Nitrogen oxides irritate respiratory passages. They are also responsible for acid rain and the creation of ozone – and, not least, they contribute to the production of smog. The main sources of nitrogen oxides are automobiles and industry – unless plant operators invest in efficient environmental technology. One salient example is an advanced sintering plant for a major steel production facility, where a NOx removal system with a highly efficient heat exchanger as the key component reduces emissions.
The author: Alfred Ernst Process Engineering and Sales, GEA Ecoflex
How is it possible to purify emitted gases (NOx) that are already highly pure? The Vienna company Strabag Energy Technologies was confronted by just this challenge when it was assigned the task of developing a DeNOx system for the waste process gas of a sintering plant. Normally, this kind of processing stage is not planned at all for this type of facility. However, the steel mill in Linz, Austria, operated by Voest Alpine, is located in a so-called fine-particle decontamination area. For this reason, Voest Alpine wanted to set new standards with the project. As a model project, it was designed to more thoroughly remove NOx downstream of the existing dry flue-gas treatment step by adding a further stage. This approach enables the substances responsible for the formation of smog and ozone (NOx) to be reduced.
Strabag was awarded the project as prime contractor to build the pure-gas SCR DeNOx facility with responsibility for the engineering, delivery, installation and start-up of the entire system. The reaction involved here – catalytic nitrogen oxide removal – takes place when ammonia is added as a reduction agent, with the result that the NOx decreases to the required level. As is often the case with downstream environmental technology, the space available for this system was severely limited. Although the NOx facility is extremely compact, it was still the size of approximately seven terraced houses. For Project Manager Klaus Weigel, therefore, the selection of the necessary components not only depended on finding an optimal balance between investment and operating costs – he was also required to consider the performance per square metre of footprint. The heat exchanger, as the core component of the facility, was expected to provide maximum output from minimum space.
Purpose of the heat exchanger
NOx limitation would not be possible without a heat exchanger. Downstream of the existing Meros dry flue-gas treatment system (a conditioned, dry desulphurisation process) the temperatures of the untreated gas are around 145 °C – too low for NOx elimination. This situation demands that the untreated gas be heated to a temperature of 280 °C. As a result, a gas-gas heat exchanger extracts heat from the treated gas downstream of the DeNOx catalytic converter and feeds it back to the untreated gas. This process raises the untreated gas to the required temperature and at the same time cools the treated gas, which is then released into the environment. The cycle created in this way keeps the temperature of the untreated gas constant.
Voest Alpine already had good experience with heat exchangers from GEA in similar projects. This made it only logical for the company to ask GEA to join them in the tendering phase. Once again, Voest was forced to admit there was no alternative to GEA’s plate heat exchangers.
It didn’t take long to decide on the best heat exchanger type. A shell-and-tube heat exchanger would have been too large. In addition, it would have required more material and therefore been expensive. The arguments for plate heat exchangers, on the other hand, were convincing, especially with regard to the investment and installation costs. Another advantage of plate heat exchangers is their small footprint compared to tube heat exchangers with the same performance. This implies lower operational weight and, in turn, simpler integration into existing plants. For the sintering plant in Linz, this was one of the most important aspects.
Densely packed and highly efficient
The solution in Linz featured a recuperative gas pre-heater from GEA Heat Exchanger Systems. This system is based on a highly efficient, fully welded plate heat exchanger for gaseous media, which operates according to the counterflow principle. The heat-transfer surfaces consist of ridged plates with a high specific output, which have been welded together to form self-supporting plate packages. The two gases involved here flow through the plates in counterflow and remain separated from each other. The prevention of leakage was an additional key benefit that tipped the scales for this heat exchanger type. This is ensured by fully welded plate packages that were manufactured with extreme precision on a fully automatic welding system.
In Linz, approximately 720,000 Nm³ of flue gas per hour flows through the powerful heat exchanger. Dimensions of 14 x 15 x 11 m are necessary to achieve this, corresponding to about 2300 m³ of enclosed space. Yet the actual size is more readily apparent if you take a look at the inside, which has a heating surface of 70,000 m². This area is the equivalent of around ten soccer fields and provides heating duty of 37,000 kW. The great advantage is most obvious during operation – and when the next bill arrives from the utility company. Downstream of the Meros system, the heat exchanger alone heats the untreated gas from 145 to 261°C. It is only the remaining 19 °C – essential to achieve the 280 °C required for NOx removal – that is the outcome of externally supplied energy.
Three factors are crucial for this impressive efficiency. The first is flow routing. The untreated gas flows through the heat exchanger vertically from the bottom to the top while the treated gas flows through the same heat exchanger, again vertically, but from the top downwards. Secondly, the high specific density of the heat-transfer surface – i.e. the surface per cubic metre – not only affects the performance of the heat exchanger but is also vital in view of the severely limited space available in the Linz plant. The untreated gas is heated 135 °C along a path of just under three metres. Thirdly, the ridged surfaces made of carbon steel likewise make an important contribution. The steel mill in Linz, by the way, produced the carbon steel required here.
Up to the top
With its approximately 700 t, the Linz heat exchanger is not exactly a featherweight. It plays a key supporting role in the entire plant. Since the space restriction required that the NOx removal plant be installed on a blasting hall of the steel mill, a clever solution was called for. The roof of the hall, originally designed to support a weight of 300 t, was reinforced so that it would hold not only the heat exchanger but also the catalytic converter. The exchanger’s rugged design features were equally beneficial. The catalytic converter and its accessories – e.g. burners, enclosures and platforms – weigh an additional 600 t which must likewise be borne by the plate heat exchanger – a support-ing role it has mastered without difficulty.
The time frame for project execution was extremely tight. Nevertheless, the heat exchanger was manufactured in less than eleven months and installation went ahead exactly according to plan. Initially, the lower hood for the bottom impingement-flow area of the heat exchangers was pre-assembled on ground level and then lifted in one piece (around 70 t) to a height of about 32 m, where it was aligned and installed on top of the blasting hall. The heat transfer surfaces of the heat exchangers were transported to Linz in convenient 50 t packages. On site, two components at a time were assembled and hoisted into place on the lower hood by a crane. The upper hood was also pre-assembled on the ground, lifted to a height of 38 m, aligned there and installed on the heat exchanger.
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