 |
|
 |
One of our major efforts on thermal science is devoted to the “reburning” for the reduction of nitrogen oxide (NO) during coal combustion. Reburning is a three-stage, in-furnace combustion technology designed for the reduction of NO by introducing a small amount of reburning fuel above the primary flame where the majority of NO is chemically reduced to nitrogen in this fuel rich environment. Wendt et al. introduced the reburning concept and their experimental results in 1973. Reburning is attractive because it retrofits old boilers; it enjoys a relatively low operation cost than the post combustion technologies, such as selective catalytic reduction (SCR). Pilot-scale and full-scale tests of reburning in the last 31 years, however, have demonstrated a 60% NO reduction floor. Thus, a single reburning technology is not sufficient for many power plants in the US to meet the stringent regulations established by laws.
Although 90% of NO can be effectively reduced in the reburning zone by natural gas, nitrogen-containing reaction products from this second stage of the process are likely the precursors of the NO production in the burnout zone, which ultimately attribute to the observed 60% NO reduction floor. During natural gas reburning, hydrocarbon free radicals, including C × , CH × and CH2 × , chemically reduce NO to HCN, a major reaction products in reburning zone. A significant portion of HCN oxidizes to NO in the burnout zone that limits the overall NO reduction efficiency. During coal reburning, a significant portion of the char nitrogen oxidizes to form NO in the burnout zone. To break these reduction barriers, advanced reburning must involve means for simultaneously minimizing NO, and its reaction intermediates, i.e., HCN and char.
With the financial of the US Department of Energy, we have been investigation reburning with a bench-scale reaction unit in the last 17 years. One of our earlier studies of various reburning fuels (see, e.g., Burch et al. , Energy and Fuel , 5 (2), 231-237, (1991); Burch et al. , Combustion and Flame , 98 (4), 391-401, (1994)) suggests that lignites from Mississippi and North Dakota have reburning efficiencies comparable to that of methane. Moreover, it has been demonstrated that, in contrast to the findings related to the bituminous coals, heterogeneous reactions on the lignite char surface contribute higher NO reduction than the corresponding gas phase NO reactions. More recently, critical reaction mechanisms were elucidated (see, e.g., Chen and Tang, AIChE Journal , 47 (12), 2781-2797, 2001; Chen and Ma, AIChE Journal , 42 (7), 1968-1976, 1996). In these studies, we found that the efficiency of heterogeneous reburning depends on the origin of the char, char preparation history, and the presence of oxidants, CO2and O2 , and the reducing agent, CO. In addition to its large internal surface area, evidences suggest that the effectiveness of lignite char is attributable to its ability to promote two consecutive reactions: 1) the catalytic gasification of char by CO2 and O2 for production of CO, and, 2) the scavenge of surface oxygen complexes, C(O), including those formed after adsorption of NO, by gaseous CO, for the regeneration of reactive sites. More interestingly, lignite ash also catalyzes the decomposition of HCN, the major intermediate during NO conversion and a major contributor of the 60% reduction floor during gas reburning.
Our elucidation of reburning mechanisms suggests the powerful application of a duel-function fuel for reburning. One of its component, natural gas, effectively converts NO to HCN, and the other, a small amount of lignite ash, converts one of its major reaction intermediate, hydrogen cyanide (HCN) to NH3 and N2 . The net impact of these reactions is expected to reduce over 85% of NO in a full, 3-staged reburning process. Plan has been made to demonstrate the mixed-fuel reburning technology on pilot- and full-scale boilers. As the first step, the Energy and Environmental Research Center (EERC) of the University of North Dakota has been invited to demonstrate our results on their pilot-scale boiler in our current DOE project in the near future.
Ashes from lignite-fired power plants are geographically limited to the Northern Great Plains (North and South Dakota and Montana), and Southern US (Texas, Louisiana, and Mississippi). Transportation of lignite ashes to boilers in other parts of the US posses a cost constraint. To render the mixed-fuel reburning technology viable to areas where lignite reserve is scarce, we recently designed a second type of mixed fuel, and its efficiency has been verified in our laboratory. It is expected to significantly overcome the 60% NO reduction floor observed in the past three decades. Most importantly, the new invention enjoys many other remarkable cost and market advantages. The University is applying for a provisional patent for this invention.
The current three-year NSF project seeks better understandings of the char deactivation process in flame by investigating the number distributions and desorption strengths of the different surface oxides formed during oxidation of these chars with the representative oxidants, O2 , CO2 , NO and H2O. Desorption of surface oxides and regeneration of reactive sites have long been considered the rate controlling steps of char oxidation. Our previous studies suggested that young lignite chars effectively reduce NO in fuel rich environments, but their reactivities decrease rapidly and substantially after the volatiles are driven out of lignite particles; oxides on these young lignite chars seem to play a dominant role in this and a number of other low NO production processes during combustion. Fundamental understandings of the number distributions and strengths of the surface oxides on young chars are, therefore, important to the development of advanced NO control and combustion technologies, and robust computer codes for the simulation and control of various combustion processes. Nevertheless, research in the correlations of reactivity with the characteristics of chars has traditionally been centered around the old chars, or the chars pyrolyzed with a long residence time, typically 1 to 3 hours. It appears, therefore, there is an urgent need to enhance our knowledge of chars in the flame region, i.e., the chars produced from pyrolysis and combustion with a residence time in an order of seconds. Moreover, we plan to measure the rates of CO scavenging of these oxides.
Initial results suggest that young char produces more abundant surface oxides than those from old char from both transient kinetic (TK) and temperature programmed desorption (TPD) over a wide range of temperatures. Desorption of oxides from the young char is much higher than that from the old char, implying the rapid oxidation of young char may attributed to not only its large amount of surface oxides but also the extremely high oxygen turn over rate on the young char surface. Desorption of large amount of stable surface oxides between 1100 and 1700 ° C during TPD was not reported before, which suggests that the existence of oxides on the basal planes during char oxidation. Desorption of these oxides on basal planes, not those oxides on the edge carbons that desorbed between 700 and 1000 ° C, is likely the rate-controlling step during char combustion in the coal combustion flame.
Abstracts of Selected Papers are available on-line.
|
|
|
10002