Some recently developed combustion systems are specifically designed for the use of opportunity fuels as residues from bio-refineries. A case study is here reported.
Production of ethanol from wheat grains is an energy demanding process which includes the milling and grinding to produce the wheat meal. The milling waste product consists of bran and small amounts of husks. Bran can be used for animal feed, but also for generating energy. An example of such combustion system is present in Wanze (BE). A vertical type boiler is fired primarily with bran (heating value 13.5 – 17.0 MJ/kg; ash content up to 7%w dry) and with natural gas, having a capacity of burning 20 tonnes bran/hour, for a total of 75 MWth, 25 MWel (100 ton/h steam at 92 bar, 520°C).
Detail of the boiler system by manufacturer Babcock & Wilcox Vølund A/S, according to their plant facts sheet, as publicly available here .
The bran is delivered to the boiler-dosing silo and fed into the boiler with screws. The combustion takes place on a water-cooled vibrating grate (approximately 430°C), suitable of handling biomass with high moisture content and of low calorific value. The grate consists of panel walls mounted on leaf springs. These panels are activated in pairs, in counter phase, by a vibrating unit. Primary combustion air is injected through holes drilled in the fins of the grate panel.
In order to avoid high temperature fouling, the boiler is designed with two empty boiler passes (about 520°C) to ensure sufficient cooling of the fuel gas before entering the convection part. In that way, ash will deposit in solid form. The empty boiler passes have been equipped with water sootblowers. Mineral additives are also used in the combustion chamber to minimize low temperature fouling. The bottom ash is removed with a submerged chain conveyor and carried to an open ash pit.
The steam from the boiler is expanded through a steam turbine to provide low pressure steam for the heating processes in the ethanol factory.
Do you know any example of modern combustion systems to process opportunity/residual solid biomass fuels?
 Plant Fact Sheet, Bran-fired steam boiler BioWanze, Wanze, Belgium, by Babcock & Wilcox Vølund A/S, 2010, here.