I for one think of stainess steel as a premium cost material and have not considered it likely to be a cost-effective material for a large commerical biogas tank, and yet in the article that follows it is clear that this material is being used for biogas plants, and reportedly, very successfully. We have included much of the article below, but please also visit the original website.
In Szeged, around an hour south of Budapest, Weltec Biopower is currently building an ultramodern biogas plant with stainless steel from ThyssenKrupp Nirosta. The 1-megawatt facility, comprising two combined heat and power plants with an output of 600 kW/h each, will go into operation at the end of 2011. The organic waste substrate will be supplied by local farmers, who in return will use the digestate left over from the anaerobic digestion /biogasproduction process as fertilizer on their fields. The generated heat will be used to heat offices in Szeged.
At the heart of the plant are two stainless steel digesters. In addition, an external stainless steel gas storage tank with a capacity of 650 cubic meters will be installed on the site. The aim is to take advantage of electricity prices, which are twice as high during the daytime. The biogas will therefore be collected in the digester tank and the external gas tank at night and the two CHP modules will run under full load in the day time.
The two 3,000 cubic meter digesters are sealed with a double membrane roof, which means that each digester also has an additional gas buffer capacity of 1,016 cubic meters. Animal waste and energy crops are used as a substrate, with special bacteria being used for the fermenting process. During biogas production, waste products such as sulfur and ammonia are also formed. "These substances are very corrosive, placing high demands on the material for the digesters. That's why we use stainless steel exclusively for our plants," says Hajo Schierhold, head of sales and marketing at Weltec Biopower, whose sole supplier of stainless steel is ThyssenKrupp Nirosta.
In the past ten years ThyssenKrupp Nirosta has supplied around 1,500 tons of material for various projects carried out by the Vechta-based plant builder. Depending on the loading, different stainless steel grades are used, e.g. for the tank walls in contact with the liquid or gas. "The majority of our deliveries have been our standard sheet steel Nirosta 4301 specially optimized for these applications," says Martin Stöckl from the ThyssenKrupp Nirosta sales team.
"What's particularly important for us is the cold-worked condition, 2H. This means thinner steel can be used to build the digesters without any loss of stability. It also means a significant cost saving". Another advantage is the extremely smooth surface. "The dense surface of Nirosta 4301 2H gives aggressive substances no opportunity to attack the material," says Weltec Biopower sales chief Schierhold. "Bacteria have no chance to take hold, so corrosion is nipped in the bud, which is an important factor for longevity and efficiency."