{"id":12001,"date":"2026-05-11T00:09:35","date_gmt":"2026-05-11T00:09:35","guid":{"rendered":"https:\/\/www.irwin.com.hk\/?p=12001"},"modified":"2026-05-11T01:27:27","modified_gmt":"2026-05-11T01:27:27","slug":"polyethylene-pe-lining-a-cost-effective-and-robustsolution-for-sulfuric-acidcorrosion-protection-in-sox-scrubbers","status":"publish","type":"post","link":"https:\/\/irlservice.com\/en\/polyethylene-pe-lining-a-cost-effective-and-robustsolution-for-sulfuric-acidcorrosion-protection-in-sox-scrubbers\/","title":{"rendered":"Material Selection for SOx\/NOx Scrubbers: Why PE Roto-Lined Steel Offers Superior Corrosion Resistance and Cost Efficiency"},"content":{"rendered":"\r\n\r\n
\r\n\r\n
\r\n\r\n
\r\n\r\n
\n\n

Material Selection for SOx\/NOx Scrubbers: Why PE Roto-Lined Steel Offers Superior Corrosion Resistance and Cost Efficiency<\/h1>\n\n

Sulfur and Nitrogen Oxides pollution abatement is continuing to be as critical as ever in the context of tighter environmental emission norms and increasing concerns of global warming. The process chemistry and unit operations associated with SOx and NOx removal techniques are standard and mature. One of the major challenges associated with the technologies is the selection of appropriate materials of construction that provide the lowest total cost of ownership and a high level of corrosion resistance. The advent of newer materials and fabrication techniques have increased the material options available. This paper takes a closer look at one of the options: polyethylene (PE) lined steel pipes manufactured by roto-lining process<\/strong>. This material, though not a recent invention, is relatively new to the field of SOx\/NOx scrubbing systems and is a strong contender among the available options.<\/p>\n\n

SOx \/ NOx Scrubbers<\/h2>\n

SOx and NOx abatement applications are encountered in coal and oil-fired power plants, steam boilers and marine engines. Though all these applications have several common features, this paper focuses on marine applications, particularly scrubbers installed in ships.<\/p>\n

A typical open loop scrubber used for installation in ships is shown in Fig. 1. The simplest scrubber system available is the open loop scrubber, where water is sourced from the surrounding sea, pumped through a filter and sprayed into the scrubber using nozzles that disperse water into droplets. An open loop scrubber is efficient only if the source of water is alkaline. This can either be done by adding an alkali chemical or by utilizing seawater, which has a natural alkalinity derived from the bicarbonate ion (HCO3<\/sub>-<\/sup>) present in seawater.<\/p>\n

The hot exhaust gas from a ship\u2019s engine is led to the inlet at the bottom of a cylindrical packed scrubber to encounter a counter-current flow of aerated sea water sprayed from the top of the tower, wherein the SOx and NOx in the exhaust gas is absorbed by the sea water and oxidized by oxygen in the aerated sea water to sulfuric acid.<\/p>\n

\n \"Schematic\n
Courtesy: W\u00e4rtsil\u00e4 Corporation. Fig. 1 \u2013 Schematic of a typical open loop SOx\/NOx scrubber system in a ship<\/em><\/figcaption>\n <\/figure>\n

The water is discharged back into the sea after particulate matters are removed. The type of solid waste generated from scrubbing is classified as hazardous waste, and therefore stored and later disposed to land-based disposal facilities. Closed loop scrubber systems are developed for no-discharge zones; this requires that the scrubber system works on a recirculated flow that is later discharged as the vessel leaves a no-discharge area. Depending on the amount of water consumed and the size of the buffer tank, the closed loop system can only run for a certain amount of time until the liquid is saturated. Therefore, to avoid saturation and increase the time span, the liquid phase is continuously bled off, adding more seawater into the system.<\/p>\n\n

Scrubber System \u2013 Chemistry and Corrosive Conditions<\/h2>\n

SOx are formed by a ship\u2019s engines during the combustion process due to the oxidation of naturally occurring sulphur in the fuel. In the exhaust gas, the major part of SOx is present as SO2<\/sub>, and a minor portion is SO3<\/sub>.<\/p>\n

Reactions involving SO2<\/sub>:<\/strong>
\n Gaseous SO2<\/sub> dissolves in seawater and subsequently is ionized, generating bisulphite and sulphite ions.<\/p>\n 

\nSO2 + H2O   \u21cc   H+ + HSO3-\nHSO3-      \u21cc   H+ + SO3^2-\nSO3^2- + 0.5 O2 \u2192 SO4^2-\n  <\/pre>\n  
The generated hydrogen ions are neutralized by the alkalinity of seawater (mainly due to its bicarbonate content), thereby consuming alkalinity.<\/p>\n  
Reactions involving SO3<\/sub>:<\/strong>
\n  Gaseous SO3<\/sub> dissolves in water, forming sulfuric acid, which dissociates to sulphate.<\/p>\n  
\nSO3 + H2O \u2192 H2SO4 (Sulfuric Acid)\nH2SO4 \u2192 H+ + HSO4- \u2192 2H+ + SO4^2- (Sulfate)\n  <\/pre>\n  
The generated hydrogen ions are neutralized by the alkalinity of seawater, thereby consuming alkalinity.<\/p>\n  
Where no effluent discharge is permitted, a closed loop scrubbing process with normal water and external alkali dosing can be employed. The Arctic, Northwest Pacific, and Norwegian ocean are some of the areas where alkalinity of seawater is inadequate for open loop scrubbing. In the interest of brevity, a detailed description of the closed loop process is not included here. The corrosive conditions encountered in the open loop process are more aggressive than those in the closed loop process (seawater is more corrosive than fresh water), hence the open loop process covered in this paper may be considered a worst-case scenario.<\/p>\n  
The surface pH of seawater usually ranges from 8.1 to 8.9. During scrubbing, the hydrogen ions generated are neutralized by the alkalinity of seawater, resulting in a pH of 4 to 5. The corresponding concentration of sulfuric acid can be read off the chart in Fig. 2.<\/p>\n  
\n    \"Sulfuric\n    
Fig. 2 \u2013 Sulfuric Acid Concentration vs Solution pH (2)<\/em><\/figcaption>\n  <\/figure>\n  
The absorption of NOx gases into the liquid phase is more complex than that of SO2<\/sub>. Not only is the gas-phase NOx in equilibrium with the liquid phase, but equilibrium and kinetic reactions also occur in the gas\u2011phase. This complicates the absorption chemistry of NOx, since different nitrogenous compounds are formed in the gas phase. These compounds can also be dissolved into the liquid phase and lead to different possible reactions in the liquid phase. NOx, in the form of NO or NO2<\/sub>, can be absorbed into the liquid phase.<\/p>\n  
\nNO(g)   \u21cc   NO \u00b7 H2O\nNO      \u21cc   [NO \u00b7 H2O]\/pNO\n\nNO2(g)  \u21cc   NO2 \u00b7 H2O\nHNO2    \u21cc   [NO2 \u00b7 H2O]\/pNO2\n\n2NO2 + H2O \u2192 HNO2 + HNO3\nHNO2    \u21cc   H+ + NO2-\nHNO3    \u21cc   H+ + NO3-\n  <\/pre>\n  
Presently ship vent gas scrubbing systems are designed mostly for SOx removal only. However, technologies such as the CSNOX Wet Scrubbing Technology developed by Ecospec, Singapore for removing SOx, CO2<\/sub> and NOx in a single wet scrubber will put ships using such technologies at a distinct advantage in the future when marine emission regulations may be extended to NOx emissions as well.<\/p>\n\n  
Selection of Materials of Construction<\/h2>\n  
From the preceding description of the chemistry involved in the scrubbing process, the materials of the various components of the scrubber system should be suitable for the respective media and temperature indicated in Table 1 below.<\/p>\n\n  
Table 1 \u2013 Material of Construction Options for SOx and NOx Scrubbers<\/h3>\n  
\n    \n      \n        \n        
SL.No.<\/th>Location \/ Part of Scrubber<\/th>Operating Temperature (\u00b0C)<\/th>Media<\/th>Material Options<\/th<\/tr>\n      <\/thead>\n      
1<\/td>Inlet Venturi and Inlet Nozzle<\/td>260 to 300<\/td>Exhaust Gas<\/td>Duplex Stainless Steel (Duplex SS) \/ Titanium<\/td<\/tr>\n        
2<\/td>Scrubber Body<\/td>50 to ?<\/td>Seawater of pH range 4 (inlet) to 8 (outlet)<\/td>Duplex SS \/ Titanium \/ PVDF \/ PFA lined Steel \u0197<\/td<\/tr>\n        
3<\/td>Scrubber liquid Effluent Piping<\/td>40 to 50<\/td>Seawater of pH 4 to 5<\/td>Duplex SS \/ Titanium \/ PE lined Steel \/ VE\u2011FRP**<\/td<\/tr>\n        
4<\/td>Scrubber Pump<\/td>20 to 50<\/td>Seawater of pH 8 to 9<\/td>Duplex SS \/ Titanium \/ PE lined Steel<\/td<\/tr>\n        
5<\/td>Effluent Tank, Sludge removal system, Effluent Pump & Piping<\/td>40 to 50<\/td>Seawater of pH 4 to 5<\/td>Duplex SS \/ Titanium \/ PE lined Steel \/ VE\u2011FRP**<\/td<\/tr>\n        
6<\/td>Exhaust Gas Blower & Duct<\/td>40 to 50<\/td>Exhaust gas & traces of Seawater of pH 8 to 9<\/td>Duplex SS \/ Titanium \/ PE lined Steel \/ VE\u2011FRP**<\/td<\/tr>\n      <\/tbody>\n    <\/table>\n  <\/div>\n  
\u0197 Non-metallic materials like PVDF\/PFA lined Steel are potential options that have superior corrosion resistance and temperature resistance as compared to PE and PP lined steel and may be potential substitutes for Duplex SS.
** Vinyl Ester FRP<\/em><\/p>\n\n  
PE Lining by Rotational Lining (Roto\u2011lining)<\/h2>\n  
The rotational lining is a process of lining the inside of pipes or other SOx components with a seamless, one\u2011piece inner layer of PE plastic. In this lining technique, the lined spool is produced by heating and rotating a carbon steel spool with a polymer (in granular\/powder form) placed inside the pipe spool. The polymer melts and forms a seamless and uniform liner on the internal surface of the carbon steel pipe. The engineered polymers used for the rotational lining process include additives which increase the bond and adhesion with the prepared metal substrate.<\/p>\n  
The choice of polymer depends on the required chemical resistance properties. Polyethylene, polypropylene, or other polymers are used for roto\u2011lining. Polyethylene is the workhorse of the segment and accounts for over 80% of roto\u2011lining applications. The lining thickness varies from 2 mm to 8 mm. The heavy lining thickness allows post\u2011machining of critical surfaces that would not be possible with a thinner lining applied by other methods. Virtually any type of metal weldment or casting may be roto\u2011lined. Typical items include tanks, carbon steel pipes, fittings, and complex welded structures.<\/p>\n\n  
Advantages of Rotational Lining<\/h3>\n  
    \n    
  • Costs for lining and tooling are relatively low.<\/li>\n    
  • The roto\u2011lining technique is easily adapted to short production runs, particularly when sets of multiple\u2011cavity molds are used.<\/li>\n    
  • Hollow, totally enclosed items, as well as pieces with openings, can be made.<\/li>\n    
  • Rotational lining eliminates the need for secondary tooling.<\/li>\n    
  • There is little or no waste due to resin scrap.<\/li>\n    
  • Wall thickness and piece weight can be easily controlled.<\/li>\n    
  • Rotational lining procedures ensure uniform wall thickness; deviations can be controlled within a tolerance of \u00b110%.<\/li>\n    
  • Pieces with intricate contours and undercuts can be easily molded.<\/li>\n    
  • Virtually any size piece can be rotationally molded.<\/li>\n    
  • There is a minimum of cross\u2011sectional deformation and warpage.<\/li>\n    
  • Rotational lining yields pieces with excellent surface detail and finish.<\/li>\n    
  • Rotationally lined items are virtually stress free.<\/li>\n    
  • Identical or similar items, or different sections on one piece, can be molded at the same time in different colors on a single spindle.<\/li>\n    
  • Plastic or metal inserts can often be lined as integral parts of the item.<\/li>\n    
  • Double wall constructions are feasible.<\/li>\n  <\/ul>\n\n  
    \n    
    \"Roto-lined
    Fig. 3 \u2013 Roto-lined tanks >10 ft diameter. (Photo Courtesy: RMB Products Inc.)<\/em><\/figcaption><\/figure>\n    
    \"Venturi
    Fig. 4 \u2013 Venturi type pipe fittings (Photo Courtesy: Irwin Rotational Lining)<\/em><\/figcaption><\/figure>\n  <\/div>\n  
    \n    
    \"S-shaped
    Fig. 5 \u2013 Roto\u2011lined S\u2011shaped pipe bends (Photo Courtesy: Irwin Rotational Lining)<\/em><\/figcaption><\/figure>\n    
    \"Internal
    Fig. 6 \u2013 Roto\u2011lined pipe fitting internal view (Photo Courtesy: Irwin Rotational Lining)<\/em><\/figcaption><\/figure>\n  <\/div>\n  
    Rotational lining is a mature technology proven in various harsh chemical applications. Figures 3 to 5 demonstrate the diverse capabilities: from large tanks to complex shaped piping.<\/p>\n  
    In addition to a wide range of sizes and intricate geometries, material options within polyethylene (LDPE, LLDPE, LMDPE, HDPE, and cross\u2011linkable polyethylene XLPE\/XLDPE) broaden the application range of PE lined equipment.<\/p>\n\n  
    PE Lining Material Options \u2013 Cross\u2011Linked Polyethylene (XLPE)<\/h3>\n  
    XLPE is a form of polyethylene with cross\u2011links. Molecules arrange in a specific pattern with actual chemical bonds formed between individual molecules, locking them into a semi\u2011rigid three\u2011dimensional shape. Heating does not weaken these bonds, so XLPE retains its physical properties at elevated temperatures. Primary advantages: improved creep, low\u2011temperature impact strength, heat resistance, improved resistance to stress cracking, and reduced permeation. These attributes allow XLPE to be used in demanding applications such as fuel and chemical tanks.<\/p>\n  
    \"Molecular
    Fig. 7 \u2013 Molecular structures of linear and cross\u2011linked polyethylene<\/em><\/figcaption><\/figure>\n  
    Key factors of XLPE for SOx\/NOx scrubbers:<\/strong><\/p>\n  
      \n    
    • Elevated temperature capability (up to 70\u202f\u00b0C)<\/li>\n    
    • Higher resistance to oxidative media like sulfuric acid and sodium hypochlorite (encountered in ballast water treatment systems)<\/li>\n    
    • Excellent stress corrosion cracking resistance<\/li>\n    
    • Excellent impact strength<\/li>\n  <\/ul>\n  
      PE has a clear edge over competing materials like VE\u2011FRP, as summarized in Table 2. Metallic options (Duplex SS, Titanium) are more expensive and excluded from this direct comparison.<\/p>\n\n  
      Table 2 \u2013 Comparative Features of XLPE Lined Steel vs. VE\u2011FRP<\/h3>\n  
      \n    \n      \n        
      Sl.No.<\/th>PE Lined Steel<\/th>VE\u2011FRP<\/th><\/tr><\/thead>\n      
      1<\/td>Higher chemical resistance. Single material homogenous layer \u2013 no dual material interface corrosion.<\/td>Prone to chemical attack at the resin\u2011glass fiber interface \u0197\u0197.<\/td><\/tr>\n        
      2<\/td>Excellent impact resistance; better resistance to vibration failure (especially important for ship scrubbers).<\/td>Impact resistance inferior to XLPE; susceptible to damage during installation and poor vibration resistance.<\/td><\/tr>\n        
      3<\/td>Design flexibility for equipment & parts with complex shapes.<\/td>Limited flexibility; not suitable for intricate shapes.<\/td><\/tr>\n        
      4<\/td>XLPE lined steel offers a safer option in case of fire \u2013 steel backing contains the process media.<\/td>Immediate failure in fire; combustion fumes of VE\u2011FRP are much more toxic than those of PE.<\/td><\/tr>\n        
      5<\/td>Conforms to classification society requirements for water\u2011tight bulkhead penetrations.<\/td>Classification societies do not permit FRP pipes in areas where scrubber piping is installed on ships.<\/td><\/tr>\n      <\/tbody>\n    <\/table>\n  <\/div>\n  
      \u0197\u0197 \u2013 Reduction in mechanical properties by water\/chemical absorption has been attributed to debonding of the fiber\u2011matrix interface, leading to delamination, cracking, and plasticizing of the matrix (5).<\/em><\/p>\n\n  
      Based on the above comparative evaluation, suggested PE lined materials for various parts of a SOx\/NOx scrubber system are shown in Fig. 8.<\/p>\n  
      \"Suggested
      Fig. 8 \u2013 Suggested PE lined material options for a SOx\/NOx scrubber system<\/em><\/figcaption><\/figure>\n\n  
      Conclusion<\/h2>\n  
      PE roto\u2011lined equipment and piping offer a robust and cost\u2011effective solution for material selection in SOx\/NOx scrubber systems. It is worthwhile to make a detailed evaluation of this option before final material choice, as it has several technical advantages over competing materials and could potentially provide the lowest total cost of ownership.<\/p>\n\n  
      References<\/h2>\n  
        \n    
      1. Yao, G. Ziegmann, \u201cWater Absorption Behavior and Its Influence on Properties of GRP Pipe\u201d, Journal of Composite Materials, Vol. 41, No. 8\/2007.<\/li>\n    
      2. Referenced pH\u2011concentration chart (source as in original paper).<\/li>\n    
      3. Rotational lining advantages \u2013 industry standard data.<\/li>\n    
      4. Cross\u2011linked polyethylene properties \u2013 industry literature.<\/li>\n    
      5. Yao, G. Ziegmann as above.<\/li>\n  <\/ol>\n\n<\/div>
        <\/div><\/div><\/div><\/div>","protected":false},"excerpt":{"rendered":"","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"categories":[27],"tags":[],"class_list":["post-12001","post","type-post","status-publish","format-standard","hentry","category-lining"],"_links":{"self":[{"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/posts\/12001","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/comments?post=12001"}],"version-history":[{"count":0,"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/posts\/12001\/revisions"}],"wp:attachment":[{"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/media?parent=12001"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/categories?post=12001"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/irlservice.com\/en\/wp-json\/wp\/v2\/tags?post=12001"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}