OCR Text |
Show In the f i e ld operation of the EPA /MIS , l arge quantities of plast ic mat e r ials were bu r ned pe r iod ic a ll y (l]. These ma t er i als were ram- fed in the rotary k i ln every 1-2 minutes. To re spond to the t rans i ent oxygen demand as a result of bu r ning these mate r i a ls , a unique oxygen feedforward - feedback control logic was designed into the Linde s ys tem. Automat ic water spray was used to modulate ki ln temperature when re qui red . Befo re the imp lementation of this 02 control feature, the MIS had difficulty in bu r ning these p lastic materials smoothly, partly due to its relatively small capacit y . Shown in Figure 2 are examples obtained during the early shakedown pe r iod . Even t hough the normal excess oxygen level was high, occasional feeding practice upsets caused puffs to occur as evidenced by the drop in the 02 conc entra t ion close to O~ and the CO spikes . Although the MIS is designed so that the was t e feed is automatically cut-off whenever the 02 level is below 4~ and/or the CO level is above 100 PPM, the waste materials already in the kiln can continue to release combustible gases for a few minutes, during which time the complete destruction of hazardous materials may not be assured. Extreme caution by operators to limit the waste feed rate and to adjust the air flow rate was used to avoid these upset conditions . This was a significant operational constraint . Immediately after the implementation of the oxygen control feature, the transient upset conditions associated with the release of the combustible gases were virtually eliminated in the operation of the MIS . As shown in Figure 3, the oxygen level of the gas entering the SCC was controlled to be within ±1~ from the setpoint of 9~ 02' while the 02 level at the SCC is maintained at about 6~ (dry). Carbon monoxide spikes were not detected. Note also in Figure 3, the oxygen flow rate responded promptly to the transient oxygen demand . This can be attributed to the fast response (short lag time) of the pure oxygen system and the ThermoX® WOG- III in-situ 02 analyzer. In addition, the high-momentum oxygen jets in the Linde burner also enhanced mixing in the kiln to eliminate any pockets of unburnt combustibles . Of course, no control scheme is a substitute for good operational discipline, including batch size reduction, avoidance of large containers of bulk liquid, etc. However, oxygen feedback flow control was shown to alleviate the common kiln upset conditions effectively . This is an important achievement for the Linde system in the full scale operation of a hazardous waste incinerator . Fuel Savings Supplemental fuel was required to provide the heat required to operate the rotary kiln at lSOO-1600°F and secondary combustion chamber at about 2100°F, because the waste materials did not have a sufficient heating value to sustain self - combustion. Specific fuel savings of over 60~ was achieved during operation of the EPA/MIS with the Linde system. This result can also be expressed as 50 MMBTU saved per ton of oxygen used, as shown in Table 1 . The economics of using oxygen to save fuel, of course, depend on the relat i ve cost of fuel and oxygen . With No.2 fuel oil costing $0.70 per gallon (or $5 . 50 per million BTU) and a fuel savings of about 50 million BTU for every - 5 - |