Getting accurate gas emissions data

Gas analysis used to be regarded as an expensive necessity, primarily to meet legislative requirements. However, companies are now realising that well-specified gas analysis systems can make a direct contribution to profit. Operators that keep their emissions of CO2, SO2, and NOx low can sell their emissions credits or allowances to others with higher emissions.

Removing the moisture from gas samples is a necessary part of sample conditioning to ensure reliable and accurate measurements. The water removal method employed can be crucial to the accuracy of the analysis, particularly when the gas involved is water soluble.

If a refrigerant dryer, or chiller, is used, moisture is removed from the gas sample by cooling it, so that water vapour condenses out of the sample as liquid water. If the process gas being analysed is soluble, it is likely that a significant part of the process gas will become dissolved in the condensate, and will therefore not be part of the dried sample subsequently analysed. Concentrations of water soluble gases such as SO2, NOx and HCl can be compromised if exposed to condensed liquid.

The recommended approach for these applications, as the alternative to a chiller, is to use a heated permeation dryer, where the sample gas passes through a selectively permeable membrane for moisture removal. In this case, water vapour only is selectively transported through the membrane. The sample gas does not come into contact with the condensate because the moisture is removed in its gaseous state.

This approach not only prevents loss of soluble sample gases but also removes the risk of damage to the gas analyser in instances where, as is the case with SO2, the dissolved sample gas is acidic and can cause corrosion of the system. Permeation dryers are also capable of drying the gas to a lower dewpoint, which results in less corrosive effects.

Comparative trials of chillers and permeation dryers for the removal of SO2 from wet gas streams in a heated sample gas conditioning line have shown that it is removed to a greater extent by a chiller. This result was presumably related to the fact that SO2 dissolves best in cold water, and that the acidity produced becomes greater as the temperature is decreased.

Considerable care and expertise is therefore necessary when specifying the water removal portion of a sample conditioning system. It is not a simple matter, and not a decision that should be based on the cost of the installed equipment alone.

Chillers are less expensive than permeation dryers and are also usually simpler to install and commission — powerful arguments to an engineer with an accountant standing behind him. Nonetheless, there are many scenarios in which a permeation dryer is substantially better and may achieve a better outcome in terms of effectiveness and environmental impact.

As an example, a large coal-fired power station was fitted with flue gas desulphurisation (FGD) equipment to remove most of the SO2 by spraying limestone slurry into the hot flue gases. The gases react with the limestone to remove at least 90% of the SO2 as gypsum.

To measure FGD efficiency, SO2 is constantly monitored by the station’s continuous emissions monitoring gas analysis installation. The sample conditioning system must be able not only to remove large water concentrations but to deal successfully with the acid gases contained in the sample stream.

The analyser systems were originally equipped with chillers, and some failed due to acid carry-over. Additionally, the system was plagued by high maintenance costs and excessive downtime. By fitting a simple permeation dryer at the outlet of the chiller, the water content of the sample gas was reduced, which improved reliability and the consistency of analysis.

Another point to consider is that analysers to measure SO2 and NOx generally use the absorption of infrared radiation at a specific wavelength. Water has a very wide absorption spectrum and its presence can interfere with the required measurement. Standard chillers can only remove water to a dew point of about 4°C (corresponding to about 8,500ppm). A permeation dryer can dry the gas to a dew point of better than -20°C (approximately 1,000ppm), removing any significant



Dow follows the laser path



Dow Chemicals is employing a new laser analyser to more accurately measure and analyse gas emissions from its polymer production and related chemicals processes, particularly those involving chlorine.

The tunable diode laser (TDL) analyser, which was developed by Yokogawa, can provide an almost instant analysis of emissions, according to Dr Sam Langridge, product manager, TDL Analysers at Yokogawa Europe. A key advantage, he said, is that it involves no conditioning of the sample -— unlike conventional analysis techniques, which must modify the sample, for example by condensing the gases being monitored.

The TDL device, branded TruePeak, was tested under harsh conditions by Dow Chemicals, where it was shown to provide fast —- from one to 20 seconds —- accurate measurements of O2, carbon monoxide, CO2 and moisture. Using a tunable diode laser as a monochromatic light source, it also operated successfully in environments with high particulate loadings.

The TDL analyser is designed to measure O2 with process temperatures as high as 1,500°C and pressures as high as 20 bar absolute. The device can also be used for measuring ppm moisture content in corrosive and aggressive process streams, including chlorine and hydrocarbons.

According to Langridge, the



laser system is generating high demand from large process players in the oil & gas and petrochemicals industries including Dow Chemicals, which is using it on all its chlorine processes. A key application, he added, is in monitoring moisture levels in chlorine, as wet chlorine can be a very expensive problem to deal with.

The instrument can also be used to optimise combustion processes by measuring excess oxygen and carbon monoxide. It also allows carbon monoxide to be accurately measured on a continuous basis at low ppm levels so that air:fuel ratios can be precisely and continually optimised.

Other target applications for the TDL device include the monitoring of carbon monoxide, methane and moisture to detect burner flame-out and process tube leaks; the measurement of oxygen on flare lines, alkylation units and gas plants; and the monitoring of carbon monoxide and oxygen on fluid catalytic cracking units for safety and catalyst regeneration.