Thermo Chemical Recuperator: Energy-Efficient Glass Melting
HyGear started the development of a thermo chemical recuperator (TCR) that can be applied to decrease the energy usage of industrial furnaces. The TCR utilizes the waste-heat of the furnace to produce a high-calorific gas mixture. The first results of this development project are discussed in this article.
Industrial furnaces are mostly heated by natural gas or fuel oil. Unfortunately, a large amount of the energy supplied to the furnaces is lost through flue gases. Even today’s most efficient regenerative or oxygen-fired furnaces generally show waste-gas heat losses of about 25-30% of the total furnace energy input.
TCR technology uses the waste heat of furnaces to convert natural gas and water into a high-calorific reformer gas mixture. The TCR finds its application in industrial furnaces for the glass, cement and metal industries.
The first development project is focused on glass furnaces and executed together with CelSian Glass & Solar with support from the National Committee Netherlands Glass industry (NCNG) and the glass companies AGC, Philips and Saint-Gobain Isover.
Currently, there are different types of flue-gas heat-recovery systems commercially available to reuse waste heat of furnaces (Fig. 1).
- Preheating of raw materials, cullet or fuel/oxygen
- Generating electricity
- Generating steam
The preheating of raw materials and cullet can lead to energy savings up to 15%. However, several problems like dust formation and degradation of refractory materials can occur.
Another possibility of preheating with flue gases is to preheat a pelletized batch. Pelletized-batch materials melt more easily and lead to an improved glass quality and less dust formation. CelSian is currently developing this solution, which has an outlook of 20% energy savings.
The flue gases can also be used to generate steam or electricity. The generation of steam is only applicable if there is a local demand for it. The efficiency of electricity generation with flue gases is relatively low and only leads to energy savings up to 5%.
TCR technology may lead to energy savings of about 20-25%. Other strong points of TCR include:
- No carryover of batch materials and less refractory corrosion
- Less batch segregation; thus, better glass quality
- Applicable for all types of glass (soda-lime, borosilicate)
With TCR, the heat of the flue gas is used to convert natural gas into hot synthesis gas (mainly CO and H2), which has higher energy content than natural gas. This valuable and hot synthesis gas can substitute for natural gas, resulting in a significant decrease of the overall natural gas consumption.
Thermo Chemical Recuperation
The core unit of the TCR is the steam reformer (Fig. 2). Conventional steam reformers are only efficient on a large scale, but HyGear has developed a reformer that is efficient on a small scale. The small-scale reformer is already deployed in HyGear’s hydrogen generation systems that generate hydrogen on-site. For the TCR, the reformer has been modified to match the waste-heat recovery system.
The steam (produced by heating water with waste heat) and desulfurized natural gas are mixed and heated with the waste-gas heat. When the mixture reaches a temperature level of 700-900°C (1292-1652°F), it is exposed to the catalyst in the reformer. In the reformer, a mixture of hydrogen and carbon monoxide – so-called synthesis gas (reformer gas or syngas) – is formed by the following endothermic reaction:
Cost and Energy Savings
The hot syngas has higher energy content than natural gas. Natural gas is added to the hot syngas, and the new fuel mixture is fed to the furnace. Using the TCR, less natural gas is needed to melt the same amount of glass. Furthermore, since the oxidation of syngas generates more heat than natural gas, the total amount of fuel is reduced. As a result, less oxygen needs to be used. Currently, oxygen is obtained by an energy-intensive process and is therefore expensive.
Determination of energy and cost savings is specific to each situation, and tailored calculations are needed for each potential TCR user. Energy savings and cost reductions have been estimated for oxyfuel and recuperative glass furnaces with different types of glass (soda-lime-silicate as well as borosilicate). Generally, energy savings can be about 20% in all cases, while cost reductions are higher for oxygen-fired furnaces, saving natural gas and oxygen at the same time.
Results of the Feasibility Study
The combustion of reform or synthesis gas in a glass furnace has an impact on the heat transfer of the flames to the surroundings and the composition of the flue gases (the moisture contents will increase). CelSian has evaluated the impact of the reform-gas combustion process on the glass melting process, while HyGear is focusing on the optimum operational conditions for the reform tubes as well as the engineering of a TCR heat-recovery unit for glass melting furnaces.
Glass Melt Results
The changed conditions in the furnace might have consequences for foaming, fining, evaporation and glass quality. CelSian has studied glass melting behavior under TCR conditions. It was concluded from experiments that there is hardly any effect on the batch melting kinetics.
The combustion of hot reform gas might, among other things, influence the heat transfer from the flames to the surroundings. The complete glass melting process (melting tank and combustion chamber) has been simulated with computational fluid dynamics (CFD).
The simulations show that the local heat flux from the combustion chamber to the glass melt slightly reduces when reform gas is used. Probably the reformate-induced flames are less sooty and thus less radiant. Nevertheless, the results show that it is possible to generate sufficient heat transfer for the melting process.
Figure 5 shows that there are no significant changes in the flow patterns. Moreover, the residence time of particles in the glass bath is the same in all studied cases, leading to a glass product with the same quality.
The optimum process conditions for syngas production have been investigated. It was shown that complete conversion and degradation of the catalyst can be avoided for temperatures above 800°C (1472°F) at a steam-carbon ratio of about 3.
In addition, the interaction between flue gases from a glass melting furnace and reformer tubes has been investigated with so-called fouling tests. Flue gases of glass furnaces contain volatile species (e.g., Na, K and B compounds) that might condense on the surface of the reformer tubes. Aggressive vapors in the flue gases might corrode the reformer tube, and the condensed deposits will influence the heat transfer of the flue gases to the reformer tube. The preliminary results of a test with a reformer tube exposed to the flue gases of an industrial glass furnace show very limited fouling and corrosion. Additional industrial fouling tests will be performed in different glass furnaces.
In addition to the previously described developments, work is ongoing on pipelines and gas skids to control fuel flow rates and leakages since syngas is composed of toxic and flammable gases (CO and H2) at high temperatures (600-900°C).
Conclusion and Next Steps
It can be concluded that the first outcomes are positive. The most important results of the feasibility study are:
- With the TCR, less natural gas and oxygen is needed to melt the same amount of glass.
- The TCR has no significant effect on the batch melting kinetics.
- The TCR gives no significant changes in the flow patterns.
- The TCR causes very limited fouling and corrosion to the reformer tubes.
In addition to this study for glass furnaces, TCR-technology investigations will begin for industries such as cement and metal.
HyGear is a clean technology company with expertise in gas processing and industrial gas system design.
The company’s lead product is Hy.Gen, a hydrogen generation system that produces hydrogen by converting natural gas with steam methane reforming. The systems can be installed on-site. Decentralized hydrogen production offers a safer, more reliable and cost-attractive alternative to conventional hydrogen supply by tube trailers or electrolyzers and significantly lowers the environmental impact.
Besides standardized products for on-site generation of hydrogen, nitrogen and oxygen, HyGear offers tailored systems for the recycling of industrial gases. Our products reduce both costs and the environmental impact of industrial gas delivery for our global customers.
HyGear’s range of gas purification systems can recover waste gas streams or upgrade the purity of hydrogen, nitrogen, oxygen or methane into purities up to 9.0.
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