Limiting NOx and SO2 emissions from an industrial-size fluidised-bed combustor

For the reported series of tests, the largest reductions in NOx emissions were achieved (together with maximum combustion efficiencies) by using a primary-to-secondary-air ratio of for the fluidised bed (FB). Under such an operating condition, high freeboard-temperatures could be attained, e.g. up to 200° C above that of the bed, so promoting the combustion of unburnt carbon-fines in the freeboard zone, i.e. secondary combustion ensued. This tended to counteract the effect of the reduced rate of carbon combustion in the bed, i.e. as a result of the increased rate of oxidation occurring in the freeboard. Reductions in both the NOx and SO2 emissions could be achieved by using such a two-stage combustion process, without affecting adversely the overall combustion efficiency. If a primary-to-secondary-air ratio of (rather than ) was employed, the rate of SO2 emissions fell, but the temperature uplift in the freeboard zone was less pronounced and a reduction in combustion efficiency of 2% was observed. The rates of emissions of NOx and SO2 could also be reduced by the adoption of wise choices for the design and operation procedure of the fluidised-bed combustor (FBC). For example, a 45% decrease in the rate of NOx emissions was achieved in one test merely by halving the bed's depth. The lower the available oxygen-concentration (including that provided by recycled flue-gases) within the bed, the smaller was this decrease. On the other hand, a doubling of the bed's depth led to a reduction in the SO2 emission of approximately 4%, provided that the in-bed oxygen concentration was maintained sufficiently high, so as to promote sulphation of the ash and any limestone present in the bed. The location of the secondary-air nozzle (i.e. in the freeboard zone) also had significant influences on the emission rates of the NOx and SO2. A 13% reduction in the rate of NOx emissions was achieved by increasing the height (above the sparge pipes) of the secondary-air's nozzle from 1·5 to 2·6 m. The larger this height, the greater the opportunity for char-NOx-reducing reactions (for which high-temperature conditions are preferred within the freeboard) to ensue, prior to the secondary-air injection. However, the rates of SO2 emission fell by ~ 10% when the height of the secondary-air's nozzle was reduced from 2·6 to 1·5 m above the sparge pipes. This was due presumably to the increased residence times of the ash and limestone sorbents (for facilitating sulphation) in the freeboard, under the oxidising condition (following secondary-air injection), which favours the complete sulphation of the sulphidated ash/lime to CaSO4. In the presently reported series of tests, the rate of NOx emissions could be reduced by up to 83% merely by adjusting the bed's depth and the secondary-air's nozzle-height, although these alterations led simultaneously to a 14% increase in the rate of SO2 emissions, and vice versa. Thus, the emissions of NOx and SOx could be controlled by the use of an appropriate design of FBC. The choice made, in practice, usually depends on which of these emission rates is critical with respect to complying with the environmental-pollution directives. Great care must be taken when comparing emission rates from the FBs, either of different sizes or incorporating alternative methods of air distribution, in order to take account of scaling effects. It is unwise, for instance, to use observations from a laboratory-size FBC to predict the quantitative behaviour of an industrial-size (i.e. large) FBC unit. This arises because of the variations in the oxygen concentration, resulting from the usually non-homogeneous, relatively less effective fluidisation achieved in the large bed; the rates of NOx emissions from the large FBC tend to be smaller (as a result of the existence of pockets of relatively low concentration oxygen in the bed) and simultaneously the rates of SO2 emission are invariably higher. In the presently reported tests, under otherwise nominally similar conditions, an approximate halving of the rate of NOx emissions resulted when using the larger FBC (rather than the small one) together with a doubling of the SO2-emission rates, even though the average percentages of oxygen present in the flue gases remained identical for both the small and large FBC units. The recommended bed-depth depends upon what emission one seeks to reduce. However, where feasible, for an industrial-size combustor, it is wise to employ a shallow bed (~ 340 mm depth, when static) in order to incur relatively low operating costs.

How to reduce pollutant emissions from small fluidised-bed combustors

As a result of 27 series of tests, it was concluded that the maximum reduction of NOx emissions occurred when the sulphur retention was also at its highest, so emphasising the important role that CaSO4 plays as a catalyst in pollution-reducing reactions. Although the minimal emissions of both SO2 and NOx (at 85 and 45 ppm, respectively) presently recorded occurred at a bed temperature of 800°C, under two-stage combustion and a low oxygen-in-the-flue concentration (=2%), the combustion efficiency under these conditions was relatively low (at 72·9% without limestone and 84·5% with limestone added to the fluidised bed). Optimal conditions for achieving maximum combustion efficiency and minimum pollutant emission occurred at the highest bed-temperature (=1000°C) employed, under two-stage combustion conditions for a moderately high (~4%) oxygen-in-the-flue concentration. Under these conditions, the CaS, formed from the lime in the substoichiometric bed, was completely oxidised to CaSO4 by the moderately high O2 concentrations in the freeboard, so optimising the reductions of both the NOx and SO2 emissions. The addition of limestone was found to increase the combustion efficiency by just under 3%, to a maximum of 91·3%, under these conditions. Further, the presence of limestone (which gave an added Ca:S mole ratio of 2), resulted in reductions in the NOx emissions of 83% (i.e. from 283 to 47 ppm) and in the SO2 emission of 74% (i.e. from 455 to 117 ppm). Both the NOx and SO2 emissions were greatly reduced by this addition of limestone, under most operating conditions. The magnitude of the reduction varied according to the bed's temperature, e.g. at a bed temperature of 800°C, under two-stage conditions, the NOx emissions were reduced by 71% and teh SO2 emission by 76%, provided sufficient limestone was added to the bed to give a 2:1 Ca:S ratio. Similarly, the use of recycled gas, to achieve bed attemperation during these tests, led not only to a reduction in the NOx emissions of 33%, compared with only 15% achieved in previous experiments, but also to a 26% reduction in the SO2 emission. The latter was a direct result of the increased residence time for SO2 gas in the contact with the limestone/ash particles within the combustor. When burning S.A. Duff with: (i) the exhaust-gas recycled back to the bed; (ii) limestone added to the bed (to give a Ca:S mole ratio of 2); and (iii) the fluidised bed operated at a relatively high bed-temperature (~1000°C) under two-stage combustion conditions with a 4% concentration of O2-in-the-flue; then a 90% overall reduction in NOx emissions (compared with those occurring under oxidising conditions with no limestone added) and a sulphur retention of 74% were achieved. Larger sulphur retentions ensued by reducing the bed temperature to 800°C and using lower oxygen-in-the-flue concentrations (~2%), but this occurred to the detriment of the combustion efficiency. Nevertheless, the lower bed-temperature of ~800°C was needed to avoid the formation of clinker when burning a low-ash fusion coal, such as Maryport smalls. By contrast, the use of a high bed-temperature (~1000°C) with low values of the oxygen-in-the-flue concentration, resulted in no sulphur retention; all the CaS being partially oxidised to SO2 under these operating conditions, with or without limestone present.

Efficient skipping of single exon duplications in DMD Patient-Derived cell lines using an antisense oligonucleotide approach

Background:Exon skipping strategies in Duchenne muscular dystrophy (DMD) have largely been directed toward altering splicing of exons flanking out-of-frame deletions, with the goal of restoring an open mRNA reading frame that leads to production of an internally deleted but partially functional dystrophin protein. Objective:We sought to apply exon skipping to duplication mutations, assuming that the inherently limited efficiency of antisense oligonucleotide-induced exon skipping would more frequently skip a single copy of a duplicated exon, rather than both and result in significant amounts of wild-type DMD mRNA. Methods:We tested this hypothesis in fibroblast cell lines derived from patients with a variety of single or multiple exon duplications that have been modified to allow transdifferentiation into a myogenic lineage. Results:Using a variety of 2’O-methyl antisense oligonucleotides, significant skipping was induced for each duplication leading to a wild-type transcript as a major mRNA product. Conclusions:This study provides another proof of concept for the feasibility of therapeutic skipping in patients carrying exon duplications in order to express wild-type, full-length mRNA, although careful evaluation of the skipping efficiency should be performed as some exons are easier to skip than others. Such a personalized strategy is expected to be highly beneficial for this subset of DMD patients, compared to inducing expression of an internally-deleted dystrophin