Allready-Box brings security to production of bioenergy

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Courtesy of ALLWEILER GmbH

Animal feed manufacturer Futtertrocknung Mindelheim eG of southern Germany has been operating a co-generation power station since 2008. During the summer months, this power station uses ecological methods to generate the heat and electricity the company needs to dry soilage. Utilizing a year-round, low-temperature heating network, it supplies local industrial firms and public institutions with district hot water for heating and feeds electrical power into the public power grid. In total, the power plant delivers about 10,000 to 14,000 MWh of useful heat yearly.

Centralized thermal oil pumps
The heart of the power plants are thermal oil pumps from Allweiler AG, a pump manufacturer based in Radolfzell, Germany. These pumps feed hot oil at temperatures up to 350 °C into a heat exchanger where the silicon oil is heated to the point of vaporization. The hot silicon-oil vapor drives the Organic Rankine Cycle (ORC) turbine to produce electrical power. The power plant is fired with wood chips provided by local landowners. The wood chips themselves are produced from scrap wood after heavy storms or during forest thinning efforts, making them a 'renewable resource' (NAWARO). The power plant produces not only electricity, but also hot water for distant heating. This system gives the Mindelheim animal feed company the most modern and efficient process available for producing power in a co-generation biomass plant. The power plant gives the cooperative a year-round source of income because it provides electrical power and heat at all times of year regardless of whether animal feed is being dried or not. Until recently, heat and power were produced for only 2000 to 2500 hours per year, but the system now runs around-theclock throughout the year. The turbine produces approximately 1.5 to 1.6 MW; the boiler’s heating capacity is between 20 and 30 MW. The plant requires approximately 50,000 metric tons of wood chips each year.

Critical pump requirements
Having centralized thermal oil pumps that are safe and dependable is critical for maintaining reliable energy production. One aspect of this are the high loads placed on the pumps. Secondly, failure of a pump can have serious consequences, especially if hot oil escapes. This makes the operation of heat-transfer systems very sensitive to leakage. For this reason, engineers walk a fine line when designing shaft seals in heat-transfer pumps. While water normally escapes through the sealing clearance in the form of vapor, escaping heat-transfer oil is always readily visible. That is why the leakage required for the shaft seal to function properly must be minimized as much as possible without overloading the seal. The unusually high loads on the pump are the result of high temperatures and temperature differentials as well as decomposition reactions that may occur when the heat-transfer liquid is subjected to excessive thermal loads. In particular, the chain-like hydrocarbons decompose over time into 'low boilers' and 'high boilers'. If the proportion of low boilers is too high, the pump may cavitate. High boilers appear in many forms, from bitumen-like consistency to extremely hard carbonized products, and accelerate wear on the pump. Both threaten the pump’s bearing and shaft seal. Using [or Use of] synthetic thermal oils, like at the Mindelheim power plant, reduces formation of low and high boilers, but the low viscosity and low lubricity of these materials mean that they have disadvantages as well. Tribological loads on the gliding components of mechanical seals are particularly high.

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