Description
As part of our Renewable Energy Program – ABC Hansen has developed a simple system for the production of 8mm pellets from wheat straw, hard grasses, , saw dust, bean stalks and other low value, high energy, low density agricultural by products.
The reasoning behind pelleting the biomass is many fold:
1) The volume of the actual biomass is reduced significantly by pelleting – making transport more effective. Typically the density on wheat straw bales are at best in the region of 175kg/m³ density. Compared to pelleted wheat straw – which can have densities of between 500kg/m³ and 1,029kg/m³ depending on the die used and pre-treatments.
2) In the pelleted form – the hay can be easily stored in conventional steel silos – and can be easily handled using conventional chain conveyors, bucket elevators, screw conveyors and belt conveyors
3) The calorific value of for example the hay actually increases by compressing it into pellets. Based on wheat straw – the calorific value goes up from around 15KJ/kg up to over 19KJ/kg. (see study here) – resulting in higher burning temperatures, longer lasting heat and less residual ash content – thus getting the most energy possible out of the hay.
The basic process is as follows:
A) Bales re received onto the bale breaker feed table where the bales are broken down into manageable pieces
B) The bale breaker then discharges the broken and cut grass/hay/biomass to a large diameter auger that transfers the grain to the hammer mill.
C)Optionally steam or water can be applied to the broken grass before milling through the hammer mill – using a steam generator or a water dosing system to increase the moisture content and/or temperature of the raw material before milling and pelleting. Moisture adding is necessary to increase the moisture content of dry straw bales – especially at the end of the season, so that the moisture content is sufficient for proper pelleting – as the moisture acts as a plasticizer in the pelleting process. By heating the raw material – and adding moisture – a much harder, stronger pellet is produced – due to the plasticizing of the moisture, as well as the higher temperature allowing easier release of lignin in the pelleting process. Capacities on the pelleting line are also increased – due to lower friction due to higher moisture content and higher temperature.
D) In the hammer mill – the raw material is cut down to particles sizes of around 5mm in length – using a combination of hammers and blades – fitted to the hammer mill shaft.
E) After milling – the raw material is fed to the pellet mill, where it is pelleted into pellets – usually of approximately 8mm in diameter. Other sizes are available, but we have found that 8mm provides sufficient compression, while keeping power consumption reasonable – at approximately 12kg/KWH
F) Hot pellets directly from the pellet mill are collected to a bucket elevator, which deposits the pellets to a counterflow pellet cooler – where pellets are cooled down using ambient air. Pellet retention time is typically between 20 and 30 minutes – depending on the initial temperature. Pellet cooling allows for rapid cooling of the pellets – consistent airflow through the cooler allows for the evaporation of the surface moisture that is prevalent on fresh pellets.
G) After cooling – pellets are graded and discharged to a bucket elevator, which loads the pellets to a hopper bottom silo – from where they will be further packaged or processed in ovens, fireplaces, boilers, burners or gasifiers.
