Measuring Hydrogen Sulfide in Syngas

Introduction

Produced through the gasification of coal, solid waste, and various other carbon-containing feedstocks, syngas is important as a fuel for certain internal combustion engines as well as a raw material for the Fischer-Tropsch process, which produces synthetic liquid hydrocarbon fuels from the syngas. The composition of syngas varies widely depending on the feedstock type, but typically comprises some mixture of hydrogen, carbon monoxide, carbon dioxide, nitrogen, and methane.

The feedstock to syngas production often contains some hydrogen sulfide, especially when produced from coal or biomass. Since H₂S degrades the syngas quality by making it corrosive and producing SO2 emissions, H₂S levels have to be carefully controlled in the syngas product. The variation in feedstock type also brings large variation in H₂S loading, thus requiring a method of continuous H₂S monitoring in the syngas production process.

The OMA H₂S Analyzer uses a high-resolution UV-Vis spectrophotometer to measure real-time H₂S concentration in a syngas process stream. The fast response of the photometric technology combined with the transparency of the syngas background in the UV-Vis wavelength domain makes the OMA an ideal instrument for online syngas H₂S analysis. This system also has additional benches available for adding analytes (e.g. COS or other sulfur compounds).

OMA Benefits: H₂S in Syngas

• Continuously measures H₂S concentration in syngas using dispersive absorbance spectrophotometer
• Syngas background (H₂, CO, CO₂, moisture) is transparent in the UV-Vis wavelength domain — no interference
• Totally solid state build with no moving parts — modern design for low maintenance
• Direct measurement of hot, wet, high-pressure sample with rugged sample flow cell
• Decades of field-proven performance in crude oil analysis all over the world

H₂S Absorbance Curve

Any single photodiode measurement is vulnerable to noise, signal saturation, or unexpected interference. This susceptibility to error makes a lone photodiode data point an unreliable indicator of one chemical’s absorbance.

As accepted in the lab community for decades, the best way to neutralize this type of error is to use collateral data in the form of ‘confirmation wavelengths,’ i.e. many data points at many wavelengths instead of a single wavelength:

In the figures above, each diamond represents a single photodiode and data point. The nova II registers absorbance at each integer wavelength within the 210-250 nm measurement range and produces an H₂S absorbance curve. After being calibrated on a full spectrum of pure H₂S, the OMA knows the absorbance-concentration correlation for each measurement wavelength; the system can average the modeled concentration value from each wavelength to completely eradicate the effect of noise at any single photodiode.

The OMA visualizes the H₂S absorbance curve in this manner and knows the expected relation of each data point to the others in terms of the curve’s structure. This curve analysis enables the OMA to automatically detect erroneous results at specific wavelengths, such as when a single photodiode is saturated with light. The normal photometer, with a single data point, is completely incapable of internally verifying its measurement.

The specifications below represent performance of the OMA-300 Process Analyzer in a typical crude oil application.

For technical details about the OMA-300 Process Analyzer, see the data sheet:
DS-001A: OMA-300 Process Analyzer

All performance specifications are subject to the assumption that the sample conditioning system and unit installation are approved by Applied Analytics. For any other arrangement, please inquire directly with Sales.

Further Reading