Biomass plays an increasingly important role in the field of renewable energies. Wood pellets are among the most significant bio fuels substituting oil, gas and coal. The price level of wood pellets was relatively constant in recent years compared to oil and gas which is why this type of fuel has developed into an attractive alternative not only for private households, but also for power plants generating energy and heat. Many coal-fired power plants use co-firing processes to replace some of the fossil fuels with wood pellets for their attractive price and to reduce the emission of climate gases like CO2.
Better machine performance for higher production volumes
The technical requirements of wood pellet presses have changed due to the increasing production volumes of wood pellets and planned new production sites with capacities of up to 1m t/a ("mega-mills"), requiring established machine manufacturers and new OEMs on the market to develop more productive and energy-efficient presses with capacities of up to 25 t/h. The availability of such high-performance machines offering higher production volumes with longer maintenance intervals are of growing importance, as long downtime results in high losses of production, especially in high-capacity plants.
The lubricant, although often underestimated, plays a crucial part in this. The optimum lubricant fitting the demands of the roller bearing will have a positive effect on the production efficiency of wood pellet presses. Klüber Lubrication München conducted comprehensive comparisons of existing products and obtained important results regarding the performance of various lubricants forming the basis for selecting the optimum lubricant for components of wood pellet presses.
Analysis of operating conditions
As various types of wood are processed, pellet press roller bearings are subject to varying loads, i.e. varying pressures, resulting in bearing temperatures of more than 120 °C. Additionally, the atmosphere surrounding the bearing is often corrosive depending on the humidity of the wood. The roller bearing lubricant must therefore ensure sufficient oil release to form a reliable lubricant film in each operating point. The lubricating film separates the bearing elements and protects reliably against wear to ensure high plant availability until the bearings are scheduled for replacement. Metered lubricant application using central lubrication systems should also be possible.
These complex operating parameters shaped the selection of mechanic-dynamic comparative tests with various lubricants performed in the tribological test laboratory of Klüber Lubrication in Munich.
Testing conditions modelled on practice
The lubricant properties analysed included mechanical stability, wear protection, load-carrying capacity, friction behaviour, corrosion protection and water resistance. Additionally, all lubricants were tested in a 500 hours FE8 test run on tapered roller bearings to compare the anti-wear effect of the additives under operating conditions similar to practice. The testers registered frictional moments, temperatures and wear rate of the rolling bearing components. This test method allows the results to transfer to actual roller bearings used in practice, as the test bearings were subject to a very high load of 50 kN at a running temperature of approx. 120 °C.
Considerable differences between the tested lubricants
Lubricants commonly used in pellet press roller bearings were tested. Only four out of the seven greases tested were able to withstand the specified load and to provide favourable results.
The friction and wear results differed considerably after a running cycle of 500 hours. As a result of the different thickeners and additives used, three greases with similar base oil viscosities brought about a temperature increase to more than 200 °C, exceeding the maximum temperature limit which caused the test rig to stop. Three lubricants, however, operated well below the specified temperature of 120 °C, illustrating the low level of internally-generated heat, which indicates their improved lubricity and exceptional wear protection behaviour. Low operating temperatures result in reduced lubricant ageing enabling extended relubrication intervals, reduced relubrication quantities and lower operating costs.
Based on the test results, mineral oil-based lubricants with polyurea thickeners and a base oil viscosity of approx. 500 mm²/s at 40°C have proven to be more efficient roller bearing lubricants in pellet presses than the common lithium or calcium soap greases. Low-wear products also showed good results in the oscillation-friction wear test. These lubricants should therefore fulfil their lubrication and wear protection tasks until the scheduled bearing replacement
The tribology experts from Klüber Lubrication conducted tests to obtain a basis for selecting the right lubricant. The requirements the bearing lubricant should meet depend on the operating conditions in the pellet press that result from the type and design of the plant and the composition of the raw material used and should be discussed with the lubricant manufacturer in each case. Tribology experts should be involved in the planning and development stages of new high-capacity pellet presses at a very early stage.