How to deal with one of the main obstracles for an increased use of CO2 neutral biomass in power plant boilers - Optimal ash deposit removal in straw fired boilers
Sustainable process-product development & green chemistry
Sustainable & Clean Technologies - III: Combustion & Emission (T1-6P)
Keywords: ash, deposits, power, combustion
An increased use of biomass as fuel in power plant boilers leads to a net decrease of CO2, but the high fuel alkali and chlorine content of biomass induce unwanted ash deposit formation on boiler hest transfer surfaces. Slagging and fouling problems that reduce boiler availability are often observed. The amount of ash deposit present on boiler heat transfer surfaces is influenced by deposit formation and shedding processes. While short time deposit formation measurements often have been performed, well controlled deposit removal experiments have rarely been conducted in biomass power plant boilers. In this study, deposit shedding was investigated by advanced probe measurements and a computer model describing deposit formation and shedding from a superheater tube was developed. Deposit shedding by both surface melting and sootblowing were investigated, and recommendations for optimal deposit removal conditions were provided.
A combined air- and water-cooled probe and support equipment were constructed, so that the weight of probe ash deposit and the heat uptake could be measured in situ in a power plant boiler. Equipment for video recording, flue gas temperature measurements and controlled soot blowing were also used. Measurements with the probe were performed in grate power plant boilers. The probe was placed near the superheaters and in the convective pass of a straw fired boiler (The Avedøre power plant) and near the superheaters of a wood and oil fired boiler (The Herning power plant).
The main shedding mechanism (at flue gas temperatures of 900 to 1100ºC) was observed to be melting of the deposit surface and ash droplet detachment. It was observed that the flue gas temperature affects the melt fraction of the deposit and thereby controls the shedding rate. A sequential deposit buildup process was observed on the probe: An initial large increase of deposit mass (deposit formation rate of 300 g/(m2h)) up to 6 hours after the clean probe was inserted into the boiler, was followed by a slow deposit weight increased in the following 5 days, and finally the deposit weight reached a reasonably stable level, with equal rates of deposit formation and shedding by ash droplet detachment.
A computer model describing the deposit formation and shedding process on the probe was developed. The model includes sub-models of mass and heat transfer, as well as calculations of deposit melt fraction and viscosity. The model describes deposit formation processes as condensation, ash particle impaction, and thermophoresis, and shedding by deposit melting and droplet detachment. The model was validated by the probe measurements and was used to calculate the influence of changed local parameters on the development of the probe deposit and heat uptake.
Well-controlled soot blowing measurements in the second boiler pass (flue gas temperature from 650 to 800ºC) of the probe was performed in order to determine the peek impact pressure needed to remove the deposits. The influence of changed probe metal temperature and residence time was investigated. At a probe temperature of 400ºC the deposit could easily be removed by the plant soot blowers. At a probe temperature of 500ºC the removability of the deposit by soot blowing decreased with time. Based on the investigation recommendations for optimal operation of straw fired boilers with respect to deposit minimization could be provided.
*corresponding author
Presented Monday 17, 13:30 to 15:00, in session Sustainable & Clean Technologies - III: Combustion & Emission (T1-6P).
