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Wobbe regulation for liquid propane with air/nitrogen
30. Nov 2022
Case study "Wobbe control with liquid propane gas": The changeover to alternative gas sources and thus a changing Wobbe index poses new challenges for burner operators. By retrofitting with a gasQS™ static, the industrial process can be upgraded quickly and cost-effectively.
For Wobbe regulation of LPG with air/nitrogen, we can offer a very simple solution, because only one gasQS™ static (see picture) is needed in the mixed gas. This measuring device determines the thermal conductivity of the gas and correlates the corresponding sweep index from it. The output value is transmitted with a 4 - 20 mA current loop signal.
There are at least two quality classes for liquid propane gas (LPG). It is never pure propane, but always contains a proportion of ethane and butane. Since the gasQS™ static only determines the thermal conductivity of the gas, it cannot distinguish between the individual gases. The mixing ratio therefore has a direct influence on the measurement error of our device. For the measuring device, ethane acts as if more air had been added (negative measuring error), butane acts as if more propane had been added (positive measuring error).
Mixture 1:
Propane ≥90 mol%, ethane ≤5 mol%, butane ≤10 mol%.
- With a minimum proportion of 55-75 mol% propane gas mixture in air (Wobbe index between 47.8 and 64.6 MJ/m3).
- Positive maximum error: 10 mol% butane / 90 mol% propane with air +2 % Ws (MJ/m3).
- Negative maximum error: 5 mol% ethane / 95 mol% propane with air -1 % Ws (MJ/m3)
For mixtures with butane, ethane and propane, the error in the Wobbe is in between.
Mixture 2 (Engine grade propane):
Propane ≥95 mol%, ethane ≤5 mol%, butane ≤5 mol%.
- With a minimum proportion of 55-75 mol% propane gas mixture in air (Wobbe index between 47.8 and 64.0 MJ/m3).
- Maximum error: 5 mol% butane or ethane / 95 mol% propane with air ±1 % Ws (MJ/m3).
For mixtures with butane, ethane and propane, the Wobbe error is in between.
The usual proportion for both quality classes is 2/3 butane and 1/3 ethane. Thus, our measuring device makes a slightly positive error in the range between 48.5 and 63.9 MJ/m3.
- Mixture 1: Mean value +0.61 % Ws (kWh/m3)
- Mixture 2: Mean value +0.30 % Ws (kWh/m3)
It can be said that the higher the Wobbe index, the smaller the measurement error.
If a higher accuracy is required for the application, a gasQS™ flonic must be used. This determines the Wobbe index over the entire DVGW G260 natural gas range to an accuracy of 1 % and is even better for this specific application. However, it requires an inlet pressure of at least 2.5 barg and has a measuring interval of 30 seconds. The question here is whether the control system can handle this dead time and whether there is enough pressure.
We hope that we have been able to give you an insight into the measuring technology. If you have any questions or ideas, please feel free to contact us.