Design of a Suspension Burner System for Forestry and Agricultural Residues (NRI) Discussion of results Feeders Combustion trials Dewatering of coir dust

Design of a Suspension Burner System for Forestry and Agricultural Residues (NRI)

Discussion of results

Feeders

Each of the three feeders tested had their individual attributes. The system chosen for the trials was the turntable feeder, but it was considered that the screw auger and vibratory feeder would have provided an equal and sometimes improved performance with some of the feedstocks. An important consideration in any choice of system is not solely its performance, but also its availability in developing countries. The vibratory feeder was a proprietary unit, manufactured in the United Kingdom. The feeder incorporates an electromagnetic vibrating table, and there may be difficulties in manufacturing this type of system locally in a developing country. Whilst the screw auger was a proprietary unit, this type of equipment is sometimes locally manufactured. The turntable feeder was an NRI design directed towards local manufacture.

Combustion trials

General
Incomplete combustion of groundnut shells and coir dust
Removal of rice husk ash
Coir dust ash fusion

General

The unit had been designed in the interests of robustness, simplicity of construction and operation. The ease with which the unit can be started and operated was demonstrated during the various trials. The use of refractory cement as the furnace lining provided a virtually maintenance-free burner. The large thermal mass of the furnace helped provide a relatively smooth energy output. The automatic relighting capability could be an important feature when considering its industrial application.

Incomplete combustion of groundnut shells and coir dust

The blue-coloured haze and black smoke in the exhaust emissions of groundnut shells and coir dust respectively were considered indicators of incomplete combustion. Numerous trials covering variations in: furnace temperature; excess air; particle size and feed rate had limited effect on the degree of emissions. From observations made during the trials it was concluded that the particles required a longer residence time to combust fully in the furnace. Laboratory analysis (Krishnan, 1990) on the pyrolysis and combustion characteristics of these materials indicated that high levels of carbon dioxide released appear to have contributed to their poor combustion characteristics. In view of the practical difficulties, it was decided not to pursue the combustion of groundnut shells and coir dust with the cyclonic burner, where particle residence time is fixed largely by the system size and design. An alternative combustion system, as detailed in the section on the dual-chamber, brick-built furnace (see below) was considered.

Removal of rice husk ash

With feedstocks of ash contents of up to 5% the furnace chamber is effectively self-cleaning, and the bulk of the ash formed from combustion is carried away in the exhaust gases. Rice husks, however, have an ash content of the order of 19% and an ash particle size similiar to the original rice husk. These factors negated the self-cleaning effect and ash quickly blocked the furnace chamber. Because of these difficulties an alternative combustion system, as detailed below, was considered.

Coir dust ash fusion

The fusion temperature of ash (1077°C) falls within the furnace temperature operating range (1000-1100°C). This produces intractable difficulties with ash fusion and glazing inside the furnace chamber, and is a potential cause of damage to furnace linings. This is likely to be a major determining factor in the success of any combustion system burning coir dust.

Dewatering of coir dust

Although it is possible to dewater coir residue to 56% moisture content, this moisture level is considered too high for efficient combustion. Moreover, the solid cake of coir residue formed from the pressing operation is suited more to burning in solid-fired systems rather than suspension burning.

The most promising source of suitable feedstock is considered to be from the mounds of coir residue waste often found adjacent to coir fibre factories. Subject to weather conditions, the top layer of material is likely to be air dried and at a moisture level more appropriate for combustion.