Cement, Energy and Environment July-Sep 2002

POLLUTION CONTROL CONTROLLED COMHl'STION Max H. Vaccaro, Pillard, E.G. C. !. The operatin g princ iple behind low NOx burners is stepwise combu st ion, whic h de lay s the mixing and input of fuel and ai r at app ropr ia te stage s in ord e r to achi eve a contro ll ed combustion process. This resul ts in a fuel-rich fl ame co re and lower peak temperatures. Extensive experience relating to low NOx burners has been made available since the early 1970s from boi le r furnace engineering. The fi rst Pillard ROTA FLAM burner de veloped to significant ly reduce NOx formation was installed in a German cement plant in 1989 and showed signifi cantl y reduced NOx emissions and in itiated the path for further development. In 1996, th e European Community fin anced a re search programme, hand led by the largest Ital ian cement producer, enti ti ed ··Nox emi ss ions from Cement Manufacture and Eva luation of Va ri ous Poss ibiliti es for NOx reduction in the Cement Industry". A series of tests were performed in a cement plant, the aim ofwhich was to demons trate th e potenti a l reducti on in NOx emissions through the optimisati on of th e main kiln burne r design, without affecting c linke r qu a lity and product ion output, but without increasing other pollutant emissions. for thi s purpose, a HOTAFLAM low NOx burner was tested in the Sarche plant. The plant has a 700 tpd Lepol grate cement kiln, and was origina' i.' ·wipped with an indirect-fired mP channel type burner firing 100% of the total combu st ion a ir. requi red (stoi chiometric basis). The ROTAFLAM low NOx burner design was installed to replace the mono chann e l burner and was commissioned in March 1996. First it was demonstrated that by design itself, the ROTAFLAM lowNOx kiln burner clearl y red uces NOx emission when compared to a mono channe l burner whi le keepi ng co and so2 levels to a mi nimum. Secondly, the influence of primary air on NOx generation has also been inves ii gated . showing th at th e reduction in primary air positively infl uences NOxemission. However primary air reduction should be kept within certain limits in order to maintain a good and s table kil n operation, especially in the case of using high sulphur and low volatile content pul veri sed fuel such as petcoke. Lastly, the geometrical feat ure of the burner is of extreme impo rtance in ord e r to avoid plugg ing problems that wo uld eve ntua ll y resul t in red uced performances. With a few mod ific~ti ons · to the coal mill ing circuit, it is possibl e to convert the firing system to semi direct i.e. by adding a cyclone on the primary air stream coming from the coal mill and red irecting this primary air to a ROT A FLAM low NOx type burner. In this case, the primary air to the burner is indeed hi gher than '.vith ind irect fir ing. however the effect offlame stability. and combus tion s tag ing, as demon st ra ted befor e, may effecti vely contribute to a reduction in NOx emi ss ions. Thi s type of semi -direct firing system was commissioned by Pillard in 200 I, on two long wet kilns at the Logansport plant in Indiana, US. Ex perience gained from the Logansport project has led to a new generat ion o f semi-direct firing systems called the DIRECTFLAM system, which aims to reduce NOx by instal ling a ROTAFLAM low NOx ki ln burn er des ign and additionally by reduc ing the primary a ir inside th e ma in fl ame co re (Pillard US patent no: 6293208) In this type of semi-d irect firing system, the primary air go ing into the main flame is limited to the amount necessary to provide the required mom entum for fl ame control. The excess primary air. i.e. that which is not necessary for name control, is evacuated inside the kiln hood before the burner tip by means of an ad dit io na l channe l sur rou nd ing the ROTAFLAM burner. In th is way , the excess primary air does not enter the main flame which can now fire with a lower amount of primary a ir lead ing to reduced NOx emissions. Experience and research in the field ofNOx reduct ion from the kiln burner standpoint is progressi ng. In pat nership with ltalcementi a new campaign oftests will be performed shortl y in Europe with the goal of looki ng at additional design features to furth er reduce NOx from the cu rrent leve ls ob ta in able today wi th a ROTAF LAM low-NOx type-burner while maintaining or •· - " .... ' ..