Ensuring gas quality in thermal processes

Materials World magazine
,
4 Jul 2016

Stephen Harrison and Kim Chapman discuss ensuring gas quality in sensitive thermal processes, particularly within glass and ceramics production.

The combustion of natural gas plays an important and diverse role in many industrial production processes. In a wide range of applications, natural gas is used to provide heat for melting or heat treatment processes – for example, in the glass, ceramics, non-ferrous metallurgy and plastics industries. Some thermal processes have proved resilient to changes in fuel gas quality. Others, however, have to be undertaken within a narrow band of optimum performance to deliver energy efficiency and product quality while meeting their emissions obligations. 

Among the various thermal processing applications, glass and ceramics production are generally acknowledged to be particularly vulnerable to fuel gas quality fluctuations, as these manufacturing processes are especially sensitive to small changes in their operating conditions. 

Both industries require an accurate temperature gradient to ensure products such as tableware, ornamentals and sanitary items obtain the right treatment. The glass industry consumes high quantities of fuel gas from natural gas distribution grids for melting glass batches. The quality of this fuel gas can fluctuate quickly – often within a day – and also shifts with seasonal cycles, according to the blended composition of regionally generated biogas, the propane vaporisation to enrich the biogas and piped natural gas from the North Sea, which has a high calorific value, or Russian natural gas. In some countries, such as the Netherlands and Germany, the biogas contribution to the grid has grown significantly in recent years and the mix of natural gas sources continues to diversify.

Highly accurate temperature control during the melting process is critical for production quality, and failures to control that temperature within a tight range can result in costly batch wastage and losses in productivity. Feed-forward adjustments to the burner operation to mitigate for changing fuel gas quality can help with this. Burner efficiency, waste gas emissions pollution control and the life of a furnace refractory lining are also important issues that can be influenced by changes in natural gas composition.

To enable such adjustments, an automated method of feed-forward process control using a fast response micro thermal conducting detector (GC-TCD) arrangement can be employed. This system analyses the fuel gas quality and computes its probable thermal characteristics using methods similar to those in ISO6976 to allow the required adjustments in burner operation to be made. The analytical matrix and the overall system control loop parameters can vary from site to site, and over time within the same site. So it is vital that the most suitable carrier gas for this application is used, as well as the most appropriate calibration gas mixtures.

Natural gas composition

The composition and calorific value of piped natural gas can change very quickly, even within minutes. It can change significantly throughout the course of a day and also has longer term macro shifts, with seasonal cycles. One reason for this change is that various sources are mixed to create natural gas. Across countries and regions there are standards within which the natural gas composition and calorific value must remain, but these are not standardised across the world or even Europe, where natural gas pipelines criss-cross the continent in a diversified energy supply network. For example, in Germany, the blended composition of natural gas arriving at the user, such as a glass manufacturer, may be a mix of regionally generated biogas (methane lean, low calorific value), intermittent slugs of high calorific value vaporised propane to enrich the biogas, piped North Sea natural gas (high calorific value) or Russian natural gas (lower calorific value). 

Since these composition changes can take place within minutes, the furnace must be able to react to these changes within a similar time frame to ensure stable and optimum operation. The measurement of natural gas calorific value can be done using flame techniques and calorimetry. This is the traditional approach, but is slow compared with the speed in which natural gas composition and calorific value can change. For a fast process control loop that is able to direct process responses at the same rate that the natural gas composition can change, a faster method of measurement is required. Analysis of the fuel gas chemical composition and computation of the implied calorific value using methods similar to those in ISO6976 has, in practical application, been shown to be a good proxy for flame methods of calorific value measurement. It does, of course, rely on a fast and accurate method of chemical species measurement and a rapid response Micro GC-TCD is best suited for the task, since it can enable a process loop response time in the order of one-to-three minutes.

Fuel process control system

The best way to ensure optimal operation of the burner is to measure and control the amount of oxygen in the burner flue gas using a feed back control loop. This ensures there is a small residual amount of oxygen emerging in the escaping flue gases. Although this is the most economically and environmentally efficient way to run the process, plant operators should guard against having a large excess of oxygen, which could impact production costs.

To achieve the right balance, oxygen should be measured in the furnace, or in the regenerator heat exchangers where the flue gases leave the furnace and pre-heat the gases coming into the furnace. 

This measurment is fed into a feedback process control loop and the measurement is typically achieved using instrumentation, such as a zirconia oxygen analyser, which is reliable and robust in this very hot operating environment. The instrument’s sensor requires periodic calibration either with ambient air, or with a specialty gases calibration mixture of percentage level oxygen in nitrogen, close to the measurement point, to ensure accuracy of measurement.

While a feedback control loop is essential to measure oxygen levels in the melting furnace and make adjustments to the oxygen or natural gas being fed in, the more sophisticated process control strategy is to use a feed-forward control loop in combination with the feedback control loop. The feed-forward loop measures the chemical composition of the natural gas (as a proxy for calorific value) coming into the furnace and enables automated predictive and proactive feed forward adjustment of natural gas flow rate and associated stoichiometric oxygen flow rate.

This process control loop ensures that the thermal input to the furnace remains under control, so that the temperature profile of the glass batch melt is controlled according to the optimum process requirements.

These feed-forward control loops usually incorporate gas chromatography instrumentation with a GC-TCD to measure the quality of the natural gas coming into the furnace and allow for feed-forward adjustments in either oxygen or the natural gas flow rates, based on the calorific or heating value of the natural gases coming in. This is a critical factor, since natural gas is fundamentally a mixture of gases whose composition changes over time, impacting on the total calorific value and furnace temperature profile.

The most suitable calibration gas mixture is a multi-component mixture of hydrocarbons with a similar composition to the natural gas stream. It is important that the composition of the natural gas stream changes, so selection of a suitable calibration gas mixture that is representative of the general fuel gas composition is recommended. 

Since this is a process control application, not a legislatively controlled emissions monitoring application or a natural gas custody transfer application, regular certification of the synthetic natural gas mixture is highly suitable. ISO17025 or ISO Guide 34 Accredited certification for these synthetic natural gas mixtures is, of course, technically feasible and commercially available, but these accredited products are more complex to manufacture and therefore add cost, which is not generally required for this process control application.

Glass and ceramics manufacturing are high-temperature, energy-intensive undertakings associated with the combustion of natural gas and oxygen. This characteristic is also found in other industries with high thermal load applications, such as iron and steel processing, oil refining, chemicals production, power generation and cement manufacture. While these industries often use liquid or solid fuels for combustion, where they do use natural gas, the principles of the combustion control application described here are transferrable.

Stephen Harrison is the Global Head of Specialty Gases & Specialty Equipment and Kim Chapman is the Global Product Manager, Specialty Gases & Specialty Equipment, Linde. 

Specialty gases required for GC-TCD operation

5.0 grade helium (99.999%) is the most typical carrier gas for gas chromatography and is the standard choice for this application. The benefits are both ease and safety of product handling, good speed of separation - essential for this feed-forward process control loop application - and broad range of applicability. The disadvantage of using helium in this application is that the natural gas being analysed also sometimes contains helium and so the use of helium as the carrier gas prevents the measurement of helium as a component of the natural gas.

5.0 grade hydrogen is an alternative carrier gas with an even faster column velocity and separation speed performance than helium. However, as hydrogen is highly flammable, there are safety concerns to be considered. The separation resolution is also not as good as helium for some species and matrices. However, use of hydrogen may be suitable in helium-rich natural gas streams.

5.0 grade nitrogen and argon are also potential GC carrier gases. Both are inert, easy to handle and are abundantly available at 5.0 grade chromatography purities on many industrial sites. However, their speed of separation is not ideal for rapid response applications and they give poor sensitivity to the TCD.