Bioenergy plant design - material decisions for tanks

Paul Boughton

As energy prices continue rise, the number of bioenergy plants across the world is increasing rapidly. However, design engineers are faced with a number of basic choices when it comes to starting a new project.

One of the major decisions is the construction material to be used in the tank design of the digester, gas storage and feedstock storage.

Essentially, three materials dominate the market, concrete, carbon steel and stainless steel, with the steel variants having a variety of coatings and construction techniques to choose from as well. The final decision for any project should be made from an informed standpoint to ensure that the most appropriate design is employed in order to deliver a reliable and durable solution.

Anaerobic digestion plants can be designed to operate in a number of ways, depending on the scale of the plant, the feedstock to be used and the aims of the project itself in terms of energy output. Clearly these must be assessed and determined first, before considering the bes-suited tank design and construction method. The essential aspects of each choice are detailed below.

Concrete decision

As a building material, concrete has two major advantages, it is strong and relatively cheap and as such it has been used for the construction of bulk storage tanks for some time. However, the process of anaerobic digestion requires much greater resistance to chemical attack than that provided by concrete. The substrates and gases involved in the processes can lead to a reduction in the expected lifespan of the tank. In addition, the standard surface finish of a concrete tank inhibits the agitation process used within the AD process.

Another consideration at the design phase of any project is the overall build time which affects the total project cost as well as the timeframe before the plant can generate income. The construction of a concrete storage facility can either be completed using prefabricated panels, lifted into position by mobile crane, or by pumping liquid concrete into a ring of preformed sections. These methods can require increased periods of time to complete and use a number of large, heavy plant vehicles.

Strong as steel

One alternative to the concrete tank is to use carbon steel which can be coated with glass linings or epoxy paint to prevent corrosion. The tanks are made from rectangular panels which can either be welded or bolted together, however both of these systems can be prone to leakage and seal deterioration. Any sealant, gasket or fastener used in tank construction needs to be properly evaluated to ensure a long service life.

The positive aspects of the steel construction are the reduced timescale for the build and the wide availability of suppliers and materials, making the market very competitive in terms of project costs. In addition, the bolted tanks can be extended, dismantled and re-sited if necessary which can increase the long term asset value.

Stainless reputation

The use of stainless steel to build storage tanks using the same construction methods as those for carbon steel has largely been ruled out on the basis of cost. However, the development of a hybrid material, Verinox, by Xaver Lipp in 1980, managed to combine the excellent corrosion resistance of stainless steel with the strength and cost effectiveness of galvanised steel. This was the first step in creating the revolutionary Lipp tank system.

Rather than bolting or welding small sections together, the Lipp Dual-Seam System uses an automated process to create a spiral tank from a roll of Verinox material. This combines an inner stainless steel skin, a boundary layer and a thicker structural outer steel layer. The edges of the sheet are folded over twice with an integral sealing compound to produce a smooth internal surface and a gas tight seal. It is this internal surface that aids an efficient agitation process as required by anaerobic digestion. The flexibility of this design allows material to be matched to the substrate and the construction can be completed in areas inaccessible to large cranes.

Since its initial development, the Lipp tank system has been used to construct over 50,000 tanks in various industries and more than 80 countries across the world. In addition, the system can be used to construct all the necessary elements of an anaerobic digestion facility, including mixing tanks, hydrolysis tanks, universal digesters, gas storage tanks and digestate storage tanks, which can be either open or covered.

Over the years, Lipp has applied its in-depth knowledge of anaerobic digestion to create more efficient processes and ultimately better gas yields from a variety of substrates. Development of the mixing tank included the creation of a number of horizontal and vertical stirring systems, to account for the individual substrate characteristics. The smooth internal surface of the base tank combined with the most efficient height-width ratio, ensures a very effective circulation with reduced energy consumption.

The use of a highly advanced hydrolysis process prior to the digestion process can improve the overall output of the plant. The Lipp hydrolysis tank pre-heats the substrate which results in the splitting up and pre-acidification of the longchain carbohydrates, fats and proteins. This reduces the retention time in the digester and achieves a higher methane  yield.

Working example

The Tannhausen farm in Germany is a mixed enterprise with both arable and livestock production and the AD system was designed to match the substrates produced by the farm. The main digester tank in a farm system from Lipp can be built to the specific requirements of the client, with a diameter anywhere between 3m and 40m.

The first step was to create a detailed layout of the entire plant based on the client's criteria. This design advocated the single stage digestion process, which allowed for a shorter retention time for the feedstock in the digester. The smooth stainless steel interior of the tank is a major advantage over traditional concrete designs, making agitation much easier as well as avoiding any build up of material on the tank walls.

The ‘Verinox’ material arrives on site as a large roll of sheet metal. This is fed into a series of rolling machines positioned around where the base of the tank will be. The rollers start to form the tank by forming the lower edge to a certain profile, then, as the leading edge completes the first lap, the lower edge is folded together with the top edge of the next layer.

This creates the first 'ring' and as the tank is rotated by the rollers so more rings are added and the tank grows in a spiral. The two edges are continuously folded together with a sealant in such a way that a gas-tight seal is produced as well as a smooth internal surface. The external fold forms the structural strength of the tank, allowing it to be self-supporting.

Once the tank reaches about 2m in height, the top ring is cut level and the roof is installed. At the same time a gas hood can be installed in the digester to capture the biogas before it is piped away to be burnt or stored. As the tank continues to gain height, any pipes or valves that are required can be installed from ground level rather than using access equipment, which would be the case in a conventional tank build.

In this project the design called for wall heating to be installed, which could also be done as the tank was formed. The hose drums were set up next to the tank and as it rotated so the hose was pulled off the drums and could be fixed to the external tank wall, again all done from ground level.

Once the design height has been reached, the bottom edge of the tank is cut level and the rollers are reversed, which lowers the finished tank into position on the foundations, where the ground fixings are installed. This allows the insulation and exterior cladding to be installed along with the pipe work, valves, pumps and agitators.

The whole project was completed on time and on budget, with the completed plant capable of producing 570KWh of electricity, sufficient for 80 households.

Lipp Systems is based Leeds, UK.

Recent Issues