Skip to main content

SORPTION PHENOMENA OF POLYCYCLIC AROMATIC HYDROCARBONS (PAH) ON COAL FLY ASH.

Objective

THE OBJECTIVE OF THE PROJECT IS TO STUDY THE DESORPTION BEHAVIOUR IN AN ORGANIC LIQUID PHASE OF PAH FROM FLY ASH AND TO STUDY THE ADSORTION BEHAVIOUR OF PAH ON FLY ASH AT TEMPERATURES OF ABOUT 100 DEGREES C.

THE ULTIMATE GOALS ARE:
A/ TO DEVELOP AND EVALUATE SUITABLE TECHNIQUES FOR THE ISOLATION OF PAH, ADSORBED ON FLY ASH, FOR ANALYTICAL PURPOSES.
B/ TO CONSIDER THE FEASIBILITY OF REDUCTION OF PAH-EMISSIONS BY TEMPERATURE-ASSISTED ADSORPTION OF PAH ON FLY ASH IN THE BAG FILTER HOUSE OF COAL-FIRED INSTALLATIONS.
Polycyclic aromatic hydrocarbons (PAH) are a group of several hundreds of organic compounds, consisting of 2 or more benzene rings. They are widespread atmospheric pollutants, are mutagenic and several are highly carcinogenic. This gives reason for concern regarding possible adverse health effects.
PAH are released into the environment by both natural and anthropogenic sources and are formed in all combustion processes.
Coal combustion for energy generation is one such a process by which PAH are emitted. PAH formed in coal fired installations may either be released into the environment in the vapour phase or adsorbed on small particles (fly ash). Fly ash is also formed during coal combustion. In most installations, fly ash is collected in 1 or more filter units. Fly ash is a highly sorptive matrix for PAH. As PAH and fly ash are formed at the same process (coal combustion), and the adsorption of PAH onto fly ash may serve as a kind of internal filter.

In cooperation with Bergbau Forschung GmbH (BBF), Federal Republic of Germany (FRG) 5 extraction techniques have been studied:
ultrasonic extraction;
Soxhlet extraction;
liquid extraction;
vacuum sublimation;
supercritical carbon dioxide extraction.

None of these techniques results in a complete extraction of the adsorbed PAH from fly ash.
The best results are obtained by using Soxhlet extraction with an aromatic solvent (benzene or toluene).
The recoveries of spiked PAH are dependent on the PAH and show a correlation with the PAH molecular weight, larger molecular weights lead to lower recoveries. The recoveries typically vary form 80 to 90% for the 3 and 4 ring PAH to 10% for the heavier PAH like anthanthrene and coronene

The adsorption of gaseous polycyclic aromatic hydrocarbons (PAH) to fly ash was studied in the temperature range between 90 C and 120 C.
The amount of adsorption of PAH on fly ash is dependent on the ash, PAH and temperature. In the temperature range studied higher temperatures lead to less PAH being adsorbed.

The adsorption process of PAH onto fly ash can be described by 4 processes:
reversible adsorption and desorption;
irreversible adsorption;
reaction of adsorbed PAH on the fly ash.

When fly ash is exposed for a prolonged time (in the order of days) to high concentrations of PAH (in the order of mgm{-3}) saturation of the fly ash with PAH can occur, most of the PAH will be reversibly adsorbed.
Using the above mentioned processes to describe the adsorption behaviour of PAH on fly ash under bagfilter conditions, the computer simulations show a substantial decrease in PAH emissions (23 to 100%) can be obtained by increasing the contact time between fly ash and PAH in the flue gas. To a lesser extend, dependent on the ash and the contact time, the emissions can be reduced by a temperature decrease in the bagfilter. The laboratory experiments and the computer model should however be validated by experiments on a real installation.
THE RESEARCH WILL RESULT IN A BETTER UNDERSTANDING OF THE BEHAVIOUR OF PAH ON FLY ASH WITH RESPECT TO AD- AND DESORPTION, AND IN RELIABLE METHODS FOR THE ANALYSIS OF THESE COMPOUNDS.
HOWEVER, IT MAY APPEAR FROM QUALITY ASSURANCE CONTROL EXPERIMENTS THAT A COMPLETE RECOVERY IS NOT POSSIBLE. IN THAT CASE AN ATTEMPT WILL BE MADE TO GIVE METHODS FOR THE ESTIMATION OF THE SYSTEMATIC ERROR.
THE RESULTS OBTAINED WILL BE USED TO CONCLUDE WHETHER A REDUCTION IN BAG FILTER TEMPERATURE WOULD LEAD TO BETTER REMOVAL OF PAH. THE ADSORPTION BEHAVIOUR AT VARIOUS TEMPERATURES IS AN IMPORTANT KEY IN THE JUDGEMENT ABOUT THE FEASIBILITY.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek
Address
97,Schoenmakerstraat
2600 JA Delft
Netherlands

Participants (1)

Bergbau-Forschung GmbH
Germany
Address
Franz-fischer-weg 61
45307 Essen