Operation Optimization of Biomass Integrated Energy System Based on Adjustable Heat-to-Electric Ratio

Considering the application of biomass energy, the Biomass Integrated Energy System (BIES) was first constructed. An integrated energy system operation optimization model was proposed with the objective functions of minimizing economic costs and maximizing clean energy utilization. Secondly, according to the characteristics of biomass Cogeneration, the scheme of adjusting the ratio of heat and power is proposed. Finally, a simulation analysis was conducted on a certain region in China. The results indicate that utilizing biomass energy in an integrated energy system can greatly reduce operating costs and improve energy utilization efficiency. After the heat-to-power ratio is adjusted, economic costs can be reduced again by 7.66%, and clean energy utilization can be increased by 6.15%.


Introduction
Since the 21st century, China's socio-economic development has been rapid, with a sharp increase in total energy demand.The contradiction between energy supply and demand has become increasingly prominent, and excessive reliance on fossil fuels has led to a series of environmental problems, such as climate change and haze.Improving the comprehensive utilization efficiency of energy, exploring ways to utilize renewable energy, and strengthening the consumption level of new energy have become inevitable choices for the future development of China's energy field [1,2].The integrated energy system coordinates distributed generation (DG) and natural gas (NG) in the form of distributed energy network [3], changing the situation of single and separated traditional energy systems.When the integrated energy system was proposed, relevant research focused on selecting equipment based on load characteristics and optimizing regional energy allocation [4][5][6].At present, in the integrated energy system, its main energy sources are natural gas and renewable energy such as wind, solar and geothermal energy [7].China's natural gas stock is limited, and excessive reliance on natural gas will limit the further development of the integrated energy system, which is not conducive to the achievement of China's "carbon peak and carbon neutrality" goals.Therefore, it is necessary to search for clean and optimized energy sources with huge reserves that have not yet been fully utilized.
The advancement of urbanization has brought a large amount of garbage and sewage, which contain sufficient low-grade energy.Fully utilizing these biomass energies is one of the directions for China's energy development.Combining biomass energy with cogeneration systems to build a biomass

Operation Mode of Biomass Integrated Energy System
The integrated energy system studied in this paper includes three subsystems: biomass treatment system, electrical system, and thermal system.As part of the integrated energy system, the biomass treatment system anaerobic processes the waste and sludge in the sedimentation tank to produce biogas required for biomass cogeneration.The sewage from the raw materials is discharged to the wastewater treatment point.A sewage source heat pump system is installed at the outlet of the treatment point to utilize the low-quality heat energy from the treatment point to meet the heat load on the demand side.Wind, photovoltaic, and biogas are the power generation sources of BIES.In order to mitigate the impact of renewable energy fluctuations, energy storage devices are added to BIES to interact and cooperate with the power grid to meet the internal power load of the system.The thermal system consists of biomass cogeneration, electric boilers, and sewage source heat pumps.The heat generated by biomass cogeneration and sewage source heat pump systems meets most of the heat load in the system, and the electric boiler is started in case of insufficient heat supply.
In addition to distributed power sources and energy storage equipment, BIES also includes biomass treatment systems and sewage source heat pumps.Among them, biomass cogeneration converts the biomass energy contained in biogas into electricity and heat energy.The sewage source heat pump utilizes low-level heat energy from the wastewater treated with biomass to provide heating and meet the heat load.
In the operation of biomass Cogeneration and sewage source heat pump systems, unlike the output power of fans and photovoltaic devices, which is difficult to control, these two devices can actively adjust the input power so that the output power of the equipment can be controlled within the range of 30% to 100%, improving the flexibility of the system.In addition, in order to further improve the energy utilization efficiency of the system, supplementary combustion facilities can be added to CHP to adjust the heating and power supply ratio of CHP, reduce the output of other equipment, and improve economic efficiency.

Principle of Thermoelectric Ratio Adjustment
In CHP, waste heat boilers can be divided into two categories based on the presence of supplementary combustion equipment: supplementary combustion waste heat boilers and non-supplementary combustion waste heat boilers.During the operation of a waste heat boiler with auxiliary combustion facilities, the natural gas supply is supplemented, and the waste is recycled for mixed combustion.The oxygen emitted by the gas turbine in the unit is used as a combustion aid, and the inlet of the waste heat boiler is used as a combustion chamber.The residual heat in the gas discharged by the gas turbine is utilized, and the heat generated by combustion is supplemented to heat the liquid in the boiler, thereby generating high-temperature steam and meeting the heating demand of the unit.
For non-supplementary combustion cogeneration units, the ratio of heating and power supply is affected by the unit performance.Generally, the ratio of heat and power is a fixed value.The supplementary fired Cogeneration unit can adjust the heat supply by supplementing the gas supply of the waste heat boiler, thus changing the heat-electricity ratio of the unit and improving the operating efficiency of the unit.Although the operation output of other heating equipment can be reduced by adjusting the heat-electricity ratio, the fuel cost of system operation will increase.When the amount of added gas reaches a certain level, the economy of cogeneration will decline.Therefore, in order to better determine the amount of gas replenishment, it is necessary to consider the cost of gas replenishment in the cost.

Objective function
The goal of optimizing the operation of biomass energy integrated energy systems is to reduce system operating costs and improve clean energy consumption rates.
(1) Objective function I: The operating costs of the system mainly include biomass processing, power supply, and heat supply costs, as shown in Formula (1): where BIES C is the operating cost of the biomass energy integrated energy system, B C is the biomass treatment cost, E C is the power generation cost of the system, and H C is the heating cost of the biomass energy integrated energy system.The above items can be represented in Formulas ( 2)-( 4): (2) Objective function II: In the integrated energy system constructed in this article, the input energy includes biomass energy, wind energy, and solar energy.To ensure the sufficient consumption of clean energy and reduce the phenomenon of wind and light waste, this article constructs a clean energy consumption objective function, as shown in Formula (5):  ( )  t and max ( ) t are minimum and maximum ratios of heating and power supply at t time.

Optimization Algorithm
Due to the integrated energy system constructed in this article containing various renewable energy sources such as biomass energy, wind energy, solar energy, etc., the system has high flexibility and uncertainty.In addition, this article constructs an objective function from the perspectives of economy and clean energy consumption.Its operation optimization is different from traditional power systems and is a multi-constraint, multi-variable, and complex nonlinear optimization problem.Therefore, this article uses a particle swarm optimization algorithm to solve the constructed IES model.When using the particle swarm optimization algorithm to solve the IES model constructed in this article, the solving process is as follows: Step 1: The basic data is entered.The main basic data includes equipment operating efficiency, equipment-rated power, energy prices, load information, etc.
Step 2: The parameters of the algorithm are initialized.For particle swarm optimization, the population size and iteration number are set, and algorithm parameters, including the learning factor, example speed, and other parameters, are adjusted.
Step 3: The fitness of the example is calculated based on the fitness function, and the objective function of IES is calculated based on the operational optimization strategy.
Step 4: Particle updates are performed.The fitness of the current particle is compared with the best position.If the fitness is high, it is updated; otherwise, it is kept the same as the original.
Step 5: It is determined whether the termination condition is met.When the solving result meets the set accuracy or reaches the set number of iterations, the solving process is terminated, and the calculation result is output.
Step 6: Analysis result is generated.The economic costs and clean energy consumption generated by IES during operation are analyzed based on the output optimization results.

Basic Data
A regional park in northern China is taken as an example for simulation analysis.The cooling demand of the park is relatively small and can be met through electricity consumption, so the cooling load of the park is not considered.In the BIES constructed in this article, the equipment includes wind turbines, photovoltaic units, biomass cogeneration units, sewage source heat pumps, energy storage systems, and electric boilers.
The heating of the park relies on biomass cogeneration and sewage source heat pumps.The biomass cogeneration unit is equipped with supplementary combustion equipment to increase heat output in the form of supplementing natural gas.In case of high heat load, additional heat supply is provided through electric boilers.The electricity load demand is provided by wind turbines, photovoltaics, biomass cogeneration, and the power grid.Table 1 shows the capacity of energy supply equipment in this area.The biomass cogeneration unit operates normally without additional supplementary combustion equipment.The optimal operation plan of each device is solved through a particle swarm optimization algorithm.
Scenario 2: When the thermal output of the equipment does not meet the thermal load, the biomass cogeneration unit's heat-electricity ratio is adjusted through supplementary combustion equipment to increase the thermal output, and the system's operation plan is obtained by solving.

Result Analysis
Figures 1 and 2 show the operating states of the electrical system under Scenario 1 and Scenario 2, respectively.Among them, the power generation equipment of the electrical system includes wind turbines, photovoltaics, biomass cogeneration, and the power grid.Due to the small installed capacity of power generation equipment and the high load in the park, the power supply relies heavily on the power grid.In Scenario 1 and Scenario 2, starting from 08:00, the load rapidly increases, and the electricity purchased from the grid increases, while the proportion of electricity purchased from the grid is relatively small at other times.Compared to Scenario 1, due to the increase in thermal output of biomass cogeneration through the operation of supplementary combustion equipment in Scenario 2, the operating power of the electric boiler decreases, resulting in a decrease in electricity consumption.Under the condition that the operating power of distributed generation equipment remains unchanged, the purchased power of the grid is reduced.During the energy storage operation process, Scenario 1 requires the purchase of electricity from the power grid.The energy storage operation strategy is to discharge during peak electricity prices and charge during low electricity prices.In Scenario 2, the energy storage operation is consistent with the optimization results of the IES operation, and charging occurs when the IES has sufficient power.In short, when the heating and power supply ratio of IES is adjusted, it can reduce the operating costs of heating equipment, reduce unnecessary grid purchases, and thus improve the utilization efficiency of IES clean energy and reduce operating costs.
The simulation results of the two scenarios are shown in Table 2.In Scenario 1, the operating cost is 87713.44yuan, and the clean energy utilization rate is 33.66%.In Scenario 2, the operating cost is 80996.25 yuan, and the clean energy utilization rate is 39.81%.The increase rates were 7.66% and 6.15%, respectively.It can be seen that in the constructed biomass integrated energy system, after adjusting the thermoelectric ratio, the dependence on purchased energy is reduced, the operation cost of the park is reduced, and the utilization rate of clean energy is improved, thus reducing carbon dioxide emissions.

Conclusion
Against the backdrop of China's proposal for the goal of "carbon peak and carbon neutrality", clean energy will gradually become the main energy to support China's socio-economic development.
Biomass energy, as a green energy source with low pollution, renewable energy, and wide distribution, has broad development prospects.Considering the application of biomass energy, this article adds biomass energy to the conventional comprehensive energy system, constructs a corresponding mathematical model, and solves it.In the IES constructed in this article, sewage source heat pumps can recover heat energy and achieve multi-level energy utilization.The solution results indicate that compared to other methods, using biomass energy for operational optimization can reduce the system's operating costs and improve energy utilization efficiency.

Fig. 1
Fig.1 Scenario 1 power system running status.Fig.2 Scenario 2 power system running status. ) es C t represents the operating cost of EES at t hour.( ) H t respectively represent the operating cost and heat of the heating equipment at t hour.bg C and ( ) bg H t represent the system's supplementary combustion cost and quantity.di C , om C , bt C , and oc C are the cleaning costs, operating costs, sewage treatment costs, and other costs of the biomass treatment system.
)) BC t is the biomass cogeneration heat to electricity ratio at t time.min loadHt are the electrical and thermal loads of the system at t time.

Table 1 .
Capacity configuration of energy supply equipment.Device

Table 2 .
Operation optimization results.Optimization