Feasibility Study of Low-Concentration NMHC Monitoring in Ambient Air Using Nutech 3000 Portable Analyzer

Background

The Nutech 3000 portable analyzer utilizes catalytic oxidation + FID (Flame Ionization Detector) analysis principle, enabling rapid monitoring for THC, CH4, and NMHC in source emission VOCs monitoring application. Recently, a new demand for low-concentration NMHC monitoring in ambient air using portable analyzer was raised. As a result, a feasibility study has been initiated to explore the performance of the Nutech 3000 for testing low-concentration VOCs.

As a global leader in VOCs analysis and testing solutions, Nutech has a profound understanding of the challenges in atmospheric NMHC analysis. Drawing on extensive experience in our portable and online system solutions, and considering the characteristics of ambient air variations, Nutech has optimized its portable solution by exploring various aspects, such as cleaner gas sources, hydrogen-air-sample mixing ratios, calibration ranges, and thereby expanding Nutech 3000’s application scope.

Nutech Testing Method

Due to the limitations of the differential method (NMHC=THC-CH4) of Nutech 3000, issues such as insufficient preheat time and the variable conditions at the field site contribute to instability in low-concentration VOCs testing. To address these issues, the following requirements are proposed:

1. Gas Source Conditions:
   • Combustion Gas: 99.99% hydrocarbon-free air
   • Fuel Gas: 99.99% hydrogen
   • Zero Gas: The above-mentioned hydrocarbon-free air can be used, with hydrocarbons not exceeding 0.3 mg/m³.
   • Calibration Gas: CH4 standard gas, with lower-concentration working standard diluted from higher-concentration primary CH4 standard.
   • Verification Gas: A CH4/C3H8 mixed calibration gas with concentrations of 74.5/5.05 ppm.
2. Instrument Operating Environment:
   • The laboratory must maintain a temperature of 25°C, with temperature variations not exceeding 3°C.
   • The instrument should be placed on a stable surface or platform.
3. Pre-Test Preparation:
   • After powering on the device, allow a preheating time of more than 1 hour before performing zero/span calibration and subsequent sample testing.

Testing Results

1. Calibration Curve

Considering that the test subject is ambient air, with CH4 around 2ppm and NMHC around 0.2ppmC, the calibration range is set to THC from 0-3ppm and CH4 from 0-2.5ppm. The following is the calibration curve corresponding to the dual FIDs.

Figure1: Calibration Curve of THC

Figure2: Calibration Curve of CH4

2. MDL

A mixed standard gas of CH4/C3H8 = 74.5/5.05 ppm is used, and dilute it by a factor of 301 for the MDL test. The working standard gas concentrations are shown in Table 1 below:

Table 1: Concentration of Each Component for MDL Testing

Table 2: MDL for THC, CH4 & NMHC

The MDL for THC, CH4, and NMHC are 0.0030 ppm, 0.0086 ppm, and 0.0077 ppm, respectively. These meet the requirements of standard HJ1012-2018, which stipulates a detection limit ≤ 0.13 ppm for Type I instruments.

3. Repeatability and Linear Error

Test results are recorded every 30 seconds, with 6 measurements taken to assess the repeatability and linear error. The formulas for calculation and the test results are provided in below.

Repeatability Calculation Formula

Where:

• Sr​ — Quantitative measurement repeatability of the instrument, %
• Ci​ — The i-th measurement value of the verification gas, ppm
• C — The average measurement value of the verfication gas, ppm
• i — Data record index (i=1～n)
• n — Number of measurements (n≥6)

Linear Error Calculation Formula

here:

• Lei — Linear error for the i-th concentration standard gas measurement, %
• ​Csi — Nominal concentration of the i-th concentration verification gas, ppm
• Cdi — Average of the 3 measurements for the i-th concentration verification gas, ppm
•  i— Verification gas index ( =1～6)
• R — Full-scale range of the instrument, ppm

3.1 Repeatability and Linear Error Test Data for THC

Table 3 shows the THC test data with nominal concentration values of 0.1489, 0.2978, 0.8935, 0.14892, 2.3827, and 2.9784 ppm. The repeatability and linear error have been calculated accordingly. The results meet the requirements in standard HJ1012-2018 for Type I instruments, with repeatability ≤ 2.0% and linear error ≤ 2.0% F.S.

Table 3: Repeatability and Linear Error Test Data for THC

3.2 Repeatability and Linear Error Test Data for CH4

Table 4 shows the CH4 test data with concentration values of 0.1238, 0.2475, 0.7425, 1.2375, 1.9801, and 2.4751 ppm. The repeatability and linear error have been calculated accordingly. The results meet the requirements in standard HJ1012-2018 for Type I instruments, with quantitative testing repeatability ≤ 2.0% and linear error ≤ 2.0% F.S.

Table 4: Repeatability and Linear Error Test Data for CH4

3.3 Repeatability and Linear Error Test Data for NMHC

Table 5 shows the NMHC test data, including concentrations of 0.0251, 0.0503, 0.1510, 0.2517, 0.4026, and 0.5033 ppm. The repeatability and linear error have been calculated accordingly. Concentrations of 0.1510 ppm and above meet the requirements in standard HJ1012-2018 for Type I instruments, with quantitative testing repeatability ≤ 2.0% and linear error ≤ 2.0% F.S. However, for the extremely low concentrations of NMHC (0.0251 ppm and 0.0503 ppm), the RSD is too large and does not meet the standard. Referring to the relevant standards, such as the “Technical Specification for Continuous Automatic Monitoring of NMHC in Ambient Air”, Section 4.2.6, the requirement for repeatability is that the RSD of six consecutive CH4 and NMHC measurements should be ≤ 5%. The test results are considered qualified.

Table 5: Repeatability and Linear Error Test Data for NMHC

4. Zero Drift

Table 6: Zero Drift Monitoring Data

As shown in the table above, within 1.5 hours, the zero drift for THC and CH4 were -0.068 ppm and -0.021 ppm, respectively. It is recommended to perform zero calibration in a timely manner during testing.

5. Ambient Air Testing Data

Continuous monitoring of ambient air for nearly 30 minutes, with data recorded every 3 seconds. The trend chart is shown below:

Figure3: Trend Chart of Ambient Air THC & CH4

Figure4: Trend Chart of Ambient Air NMHC

Conclusion

Using high-purity hydrocarbon-free air and high-purity hydrogen as the gas sources, Nutech 3000’s calibration curve linearity, repeatability, and linear error are all satisfactory. For baseline drift, zero calibration every half hour can ensures the accuracy of the test data. In summary, using this approach and under stable lab conditions, Nutech 3000 can accurately test low-concentration NMHC in ambient air.