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Coulometric titration was used to test the sulfur content in the coal. With iodine as the titrant, the coal sample at a high temperature of 1150 °C, the sulfur in the coal will be converted to SO2 and SO3 gas; the gas will lead to all the electrolytic cell, SO2 and water to generate sulfurous acid, the oxidation of iodine to sulfuric acid. The instrument uses a double platinum electrode to indicate the end point. According to the amount of electricity consumed in the electrolytic iodine process, Faraday's law can calculate the sulfur content in the coal: 
2 The composition and working principle of the instrument The instrument consists of a PC, an intelligent controller, an injection device, a high-temperature combustion furnace, a power supply system, an air delivery and purification system, an electrolytic cell, and an agitator.
The PC mainly completes the setting of control parameters, the sending of control instructions, the processing of measurement data, the display of temperature and electrolytic current change curves, and the storage and printing of measurement results.
The intelligent controller is a key part of the sulfur analyzer. It can receive the control commands and control parameters sent by the PC through the USB interface, and completes the heating control and temperature measurement of the high-temperature furnace together with the electrolytic cell, silicon carbon tube, and thermocouple. The measurement and control of electrolysis, the delivery and withdrawal of samples and other functions, and the electrolysis current and temperature changes in the measurement process as well as the measurement results of the electricity sent to the PC through the USB interface.
The heating element in the high-temperature combustion furnace uses a silicon carbon tube and uses a thermocouple to measure the furnace temperature. Air transport and purification systems consist of electromagnetic pumps, air flow meters, drying tubes and desiccants. The electrolytic cell case is made of plexiglass, and a pair of electrolytic electrodes and a pair of indicator electrodes are fixed on the upper cover. A magnetic stirrer is placed in the electrolytic cell, and it is rotated at a speed of about 500r/min under the drive of a magnetic stirrer to ensure uniform distribution of the electrolyte in the electrolytic cell.
3 Intelligent controller design The intelligent controller is at the core of the sulfur analyzer. The data collection of the thermocouple temperature, the cold junction temperature, the indicator electrode voltage, and the electrolytic current, and the control of the furnace temperature and the electrolytic current are completed. Measure the integration time during the process and send the integrated result of the power to the PC through the USB interface.
The block diagram of the intelligent controller is shown in Figure 1. The microcontroller with USB interface in the picture uses CY7C68013A. The chip contains an enhanced 8052 core and 16KB of RAM. The program stored in EEPROM is automatically loaded into RAM through a serial interface at power-on. The frequency can be as high as 48MHz; at the same time, it also has a USB2.0 interface, which can not only achieve serial communication with other devices, but also realize online update of the program through this interface, which brings great convenience to the debugging and upgrading of the system. 
3.1 Electrolytic Current Control Module Since the sulfur content in the Coulomb titration method is the result of the integration of the electrolysis current, it is particularly important to control and accurately measure the electrolysis current. In the sulfur meter, the electrolytic current is controlled based on the measurement result of the voltage of the electrolytic electrode. When the voltage of the electrolysis electrode exceeds the equilibrium voltage, the electrolysis process starts; normally, the electrolysis current is proportional to the electrolysis electrode voltage. Electrolytic currents can also be controlled in sections because of the large voltage fluctuations in the electrolysis electrodes.
The electrolytic current data output from the microprocessor is first converted into an analog voltage by the DAC0832 and the IV conversion circuit, and then input to the non-inverting terminal of the voltage comparator; the negative terminal of the voltage comparator is connected to the sampling resistor R to constitute a negative feedback. When the voltage on the sampling resistor R is reduced by the output voltage of the D/A converter, the output of the voltage comparator is positive, and the composite tube composed of the field effect transistor and the triode is turned on, and the voltage drop across the sampling resistor R increases; When the voltage drop across the sampling resistor R is higher than the output voltage of the D/A converter, the output of the voltage comparator is negative, the composite tube is turned off, and the voltage drop across the sampling resistor R decreases. Since this process is automatically adjusted by the hardware, the voltage drop across the sampling resistor R is extremely small and always equal to the output voltage of the D/A converter. As can be seen from Figure 1, the current flowing through the sampling resistor R is equal to the current flowing through the electrolytic cell. Therefore, when the sampling resistance R is kept constant, the voltage drop across the sampling resistor R is proportional to the electrolytic current and can be passed. The D/A converter controls the electrolytic current.
Since the DAC0832 has only 8-bit conversion accuracy, and there is a certain non-linearity, in order to ensure the accuracy of the instrument, the value of the electrolytic current is not calculated directly from the data sent from the microprocessor to the DAC0832, but instead using 12-bit A/D conversion. The MAX1247 performs measurements while the entire integration process is controlled by the microprocessor's timer to ensure the accuracy of the charge integration.
3.2 Furnace temperature control module In order to ensure the measurement accuracy, the furnace temperature must be accurately measured and controlled during the experiment. In the sulfur meter, a thermocouple was used to measure the temperature of the furnace, and the AD590 was used to measure the ambient temperature to achieve cold junction compensation for the thermocouple.
When the furnace temperature is within 1100°C, the combustion furnace is heated with a 70% duty cycle PWM signal in a period of 2 seconds; when the furnace temperature is higher than 1100°C, in order to avoid frequent heating and power failure of the heating element The silicon carbon tube life, carries on PID control to the furnace temperature, its proportion, the integral and the differential coefficient are respectively:
KP=2.45, KI=2.5, KD=1.25(2)
During the test, the furnace temperature was stably maintained at (1150±2)°C.
3.3 Data acquisition module In the intelligent controller, it is necessary to complete the collection of the four-way signal of electrolytic electrode voltage, electrolytic current, thermocouple temperature and cold junction temperature. Analog-to-digital conversion is implemented using the 12-bit serial A/D converter, the MAX1247, which includes four analog switches and a sample-and-hold circuit that converts four analog input signals to digital signals and sends the conversion results to the microcontroller. .
The voltage of the electrode voltage is generally between 20mV and 200mV, and since it is taken directly from the electrolytic cell, it is amplified with a precision instrumentation amplifier before being sent to the A/D converter, and then isolating it from the electrolytic cell using an isolation amplifier. . Thermocouple output voltage is very weak, so the use of two-stage precision amplifier to amplify 400 times and then into the A / D; AD590 used to measure the temperature of the thermocouple cold junction, the need to use 10kΩ precision resistor to convert the output current to voltage The signal is then sent to A/D; the last input signal of the A/D converter comes from the voltage drop across the sampling resistor R (proportional to the electrolytic current).
4 Test Results and Accuracy Analysis Using the designed sulfur analyzer, 10 tests were performed on sample 1 (standard sulfur content 0.88%) and sample 2 (standard sulfur content 4.24%). The test results are shown in the table. 1 and Table 2 shows. 
It can be seen from the table that the maximum error of the test result of Sample 1 is 0.02%, which is lower than the margin of error (0.05%) of low-sulfur coal (sulfur content below 1%) specified in the national standard; The maximum error of the test results is 0.05%, which is also lower than the margin of error (0.2%) of the high-sulfur coal (sulphur content above 4%) specified in the national standards.
5 Conclusion The Coulomb method is a commonly used measure of sulfur content. In the Coulomb method, the sulfur content is determined based on the integral of the amount of electrolysis electricity. The structure and working principle of the sulfur meter based on USB interface were introduced. The data acquisition module, electrolytic current control module and furnace temperature control module of the instrument were analyzed. The measurement accuracy was analyzed. The analysis results show that the measurement accuracy of the instrument has reached the national standard.
Design of Sulfur Tester Based on USB Interface
1 Introduction Coal with high sulfur content will bring great harm when used for combustion or gasification or coking coal. For example, when high-sulfur coal is used as a fuel, the sulfur dioxide gas generated after combustion not only seriously corrodes the boiler pipeline, but also seriously pollutes the atmosphere. In the coking industry, the impact of sulfur content is even greater. On the one hand, the sulfur content in coal is high and the coke is high. The sulfur content is also high, which directly affects the quality of steel. On the other hand, in order to remove the sulfur in the steel, more limestone must be added to the blast furnace, which will reduce the effective capacity of the blast furnace and increase the amount of slag discharge. Therefore, in order to use coal resources effectively and economically, it is necessary to understand the content of sulfur in coal.