Abstract: Biomass is an important renewable energy source. Currently, the main resource that can be developed and utilized is the waste produced by crops - straw. Compared with coal-fired power generation, straw power generation has very low pollution emissions and can be recycled, making it a clean renewable energy source. The main difficulty in the integrated automation control of biomass power generation is that the instability of fuel calorific value makes it difficult to control boiler combustion economically and stably, and the problems in straw collection, transportation, and storage make it difficult for the material conveying system to operate safely and continuously. This paper takes Shanxian biomass power plant as an example, introduces the system structure and control strategy of integrated automation of biomass power plant, and conducts a preliminary discussion on the problems exposed in the commissioning and operation and their improvement schemes. Keywords: biomass integrated automatic control control strategy 1. Introduction Changing the energy structure, maintaining energy security, and developing renewable energy are important measures for countries around the world today. Since the oil crisis in the 1970s, the development and utilization of biomass energy has begun to attract attention from various countries [1]. China has abundant biomass energy resources. According to estimates, China's theoretical biomass energy resources are about 5 billion tons. Currently, the main resource that can be developed and utilized is the waste produced by rice, corn and wheat - straw. The National Development and Reform Commission has listed biomass direct combustion power generation as an important aspect of the development of the renewable energy industry [2]. With the promulgation and implementation of the National Renewable Energy Law, biomass power generation will surely usher in a peak of development. Unlike the integrated automation control system of traditional thermal power plants, the process and equipment of the integrated automation system of biomass power plants have their own special characteristics, including alkali metal corrosion, ash coking, slagging, tar and other problems [3]. At the same time, the calorific value of biomass fuel is very unstable with the change of humidity and other characteristics. Therefore, the amount of fuel to be put into the plant cannot be calculated according to the load by conventional control methods. Instead, the feed amount should be adjusted by calculating the air volume, which essentially changes the control method of conventional coal-fired power plants. In addition, the composition of biomass fuel and coal powder are very different. Therefore, the process parameters and environment for coking and corrosion are different from those of ordinary coal-fired boilers, which puts forward different requirements for the process design and automation control of boilers and their auxiliary equipment. Due to the many problems in the transportation, storage and cutting processes of fuel, the feed system of biomass power plants has a high dust content, which poses a great safety hazard. In addition, the existing feed system often clumps and blocks, which cannot guarantee the continuous and stable operation of the boiler. The localization of biomass power generation equipment and integrated automation control, as well as the existing problems, are urgent issues that the biomass power generation industry needs to address in the future. 2. Structure of the Integrated Automation Control System for Biomass Power Generation Currently, the key primary equipment used in biomass power generation has basically been localized. Taking the Shanxian project as an example, the feeding system has been completely localized. The power plant boiler is based on Danish BWE boiler technology, using a 130t/h, vibrating grate, four-pass (M-type flue gas loop), natural circulation domestic drum boiler. The turbine and generator equipment are basically similar to those in conventional small power plants, and their control methods are also basically similar. Based on the characteristics of the straw power generation project and combined with the development trend of automation control in China, the automation control system can adopt hierarchical relay control. The entire system can be divided into three control levels: the station control layer, the intermediate layer, and the interval layer. 2.1 Station Control Layer The station control layer receives various information transmitted from the intermediate layer and issues the operators' instructions to the corresponding intermediate layer controllers, serving as the human-machine interface of the entire system. Operators can intuitively grasp the operation status of the entire system in real time through the display screen. From a security perspective, the station control layer can adopt a dual-machine, dual-network structure, using 100M high-speed Ethernet for communication. The two servers serve as hot backups for each other, improving the real-time performance and reliability of the entire system. 2.2 Intermediate Layer The intermediate layer acts as the bridge of the entire automated control system, responsible for collecting various information transmitted from the interval layer, running corresponding control logic, transmitting various information to the station control layer, and receiving various instructions from the station control layer to trigger corresponding logic flows. The intermediate layer can also adopt a dual-machine, dual-network structure, using 10M/100M adaptive Ethernet for communication. The two front-end controllers serve as hot backups for each other, ensuring a system availability of no less than 99.9%. 2.3 Interval Layer The interval layer is mainly responsible for real-time acquisition of information from field equipment and transmitting the acquired information to the intermediate layer, forming the foundation of the entire automated control system. The interval layer can adopt different network configurations depending on the installation and configuration of the primary equipment in the field. Common configurations include Ethernet, CAN bus, and remote I/O. This layer can be configured with a high-security PLC or PCC, ensuring high reliability for the entire system. A schematic diagram of the automated control system structure is shown in Figure 1. [align=center]Figure 1 Schematic diagram of the integrated automatic control system for biomass power generation[/align] 3. Research on integrated automatic control strategy for biomass power generation Biomass power plants are a completely new concept in China. The biggest difference between biomass power plants and conventional thermal power plants is the fuel, especially since biomass fuel has a high and unstable moisture content, resulting in inconsistent calorific value and leading to differences in fuel processing, transportation, and boiler combustion methods. However, the control methods for systems such as turbines, generators, electrical systems, and auxiliary workshops are not significantly different from those of conventional power plants of the same scale. Based on the investigation and research of automated control systems in domestic and foreign biomass power plants, a brief analysis and discussion of the main control functions of boilers in biomass power plants is presented. 3.1 Load Control The load control approach of biomass power plants differs fundamentally from that of conventional power plants. The unit control adopts a turbine-boiler adjustment method, where the boiler load is adjusted, and the turbine maintains a relatively constant steam pressure as the boiler load changes. Because the calorific value of biomass fuel is not fixed, the feed rate cannot be directly determined based on the boiler load. However, the air supply rate can be determined. Then, based on the actual evaporation rate of the boiler and the oxygen content in the tail flue, the air supply rate and feed rate are adjusted through calculations by the load controller, ultimately achieving load regulation on the boiler side. This control method can solve the problem of precise and automatic load control for boilers using biomass, a fuel with an uncertain calorific value. 3.2 Boiler Airflow Control: The boiler airflow is directly determined by the load, which in turn determines the opening degree of the blower damper and the blower speed. Closed-loop control of the boiler airflow is achieved by monitoring the total airflow. The air is rationally distributed into grate air, secondary air below the front and rear walls, secondary air on the front and rear walls, and feeding air. Each airflow path is automatically controlled by its own flow rate or pressure. 3.3 Boiler Feeding Control The boiler feeding system consists of two feeding lines. Each line comprises a pre-furnace silo, a reclaimer, a conveyor, a batching machine, three buffer silos, and three screw feeders. Biomass fuel is ultimately fed into the furnace by the adjustable-speed screw feeders. The feeding system is designed with a margin; each feeder operating at approximately 80% load is sufficient for full boiler operation. The boiler load is evenly distributed across each feeder, allowing operators to adjust the output of each feeder to a certain extent. When the load changes, the control system automatically adjusts the load of each feeder, evenly distributing the load change across all operating feeders. If any feeder malfunctions, the control system can quickly and automatically redistribute the load of the faulty feeder to the other operating feeders, maintaining a relatively stable boiler load and ensuring the safe and stable operation of the power plant. The rotational speeds of the reclaimer, conveyor, and batching machine are proportional to the rotational speed of the feeder, ensuring that each feeder's buffer hopper has sufficient fuel. The material level in the buffer hopper corrects for the rotational speed. The control principle of the feeding system is shown in Figure 2. [align=center] Figure 2 Feeding System Control[/align] 3.4 Furnace Pressure Control The furnace pressure control is a simple single-loop closed-loop control. The operation and commissioning personnel set the furnace pressure and adjust the opening of the induced draft fan inlet damper and the induced draft fan speed according to the pressure setpoint and furnace pressure feedback. The furnace pressure control principle is shown in Figure 3. [align=center] Figure 3 Furnace Pressure Control[/align] 3.5 Main Steam Temperature Control The boiler's main steam temperature control system consists of a four-stage superheater and a three-stage spray desuperheater. The purpose of the superheater and spray desuperheater is to stabilize the main steam temperature at the set temperature. The temperature controller for each superheater is designed as a typical dual-loop controller. The response record of the superheater inlet temperature serves as the reaction to the superheater outlet deviation record. The reaction delay stops, similar to the actual process response of the superheater. This can be achieved through the integrator and Pt1 filter in the loop. The principle of main steam temperature control is shown in Figure 4. Unlike conventional power plants where the temperature setpoint is manually set at the outlet of each superheater stage, this system only sets the temperature setpoint at the outlet of the fourth-stage superheater. The temperature setpoints at the outlets of the first two stages of superheaters are taken from the inlet temperature of the next stage desuperheater. In this way, the entire system is integrated into a whole to control the main steam temperature, shortening the control time, enhancing temperature stability, improving control efficiency, and improving the quality of the main steam. [align=center] Figure 4 Main Steam Temperature Control[/align] 3.6 Feedwater Control The boiler feedwater system mainly consists of a steam drum, a feedwater regulating valve, and two feedwater pumps. The two feedwater pumps are speed-adjustable pumps, one in use and one on standby. The steam drum liquid level determines the speed of the feedwater pump, while the main steam flow rate and feedwater flow rate are also important factors affecting the speed of the feedwater pump. The feedwater pressure is adjusted slightly for a short time by throttling the feedwater regulating valve. Normally, the feedwater valve is fully open. It is only when the boiler starts up or the desuperheating water pressure is insufficient that the feedwater valve is throttled to increase the feedwater pressure. The boiler feedwater control principle is shown in Figure 5. [align=center]Figure 5 Boiler Feedwater Control[/align] 3.7 Air Preheater Control Unlike conventional power plants, the system is equipped with a set of high-pressure air preheaters, high-pressure flue gas coolers, and a set of low-pressure air preheaters and low-pressure flue gas coolers. The system uses water as a medium to transfer the heat of the flue gas to the air entering the furnace. This system arrangement ensures good heating of the air while preventing excessively low flue gas temperature, low-temperature condensation, and corrosion of the air preheater pipes. 3.8 Fuel Storage and Transportation System Control The fuel used in biomass power plants is mainly various types of straw or fast-growing timber. Pre-treatment such as fuel crushing is completed by the fuel collection and storage unit. The fuel storage and transportation system of a power plant mainly consists of the following parts: silos, emergency hoppers, bucket elevators, screw conveyors, mobile distribution belts, electronic truck scales, double-row scraper conveyors, belt conveyors, belt separators, and boiler feed silos. The fuel storage and transportation system is shown in Figure 6. During normal operation, the system starts with the forward flow and stops with the reverse flow; under fault conditions, the reverse flow protection trips. The speed of the bucket elevator and linear screw conveyor is determined by the speed of the boiler feeder, and the height of the boiler feed silos is used as an auxiliary adjustment parameter to ensure uniform fuel delivery while meeting boiler load requirements. [align=center]Figure 6 Fuel Storage and Transportation[/align] 4. Discussion of Control Challenges and Improvement Schemes Practice has proven that the above control strategies can achieve automated control of biomass power plants and ensure the safe and stable operation of the power plant. However, during commissioning and operation, several problems with the control system were still exposed. 4.1 Dust Control and Fire/Explosion Prevention Currently, fuel storage and transportation in biomass power plants are conducted under normal pressure. Due to the inherent characteristics of biomass fuel, a large amount of dust is generated during its crushing or transportation when there are drops in elevation. This results in high dust content in the feeding system and boiler feed system, with dust concentrations even reaching the explosion limit, posing a significant safety hazard. To address this, we need to modify the production process in the fuel crushing, transportation, and feeding stages based on the domestic fuel supply situation. This includes adopting closed-loop negative pressure storage and transportation; installing dust removal devices at locations with significant elevation drops; adding dust concentration sensors for real-time dust monitoring; maintaining good ventilation in the silos; and monitoring and controlling the temperature and humidity of the silos. 4.2 Simplification of the Fuel Conveying System Currently, the fuel conveying system and boiler feed system involve many steps and complex processes, with frequent blockages in screw conveyors and bucket elevators. Failures in the fuel conveying system can lead to fuel shortages in the furnace silos, failing to meet the fuel supply requirements under boiler load. In order to avoid this phenomenon, we can consider improving the existing feeding process, reducing the feeding links, not using bucket elevators, but using trestle and belt to directly transport the material from the silo to the furnace front silo. At the same time, we should strictly control the fuel humidity and particle size to prevent fuel agglomeration and entanglement, and improve the automated control methods to ensure the continuous and stable operation of the conveying system. 4.3 Coking and Corrosion The composition of biomass fuel is very different from that of coal powder, especially the ash contains a large amount of alkali metal salts. These components cause its ash melting point to be lower than that of coal powder, which makes it easy to produce fouling, coking and corrosion. Therefore, the operating parameters of biomass boilers that produce coking and corrosion are different from those of ordinary coal-fired boilers. We should put forward different requirements for the process design of boilers and their auxiliary equipment according to the different fuel properties and combustion characteristics, and improve the relevant automated control to make the process operating environment meet the requirements of existing equipment. 5. Conclusion Biomass power generation, as a clean renewable energy source, is of great significance for improving China's energy structure, reducing China's dependence on fossil fuels, and thus reducing the emission of pollutants such as CO2 and SO2, and finally alleviating the pressure of energy consumption on the environment[4]. To achieve the localization of biomass power generation equipment and its integrated automated control, to carry out the research and development of core equipment, to develop complete sets of biomass power generation technology and equipment, and finally to form biomass power generation technology with independent intellectual property rights in China, there is a broad prospect for development! References: [1] Report on the development and utilization of biomass energy in Sweden, Denmark, Germany and Italy http://nyj.ndrc.gov.cn/dcyyj/t20050928_44084.htm [2] Guidance Catalogue for the Development of Renewable Energy Industry of the National Development and Reform Commission 2005, 11 [3] Duan Jingchun Xiao Jun Research on biomass and coal co-combustion power plant system engineering 2004.1 [4] Yin Xiuli Wu Chuangzhi The role of biomass gasification in reducing CO2 emissions [J]. Acta Energiae Solaris Sinica, 2000, 1