Stainless steel storage

Materials World magazine
,
1 Jun 2017

Nancy Baddoo looks at the design, fabrication and installation of stainless steel bio-digester tanks.

Biogas is an established form of renewable energy, where biomass, typically manure, livestock slurry and other organic materials are broken down by anaerobic digestion, releasing biogas, a mixture of methane, carbon dioxide and traces of other gases. The methane is then burnt in a power station to produce electricity, or heat and electricity in a combined heat and power (CHP) plant, or cleaned to use as natural gas or vehicle fuel.

The economics of biogas are dependent on the digestion process continuing undisturbed, so a low-maintenance material that can withstand the corrosive environment is needed for the digestion tank. Molybdenum-containing stainless steel is strong, easy to fabricate and inherently corrosion resistant, even in the most aggressive internal areas, making it a viable choice for reliable, continuous operation. 

Biogas is an ideal renewable energy source. It uses generally ‘free’ waste products to generate energy and produce a residual organic material, resulting in an organic fertiliser that has fewer odours than slurry. It also reduces the greenhouse gas potential of farming activities, as it captures the more environmentally significant methane for burning rather than releasing it directly into the atmosphere. Investing in biogas enables farmers to diversify, producing a steady income stream, while reducing fossil fuel reliance and replacing expensive artificial fertilisers with an organic by-product. External food processing and catering waste can also be added to the feedstock, boosting gas production, improving overall economics and providing an environmentally responsible alternative to landfill.

The anaerobic digestion process is straightforward but requires careful management to ensure that it operates at its full potential. Most systems in use have an operating temperature at 25–45°C. Higher temperatures (50–60°C) are possible and produce gas quicker, but are more expensive to operate. Gas production is maximised when the feed mixture is 6.5–8pH.

The specification of the tank is dictated by the production of acetic acid and trace gases such as hydrogen sulphide (H2S) and halides. The level of H2S must be substantially reduced if the biogas is intended for CHP use, otherwise excessive corrosion can occur. Biological desulphurisation works by adding small volumes of oxygen, creating sulphur deposits on the upper parts of the tank. Sulphurous acid may also be produced. Chemical desulphurisation occurs by adding iron hydroxide (Fe(OH)2) leading to the precipitation of iron sulphide (FeS). High levels of sulphur will also be produced if the feedstock contains meat or fish processing residues. Convenience products and leftover food will also produce high levels of sodium chloride (NaCl).

These desulphurisation methods add to the already corrosive environment inside the tank and make the specification of a suitably resistant material even more important. Concrete and carbon steel are popular choices, but must be lined or coated with a protective material.

Alternative options

With a wide variety of grades and alloying combinations, a correctly specified and fabricated stainless steel tank will deliver corrosion-free performance. Generally speaking, the higher the chromium, molybdenum and in some cases nickel content, the better the corrosion resistance.

The most corrosive area inside a biodigester tank is the splash or tidal zone, where corrosive substances can concentrate. For this reason, the upper part of a digester tank is usually made from a more highly alloyed grade, while the permanently submerged lower part can be made from less alloyed steel. Corrosion resistance is also affected by the surface finish – generally, the smoother it is, the more resistant it is to corrosion, particularly in the tidal zone.

The correct grade of stainless steel depends on the corrosivity of the environment inside the biodigester, which itself depends on a variety of variables including the operating conditions, design and location within the tank.

Corrosion resistance

BIOGASS, a three-year EU study carried out with financial support from International Molybdenum Association and the Nickel Institute examined the variables and its effects using artificial solutions and real feedstocks to study corrosion resistance in a number of laboratory tests on a range of stainless steel grades.

The same grades were then tested in field trials using digesters in Finland and Germany. Samples of the different grades were placed in various positions in the tanks, some submerged, some in the tidal zone, and others in the gas phase. After five and six months, respectively, the samples were removed and analysed for corrosion. The Finnish biodigester used a manure feedstock and removed H2S with oxidation in a separate tank – only the lowest alloyed grade (1.4003) showed any signs of corrosion. 

The German biodigester used corn and manure as feedstock and was desulphurised with Fe(OH)2 and oxygen. The conditions inside the German tank were more corrosive, as samples that had not corroded in the Finnish tank showed evidence of corrosion in the German tank, with the most severe corrosion in creviced areas of samples in the tidal zone. Both the type of feedstock and the method of desulphurisation were demonstrated to have a significant effect on the corrosivity of the environment.

As a result of the laboratory investigations and field trials, the study made a number of material specification recommendations for bolted stainless steel biotanks. The 2205 duplex stainless steel (1.4462) did not corrode under any of the relevant electrochemical, laboratory or field tests, making it a suitable material for the most aggressive conditions such as feedstocks containing food waste with high levels of NaCl.

The global market for biogas was estimated to be worth nearly US$20 billion in 2015. Europe is currently the dominant global region, with more than 7,500MW of capacity registered in 2015. Industrialisation in India and China and the increasing favourability of renewable energy at government level is expected to create significant demand in Asia-Pacific, driving an estimated growth of more than 6% per year to 2023 and an estimated total value of US$32 billion. The high corrosion-resistance, low maintenance and longevity of molybdenum-containing steel is playing a key role in building this market by making the logistics and economics of biogas production more favourable.

This article is partly based on Stainless steel tanks for biogas production, the final report of the BIOGASS project, carried out with the financial support from the Research Fund for Coal and Steel of the European Community, the International Molybdenum Association and the Nickel Institute. The full report is available to download at www.steel-sci.com 

Nancy Baddoo is an Associate Director at The Steel Construction Institute, UK, and a consultant to the International Molybdenum Association. She chairs the European technical committee responsible for the stainless steel Eurocode and wrote the American Institute of Steel Construction Design Guide Structural Stainless Steel.