Straw pellets for small-scale combustion units
Compared to wood, straw shows strong variations with regard to combustion relevant fuel properties. In particular, the high ash content combined with often unfavorable ash melting properties and high concentrations of problematic elements (e.g. N, S, Cl, K,...) often lead to increased emissions or a breakdown due to slag formation or corrosive attacks on boiler materials.
In the presented work, combustion experiments are conducted in a 15 kW and a 150 kW boiler respectively. Gaseous and particulate emissions are measured and the effect of varying operating conditions in terms of air staging, air ratio and flue gas circulation on the emission values is observed. Moreover, a novel heat exchanger technology - the Schräder Hydrocube® - is investigated focusing on the potential to reduce these emissions.
Another topic in the presented project is the investigation of corrosive attacks on refractory materials in the combustion chamber. In order to study fuel ash - material - interactions, pellets with different types of additives are used in combustion chambers made of varied refractory. Furthermore, corrosive effects are investigated related to the conditions in the combustion chamber, taking into account that atmosphere and temperature vary dependent to the particular position.
The raw material has been harvested and processed under well controlled conditions. Even though, strong variations with regard to fuel quality can be observed, particularly regarding the ash content and some ash forming elements respectively.
The composition of the fuel as well as of the refractory material together with the combustion chamber temperature represents the most important parameters of corrosion of the refractory material. Furthermore it could be found that for relatively short hours of service the parameters of combustion athmosphere and the interaction of slag with the refractory material don't have significant impact on the degree of material corrosion.
NOx-emissions are reduced significantly with increasing flue gas recirculation. Dust emissions showed a clear correlation with the burnout of the flue gas, although CO-emissions in general were very low. Moreover, a decrease of dust emissions with an increasing percentage of secondary air and less excess air could be observed.
Particulate emissions could be reduced by 20 - 25% using the Hydrocube®, and the main separation effect of fine particles was identified to be due to the heat exchanger part of the system. Though reducing most particle fractions, a strong increase of the smallest fraction of 30 nm was measured. Applying the same system with an additional electronic charger, the separation efficiency was increased up to 65%. A significant reduction of HCl and SO2 emissions could be observed in all experiments using the Hydrocube®.
DI Elisabeth Wopienka
Austrian Bioenergy Centre GmbH
- KWB - Kraft und Wärme aus Biomasse
- Intocast GmbH
- AWS - Air Water Systems AG
- Österreichisches Forschungsinstitut für Technik und Gesellschaft
- IVU - TU Wien
- Francisco Josephinum BLT