Novel crusher to enable automatic RUF briquette-to-boiler feed

Wood briquettes have many strengths: they can be produced easily, flexibly and inexpensively with modern machinery precisely where wood residues occur. However, a disadvantage is that small-scale biomass boilers typically used in residential and light commercial space heating applications cannot be feed continuously and automatically with briquettes. This has a limiting effect on the potential local market for the briquette producer.

Briquetting machine specialists RUF, Zaisertshofen along with DBFZ in Leipzig, started a joint project to resolve this issue. According to Andreas Jessberger, Sales Manager of RUF GmbH, the advantages of the briquettes – namely cheap, flexible production – need to be combined with the advantage of pellets – good conveying characteristics.

The task of the innovative project was two-fold. Firstly, to find out how continuous briquette combustion can be achieved and whether there were already marketable solutions for this. Secondly to look at the emissions occurring during combustion and optimise if necessary.

Slow speed twin shaft crusher

Through extensive product and patent research, DBFZ discovered that there is currently no automatic, continuous feeding of wood briquettes for a small-scale furnace. There is a patent, which describes the irregular, requirement-oriented feeding of wood-fuelled boilers with paraffined wood briquettes thus reason enough to start the project.

First of all we must crush our RUF briquettes to implement our idea so that they can be conveyed evenly and automatically by means of a screw, said Jessberger.

A prototype briquette crusher developed by RUF engineers accomplishes this task. According to the manufacturer, the briquettes used for feeding the machine have a fuel value 5.0 kWh/kg, the cross-section is rectangular (15.3 cm × 6.3 cm) and the height is 9 cm.

The crusher consists of two electrically driven counter-rotating slow speed drive shafts, which are located inside the machine. Toothed rings are located on the opposite side of the drive shafts and have been welded in a staggered form. These have a dual function to grip and crush the briquettes that have fed into the crusher by a conveyor. A small Siemens SPC is used to monitor the drive unit

The crushed fuel falls into a storage vessel which is monitored by filling level sensors. From the storage unit, the fuel is transferred into the small-scale boiler via a screw auger. A “C0” model Ökotherm boiler from the company A.P. Bioenergietechnik GmbH with a nominal output of 49 kW was used as the combustion unit in the showcase RUF-DBFZ project.

Carbon monoxide (CO) emissions in compliance

Classifying the emissions that occur during the combustion of the crushed wood briquettes was the second task of the project. This research was led by Dr Ingo Hartmann, a scientist at DBFZ. A test bench to measure emissions was constructed for this at the DBFZ’s site in Leipzig.

The necessary equipment with a briquetting press and a briquette crusher operated continuously without any disruptions so that it was possible to compile representative results.

Our measurements immediately showed average carbon monoxide (CO) emissions of 0.355 g/m³. This means we are within the statutory framework of 0.4 g/m³ without the need for any further adjustments, said Dr Hartmann.

Also with regard to the dust emissions, the test structure only just missed the statutory dust limit of 0.02 g/m³ with its average value of 0.026 mg/m³.

Similarly to the use of wood chips, this problem can be kept under control with corresponding reduction measures, Dr Hartmann remarked

Whether and in which form this is even necessary, is to be shown by additional tests. There are already indications now that the fine particle fraction from the crunching of the briquettes is partly responsible for the slightly higher dust concentrations. If this is confirmed, separation units on this machine could provide a particularly cost-efficient remedy.