The transportation strategy with the lowest carbon tax cost is: 5 vehicles departing from the transportation center, and the first vehicle path is 0-1-10-5-4-0. The second vehicle path is 0-7-12-13-0. The third vehicle path is 0-11-3-0. The fourth car has a path of 0-9-8-0, and the fifth car has a path of 0-2-6-0. The shortest path under this strategy is 1989KM, the comprehensive cost is 4456 yuan, and the carbon tax cost is 4009 yuan.
The shortest transportation strategy for the distribution route is: 4 vehicles from the transportation center, and the first vehicle route is 0-1-10-5-4-0. The second vehicle path is 0-2-11-3-0. The third vehicle path is 0-6-8-9-0, and the fourth vehicle path is 0-13-7-12-0. The shortest path under this strategy is 1896 km, the comprehensive cost is 4537 yuan, and the carbon tax cost is 4186 yuan.
The results of two different experiments are shown in Table 2. From the analysis in Table 2, it can be known that the shortest distribution path may not necessarily reduce the carbon tax cost and comprehensive cost. Experimental results show that although the distribution path under the carbon tax model is increased by about 0.49%, the overall cost is reduced by 1.7% and the carbon tax cost is decreased by 4.2%, which verifies the effectiveness of the model.
Comparison of carbon tax cost results in different situations
In order to test the performance of QBFO and its improved strategy for solving low carbon cold chain distribution, and to compare with the performance of the popular classical quantum, simulation experiment is run in this section. On the basis of the above testing, the classical Solomon [32] examples are used to compare QBFO with other variations of algorithm. There are 6 kinds of examples in Solomon, and one standard problem from each kind of examples is selected and executed for 20 times using QBFO. The efficiency will be verified through the comparison with existing QPSO (Quantum particle swarm optimization) [33], QACO (Quantum ant colony optimization) [24], IBFO (Improved bacterial optimization) [18] and BFO.
The maximum iteration is 100 and the convergence curve of the optimal solution of the five algorithms can be shown in Fig. 3. It can be seen from Fig. 3 that the convergence speed of the proposed QBFO is faster than the other four algorithms obviously, and it has also better global optimization capability.

The fitness convergence curve of 5 algorithms.
Moreover, in order to further proof the validity of the algorithm, the above experimental parameters were selected, and comparative analysis of the proposed QBFO with QACO and BFO. Besides, the experiment was run randomly for 20 times. The combined cost and carbon tax cost results of different algorithms are shown in Fig. 4.

Comparison of comprehensive cost and carbon tax cost.
Derived from the experimental results in Fig. 4, for the problem of cold chain logistics distribution path optimization under the carbon tax mechanism studied, the experimental results of its algorithm are compared with the basic BFO in this paper: the minimum comprehensive cost is reduced by about 3.4%, and the carbon tax cost is reduced by about 8.3%. Similarly, compared with the QACO, the minimum comprehensive cost is reduced by about 2.7%, and the carbon tax cost is reduced by about 4.5%.
Conclusion
(1) Consider carbon tax costs. By comparing and analyzing the shortest distribution path and the smallest distribution cost, the experimental results show that the shortest distribution path cannot effectively reduce the carbon tax cost and the comprehensive cost.
(2) Local optimization analysis. Through comparative analysis of different situations of initializing pheromone, the experiment results show that the proposed QBFO makes the algorithm jump out of the problem of local optimization. In addition, the carbon tax cost and comprehensive cost during transportation are also reduced. Choosing different values for the algorithm’s data for analysis has little effect on the experiment results, verifying the effectiveness and stability of the proposed algorithm.
(3) From the method based on mathematical model and experimental data, compare the proposed QBFO, QACQ and basic BFO, it proves that the method proposed in this study can effectively decrease the carbon tax cost and comprehensive cost in the cold chain transportation process, which further shows that the model and algorithm proposed are effective.
Future directions
Except the methods used in the paper, some of the most representative computational intelligence algorithms can be used to solve the problems, like monarch butterfly optimization (MBO) [34], earthworm optimization algorithm (EWA) [24], elephant herding optimization (EHO) [35], moth search algorithm(MSO) [36].
Based on this article, additionally, the occurrence of uncertain interference events such as traffic congestion and vehicle failure during vehicle transportation will affect the overall cost and carbon tax cost. Therefore, how to reduce the impact of uncertain interference events on costs is the direction of this paper’s next research.
Declaration of competing interest
None. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Footnotes
Acknowledgments
Foundation items-Project supported by Liaoning Provincial Natural Science Foundation (20180550499, 2019-ZD-0109), the Fundamental Research Funds for the Central Universities (110084) and Education Department Project of Liaoning Province (JDL2019022, L2020006).
