Device for formaldehyde analysis in hydrogen energy

In this paper, a calibration device for the analysis of formaldehyde in hydrogen energy is designed, and the standard formaldehyde gas of different concentrations is obtained by using the principle of osmotic tube, and the standard formaldehyde gas obtained by gas chromatograph analysis is combined. The results of the experiments show that the concentration of formaldehyde standard gas obtained by this calibration device has high accuracy and excellent stability. This overcomes the difficulty of formulating low-concentration standard gas due to the adsorption of formaldehyde gas, replaces the traditional method of formaldehyde standard gas preparation, and provides a quantitative basis for the calibration of formaldehyde analysis in hydrogen energy.


Introduction
Under the objective of carbon neutrality, hydrogen energy has swiftly evolved as a nascent zerocarbon secondary energy source, presenting rare opportunities for advancing the power system.First and foremost, hydrogen is generated through renewable energy, thus promoting the absorption of renewable energy.Secondly, leveraging the energy storage characteristic of hydrogen enables largescale, long-term, cross-seasonal electricity storage.Thirdly, the rapid response capability of hydrogen power stations provides flexible regulation methods for the new power system.Fourthly, it facilitates the interconnection of cross-domain, multi-type energy networks, expanding the comprehensive utilization pathways of electricity [1,2] .As a flexible and efficient secondary energy source, hydrogen, through electrolyzers and fuel cells at energy consumption terminals, realizes the interconnection, complementarity, and coordinated optimization of power, heating, and fuel energy networks by converting electricity to hydrogen, thus propelling the development of distributed energy and enhancing terminal energy utilization efficiency [3,4] .
Moreover, given the significant potential of hydrogen fuel in emission reduction, hydrogen energy and fuel cell vehicle technologies have garnered widespread attention globally.In the forthcoming years, the industry of fuel cell vehicles is anticipated to witness explosive growth, with a concurrent rapid increase in hydrogen demand, thereby elevating the concern over the impact of hydrogen quality on fuel cell performance.Studies indicate that excessive formaldehyde in hydrogen may combust to produce CO and CO2.While diluting hydrogen fuel, CO2 forms CO through a reverse conversion reaction, leading to the poisoning of the anode platinum catalyst.CO adsorbs on the platinum catalyst, occupying the platinum catalytic active sites necessary for the oxidation reaction of hydrogen, consequently significantly diminishing fuel cell performance [5,6] .As per the stipulation in China's national standard GB/T 37244-2018 "Proton Exchange Membrane Fuel Cell Vehicle Fuel Hydrogen", the maximum concentration of formaldehyde should not exceed 10 ppb.Therefore, accurately measuring the formaldehyde content in hydrogen energy is of paramount importance.
Usually, when utilizing analyzers to detect formaldehyde content, it is requisite first to prepare lowconcentration formaldehyde standard gas for calibration.The preparation methods of formaldehyde standard gas generally fall into static and dynamic methods.Due to the unique adsorption properties of formaldehyde, when packed and stored in high-pressure cylinders, it easily adsorbs on the container body, valves, etc., making it challenging to obtain accurate and stable low-concentration formaldehyde standard gas, thereby increasing the difficulty of tracing the measurement value of formaldehyde gas detection instruments.Konopel'ko et al. [7] achieved a 2 mmol/mol level gas mixture of formaldehyde in nitrogen using a dynamic gravimetric method with trioxane vapor depolymerization.Liu et al. [8] prepared standard gas from formaldehyde aqueous solution standard substance through a static gas mixing device and measured the indication error of the formaldehyde gas detection instrument.Jiao et al. [9] prepared formaldehyde standard gas through heating catalytic decomposition of trioxane vapor.Liu et al. [10] placed formaldehyde solution in a D-10 diffusion tube, and with a standard gas generation device, a constant low-concentration formaldehyde gas could be generated.
Based on this, this paper designs a calibration device for formaldehyde analysis in hydrogen fuel using the principle of permeation tube, obtaining various concentrations of formaldehyde standard gas.Subsequently, the obtained formaldehyde standard gas, when tested with a gas chromatograph, showed high accuracy and excellent stability, providing a quantitative basis for calibrating formaldehyde analysis in hydrogen fuel.

Instruments
Gas Chromatograph: GC 9560 Gas Chromatograph (equipped with a He purifier and PDHID detector), manufactured by Shanghai Huai Chromatography Analysis Co., Ltd.

Standard Gas
The details of standard gas are shown in Table 1.

Operational Procedure for Formaldehyde Standard Gas Analysis and Calibration Device
The schematic diagram and actual image of the calibration device designed for formaldehyde analysis in hydrogen fuel in this paper are shown in Figure 1 and Figure 2 respectively.It mainly includes a purge gas, a mass flowmeter, a formaldehyde permeation tube assembly, a temperature regulation and circuit control assembly, and a gas chromatograph.First, open the purge gas and record the flow rate through the mass flowmeter.The flow rate adjustment range of the mass flowmeter is 0-2 L/min.The purge gas is 99.999% high-purity hydrogen.Then, set the temperature of the formaldehyde permeation tube.Different temperatures of the formaldehyde permeation tube result in different release rates and concentrations.The formaldehyde permeation tube assembly consists of an aluminum plate embedded with a heating rod, platinum resistance, and a formaldehyde permeation tube.The heating rod and platinum resistance are connected to the circuit board to control and display the temperature.The formaldehyde permeation tube includes a 316 L stainless steel outer shell and a 1/8 diameter SPK polyester tube filled with 5 ppm formaldehyde standard gas, sealed with stainless steel ferrules at both ends.The different concentrations of formaldehyde standard gas obtained are analyzed and tested using a helium ion gas chromatograph.

Chromatographic Analysis System
The chromatographic analysis system primarily consists of a gas flow system comprised of a ten-way valve I, a formaldehyde analysis chromatographic column, and a helium ion detector.The analysis process is illustrated in Figure 3.

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Carrier gas flow rate of carrier gas 1: 50 mL/min.

Selection of Formaldehyde Permeation Tube Conditions
Principle of Permeation Tube: The permeation tube is placed in an environment with precisely controlled temperature.Volatile substances inside the permeation tube diffuse into the surrounding gas, precisely controlled regarding flow rate.This results in the production of the required gas mixture.Therefore, the concentration of formaldehyde standard gas obtained varies at different temperatures and purge gas flow rates.
According to experiments, it was found that under 60°C, the permeation tube releases formaldehyde at a rate of 165.82 ng/min, which is then diluted by hydrogen purge gas at a fixed flow rate.The results are shown in Table 2, and the concentration of the diluted sample is calculated using Equation (1).
where:  C: Diluted Formaldehyde Concentration/ppm. Vf: Formaldehyde Release Rate from Permeation Tube/ng*min -1 . M: Formaldehyde Molar Mass/g*mol -1 . R: Gas Molar Constant/J*mol -1 *K -1 . T: Ambient Temperature/K. Vc: Carrier Gas Flow Rate/ml*min -1 . P: Ambient Pressure/kPa.According to the data in Table 2, the linear relationship between peak area and concentration is established by the standard curve method, the linear diagram and linear regression equation are shown in Figure 4.It can be seen that when the flow rate of helium gas passing through the permeation tube is high, the linearity error is relatively large.Therefore, divide the flow rate into two ranges, the high flow rate is >171 mL*min -1 and the low is 24~171 mL*min -1 , the linear diagram are shown in Figure 5 and Figure 6 respectively.

Chromatogram of Standard Formaldehyde Gas at the Outlet of the Calibration Device
The chromatogram of formaldehyde gas under the optimum condition is shown in Figure 7, and the formaldehyde chromatographic peak in the Figure 7 is preceded by the air peak.

Repetition Test
Under the optimal chromatographic analysis conditions, the analytical data results obtained from continuous injection of 1.7 ppm formaldehyde standard gas six times are presented in Table 3.The relative standard deviation (RSD) for each component is calculated.The results indicate that the RSD for peak areas of all components is less than 3%, indicating good stability of the formaldehyde standard gas obtained by this device.

Discussion of Data Results
The experimental data presented above indicates that selecting the permeation tube at 60 results in the release of formaldehyde at a rate of 165.82 ng/min.Within the low flow rate range of the formaldehyde permeation tube (i.e., 24~171 mL*min -1 ), R 2 =0.99985, demonstrating good stability and high accuracy of the formaldehyde standard gas is obtained by this device.However, high purge gas flow rates and reduced permeation tube temperatures can affect linearity and result in lower accuracy of the formaldehyde standard gas.
The repetition test data show that the formaldehyde standard gas obtained using this device is accurate and stable.

Conclusion
This study presents the design of a calibration device for formaldehyde analysis in hydrogen energy applications.The device, grounded in the principle of permeation tubes, can prepare standard formaldehyde gases of varying concentrations.Compared to traditional methods, this approach boasts superior accuracy and stability.Experimental data indicate that, at lower flow rates of the permeation tube, the device yields highly accurate and stable concentrations of standard formaldehyde gas.However, at higher flow rates, a decrease in the temperature of the permeation tube can impact linearity, leading to reduced accuracy in the prepared standard formaldehyde gas.Additionally, repeatability tests further validate the accuracy and stability of the formaldehyde concentrations prepared by this device.
The invention of this device offers a novel and effective calibration method for formaldehyde analysis in hydrogen energy.Given the significance of formaldehyde in hydrogen energy and its potential impact on fuel cell performance, this research provides crucial technical support to ensure the quality and safety of hydrogen energy.

Figure 1 .
Figure 1.Schematic diagram of the calibration device.The key components in the figure are labeled as follows: 1. Purge Gas. 2. Mass Flowmeter.3. Formaldehyde Permeation Tube Assembly.4. Temperature Regulation and Circuit Control Assembly.5. Gas Chromatograph.

Figure 2 .
Figure 2. The actual image of the overall device.

Figure 7 .
Figure 7. Detection of formaldehyde chromatogram by gas chromatography under preferred conditions.

Table 2 .
Formaldehyde concentration at different purge air flow rates.