Unforeseen Dioxin Formation in Waste Incineration

- by  Ingrid Söder­bergh, Sep­tem­ber 18, 2014, Phys.org

Diox­ins forms faster, at low­er tem­per­a­tures and under oth­er con­di­tions than pre­vi­ous­ly thought. This may affect how we in the future con­struct sam­pling equip­ment, flue gas fil­ter­ing sys­tems for waste incin­er­a­tion and how to treat waste incin­er­a­tion fly ash. These are some of the con­clu­sions Eva Wei­de­mann draws in her doc­tor­al the­sis, which she defends at Umeå Uni­ver­si­ty on Fri­day the 26 of September.

Diox­ins is a col­lec­tive name for a spe­cif­ic group of chlo­ri­nat­ed organ­ic mol­e­cules where some exhib­it hor­mone dis­rupt­ing and car­cino­genic prop­er­ties. Diox­ins can form in waste incin­er­a­tion, as the flue gas­es cool down.

“When you incin­er­ate waste, some diox­in for­ma­tion is inevitable, but with the mod­ern flue gas clean­ing sys­tems the emis­sion through the stack is min­i­mized, The diox­ins are fil­tered from the flue gas­es and end up in the fly ash”, says Eva Weidemann.

That diox­ins form is known since the 80’s but in the the­sis work Eva Wei­de­mann shows that these tox­ic sub­stances can form under pre­vi­ous­ly unseen con­di­tions. Amongst oth­er find­ings she describes for­ma­tion of diox­ins with­in the flue gas fil­ters of a full scale waste incin­er­a­tion plant.

“The intend­ed func­tion of the fil­ters is to remove the diox­ins from the flue gas, but I found that they actu­al­ly formed instead. The diox­in emis­sions from the plant still falls below the leg­isla­tive lim­its, but that the for­ma­tion takes place in the first place is bad news. We have iden­ti­fied key para­me­ters for the for­ma­tion and approx­i­mate mechan­ics. My hope is that our find­ings can con­tribute to bet­ter fil­ter design in the future,” says Eva Weidemann.

Anoth­er prob­lem addressed by the the­sis is that diox­ins can form with­in the sam­pling equip­ment used dur­ing high tem­per­a­ture sam­pling and Eva Wei­de­mann has inves­ti­gat­ed how to car­ry out high tem­per­a­ture diox­in sam­pling to avoid this occur­rence. The solu­tion is more effi­cient cool­ing at a crit­i­cal stage, which then pre­vents the for­ma­tion of dioxins.

Eva Wei­de­mann have also looked at how diox­ins in waste incin­er­a­tion fly ash is influ­enced by dif­fer­ent of hot and cold treat­ments to find pos­si­ble meth­ods to detox­i­fy the ash­es. The results are not entire­ly con­clu­sive, but they pro­vide puz­zle pieces that can help.

“If we could find a good detox­i­fi­ca­tion method for the fly ash­es, it would be an envi­ron­men­tal ben­e­fit from a diox­in per­spec­tive but also in oth­er aspects such as recy­cling”, says Eva Weidemann.

Waste incin­er­a­tion is despite the diox­ins a good option to uti­lize the ener­gy in waste that can­not be sort­ed and recy­cled. The waste is reduced in weight and vol­ume, and bac­te­ria and odor dis­ap­pears. In addi­tion, com­bus­tion a more cli­mate friend­ly han­dling method in com­par­i­son to land­fill­ing. The methane gas that forms as the waste decays is a worse green­house gas than car­bon diox­ide formed dur­ing com­bus­tion. The pol­lu­tion prob­lem attrib­uted to the method in the 80’s and 90’s are today near­ly elim­i­nat­ed with the help of advanced fil­ters and purifi­ca­tion sys­tems, as well as peri­od­ic emis­sion controls.