Flue gas heat recovery through the acid dew point
Polymer based heat exchange technology enables more heat recovery from flue gas by addressing acid corrosion issues.
Pim Van Keep and ROBERT SAKKO
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Crude oil refining is an energy intensive process. On average, the thermal energy consumption of a refinery is in the order of 350-400 MJ/bbl, based on 2017 data from environmental agencies in the Netherlands, with some refiners achieving a higher energy efficiency than others. In order to reduce operational expenditure as well as the carbon footprint, refiners engage in projects to improve energy efficiency and process sustainability.
For a 100000 b/d refinery, the typical thermal energy consumption equates to 400 MW. Of this energy, most is consumed in endothermic processes. Approximately 10% however is lost through the stack as waste heat. Historically, these stack losses of flue gas being emitted at high temperatures were accepted as a given because further heat recovery would lead to acid corrosion and related operational reliability issues.
These corrosion and reliability concerns can now be overcome, allowing refiners to recover about a third of the waste heat from their flue gases (14 MW/100 000 b/d) in an economical way. As a result, an improvement of the overall refinery energy efficiency of 3-4% can be achieved.
In this article, we look at the issues traditionally linked to recovering heat from flue gases. We review the solution that allows more heat to be recovered by cooling the flue gas down to temperatures below the acid dew point, and we discuss what the implications are for existing downstream equipment.
What is the issue with corrosive flue gas?
In refineries, a wide range of fuels are combusted in thermal processes, ranging from natural gas, off-gases, LPG to naphtha and fuel oil. Most of these fuels contain sulphur components like H2S, mercaptans and thiophenes, which are readily converted to SOx in the combustion chamber. Mainly SO2 is formed, but part of this SO2 (typically about 2-4%) oxidises further to SO3. This SO3 reacts with H2O in a condensing reaction to form sulphuric acid, when the flue gas cools below the acid dew point (ADP):
SO3 (g) + H2O (g) -> H2SO4 (l) 
The acid dew point temperature depends on the levels of reactants present in the flue gas. It generally lies in the range 100-150°C. Figure 1 shows the relationship between the ADP and the SO3 level, calculated at a typical H2O level of 15 vol% by using a number of different approaches proposed in the literature.
Sulphuric acid is highly corrosive and affects susceptible equipment surfaces. For example, on cold surfaces in metal air preheaters, local temperatures drop below the ADP, leading to sulphuric acid condensation, which results in rapid corrosion and breakdown of plates and tubes. This phenomenon is known as cold spot corrosion. The degradation will go unnoticed for a period, but in the end leaks will result in a short cut between combustion air and flue gas and thereby an energy loss through a loss in recovery efficiency as well as an increase in power consumption by the combustion air fan. The leakage can impact the production rate, once the combustion air fan reaches its limitation.
Such cold spots can occur even if the flue gas bulk temperature is still as high as 250°C, because of the cold ambient air at the other side of the air preheater surface. The excessive cooling leads to flue gas side surface temperatures below the acid dew point. Cold spot corrosion can be aggravated by varying levels of sulphur in the fuels. As sulphur levels increase, the acid dew point increases accordingly. Even if the refiner targets to remain 10-20°C above the typical acid dew point, these variations in fuel sulphur content can lead to sulphuric acid condensation during sulphur peaks, resulting in loss of equipment integrity and a reduction in plant performance.
What are the challenges with conventional equipment?
To deal with acid corrosion, the industry has implemented different approaches with mixed success. The first approach is to stay away from the acid dew point by keeping stack temperatures up. But as described above, variations in fuel sulphur levels and localised cold spots can still lead to corrosion. To avoid over-cooling of exchange surfaces by cold air, part of the heated combustion air can be recycled to the inlet of the forced draught air fan which will lift the air temperature in the air preheater and thereby reduce the risk of cold spot corrosion. This however requires extra ducting, demands more power on the air fan and reduces the heat recovery on the air preheater. Alternatively, the combustion air can be first warmed up with a steam air preheater. This approach results in additional costs of steam and reduces the heat recovery. Both of these mitigation approaches still limit the recovery of the heat in the flue gas to approximately 20°C above the acid dew point.
To recover more energy from the flue gas, it has to be cooled down below the original acid dew point. Around the acid dew point, corrosion rates are high, but as the flue gas is cooled further down through the corrosive temperature range (see Figure 2), rates of corrosion become manageable again. Below 90-100°C, the corrosiveness of the flue gas is significantly lower compared to the corrosiveness just below the acid dew point temperature.
Under the acid condensing conditions arising from cooling down through the acid dew point, standard metal heat exchangers are not suitable. Special alloys need to be used. The cost of these, however, makes the heat recovery uneconomical. Alternative materials of construction like glass or enamel coated metal have been implemented at times. These solutions are, however, susceptible to flow induced vibrations and thermal shocks, which lead to damaged enamel coatings (allowing the acid to reach the underlying metal surface and corrode it away), tube breakage or rupture. The subsequent short cut between combustion air and flue gas reduces heat recovery and increases the load on the air fan as described before.
Polymer heat exchanging tube bundles
HeatMatrix Group has developed an innovative polymer based heat exchange technology that allows recovery of heat from corrosive and/or fouling flue and exhaust gases, to preheat combustion air and thereby improve overall process energy efficiency. This technology allows operators to recover even more heat from the flue gas down to temperatures well below the acid dew point, or replace their existing glass tube or glass lined air preheater or steam air preheater with a reliable, more efficient solution.
The HeatMatrix APH air preheater is based on Polymer Honeycomb technology. In this technology, multiple tubes are connected to each other over a significant length of the tube, forming a honeycomb modular bundle (see Figure 3, left). Multiple corrosion resistant polymer tube bundles are mounted into a metal casing (see Figure 3, right) to give the required heat exchange area.
Polymer Honeycomb technology provides a strong and rigid heat exchange matrix that is able to resist high gas velocities and thermal shocks. The geometry creates a 100% counter current flow configuration between the flue gas and air streams. This configuration improves the heat transfer by up to 20% compared to cross flow type exchangers. Flue gas flows from top to bottom through the tubes (see Figure 3, red arrow) and combustion air flows in the opposite direction around the tubes (see Figure 3, blue arrow). Inside the polymer tubes, the flue gas will go through its acid dew point and acid condenses on the tube wall. At the ADP, the concentration of sulphuric acid is high. As the acid travels down the tube, it will absorb water and the resulting condensate is collected in and drained from the bottom section of the air preheater. This bottom section has been designed to disengage condensate from the flue gas. Whereas the top three sections of the exchanger are carbon steel based, the bottom section is specifically designed to withstand the diluted acidic condensate (approximately 1% H2SO4 content).
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