Large-scale coal-over-coal reburn successful at Italy’s Vado Ligure23 March 2001
Controlling emissions in coal-fired plants by reburn using gas is a well established concept. Following a programme of modifications the Vado Ligure power plant in Italy is now showing that coal can be used as reburn fuel at large scale, with very positive results. G Hesselmann, Mitsui Babcock, Technology Centre, Renfrew, Scotland, UK; S Pasini, ENEL Produzione - Ricerca, Pisa, Italy; G Mainini, Ansaldo Termosud Unita Caldaie, Legnano, Italy
Reburn is a process which offers the prospect of a significant reduction in the emissions of NOx arising from the combustion of pulverised coal in large utility furnaces by means of staging the addition of both the fuel and the combustion air supply. Gas-over-coal reburn was successfully demonstrated at Scottish Power’s Longannet power station. The use of pulverised coal as the reburn fuel represents a significant development of the technology.
What is reburn?
A schematic representation of the reburn process is shown in Figure 1.
The bulk of the thermal input to the furnace, typically 80 per cent, is provided by the main pulverised coal supplied to the primary zone, where it is combusted in a globally oxidising environment. The main coal can be fired through either high intensity swirl burners or low NOx burners (for example Mitsui Babcock’s Mk3 or Mk5 design, or the TEA-C design of Ansaldo Termosud as installed at Vado Ligure). It is generally accepted that minimising the NOx produced in the primary zone gives rise to a lower emission overall, and the use of low NOx burners in conjunction with reburn is therefore preferred.
The remaining thermal input (20 per cent) is then introduced downstream of the primary zone, ideally conveyed in an inert carrier stream containing minimum oxygen, so as to create a region of low stoichiometry. Under the reducing conditions to be found in the reburn zone the fuel is partially decomposed into hydrocarbon radicals (CHi) which react with the NOx previously formed in the primary zone, so destroying it. Mixing of the reburn fuel is an important factor in the performance of the process, and this may be facilitated by the use of flue gas recycle (FGR) to provide the additional momentum required.
Finally, the remaining air is supplied to the burnout zone via overfire air (OFA) ports to complete the combustion process.
The Vado Ligure plant
Vado Ligure power station (Figure 2) is located near Savona in Italy, and comprises four identical units, each of which is rated at 320 MWe. The boilers are designed for coal firing, and operate on a wide range of supplies, from the USA, South Africa, Europe and Colombia. The steam cycle is once-through sub-critical with the main steam conditions being 1050 t/h at 538ºC, 170 bar, and reheat steam being produced at 538ºC, 35 bar.
The boiler was originally operated in forced draught mode, with flue gas recycle extracted from the economiser outlet for the purposes of reheat steam temperature control (FGR to hopper) and gas tempering, introduced just below the furnace arch, to maintain an acceptable furnace exit gas temperature (FEGT).
The original firing equipment consisted of 30 burners arranged in 15 cells of two burners each (seven cells on the front wall, eight on the rear) supplied by five 8.5E10 pulverising mills, with four mills being required to achieve full load on the design coal.
At the commencement of the coal reburn retrofit Vado Ligure power station was owned and operated by the Italian state utility ENEL. It is now owned by the devolved company Interpower, formed from the demerging of ENEL in preparation for privatisation.
Baseline and target performance
Baseline performance with the “as installed” cell burners firing bituminous coal was typically of the order of 1200 mg/Nm3@ 6 per cent O2, with carbon in ash (CIA) levels around 6 per cent. The cell burners were subsequently modified for low NOx operation by supplying all the coal to the lower nozzle in each cell, with the upper nozzle being replaced by a NOx port. After this modification the NOx was reduced to around 800 to 1000 mg/Nm3@6 per cent O2, with CIA increased to 8 per cent.
From the outset it was recognised that the compact arrangement of the furnace configuration at Vado Ligure would constrain the implementation of the coal-over-coal reburn process. In particular it was understood that the achievable NOx reduction would have to be compromised in order to ensure that acceptable CIA levels could be obtained. Based on a preliminary assessment of the Vado Ligure furnace, the target performance for the coal reburn retrofit was specified as being:
• NOx ~ 425 mg/Nm3@6%O2.
• CO < 200 mg/Nm3@6%O2.
• CIA <= 7 per cent
The process design for the application of coal reburn to Vado Ligure was established by means of an extensive programme of pilot scale testing and computational fluid dynamics (CFD) modelling – the work undertaken allowed the specification of the stoichiometries, residence times, disposition of the injectors, injection velocities etc, after due consideration of practical physical constraints (eg the location of buckstays, etc).
From the pilot scale testing, it was found that the key parameters of interest (ie those having the greatest impact on NOx and CIA) were the stoichiometry in the reburn zone, and the residence times in the primary, reburn and burnout zones.
Given a reasonable residence time in the reburn zone, NOx emissions of about 250 mg/Nm3 @ 6% O2 can be achieved. Generally, longer residence time and lower stoichiometry in the reburn zone gives lower final NOx emissions.
Based on the pilot scale test results and practical (ie layout) considerations the elevation of the reburn and OFA injectors and the reburn zone stoichiometry were fixed.
CFD modelling was undertaken (by a number of independent organisations using different codes) to identify the optimum disposition of the reburn and OFA injectors around the furnace perimeter, the injection velocity of each inlet stream, and the quantity of FGR required to ensure adequate mixing and penetration of the reburn coal across the furnace cross-section. Furthermore, the CFD predictions of CIA were used to develop the specification of the main and reburn coal particle fineness. Significantly, it was determined that adequate fineness to achieve acceptable CIA could be obtained from conventional mills and classifiers; there was no requirement for the very fine coal size distribution delivered by a microniser for the reburn coal.
Based upon the pilot scale tests and the CFD modelling activities, the key process design parameters arrived at for Vado Ligure were:
• Reburn fuel fraction – 20 per cent (ie one mill out of five installed).
• Reburn zone stoichiometry – 0.89.
• Reburn zone residence time – 0.47 s.
• Reburn coal injectors on front and rear walls (five per wall).
• OFA injectors on side walls (five per wall).
The retrofit project
At the same time as the coal reburn system was installed (Figure 3), the Vado Ligure plant was extensively upgraded. The work included the installation of a wet FGD scrubber, SCR system, new C & I, improvements to the electrostatic precipitators, the virtual rebuild of the lower furnace, conversion of the unit from forced draught to balanced draught operation, new bottom ash handling equipment, etc. Only those aspects pertinent to the reburn system are discussed here.
• Low NOx burners. A total of 24 swirl stabilised low NOx burners were installed on the front and rear walls to replace the existing cell burners. This allowed a lower baseline NOx emission, of around 650 mg/Nm3@ 6 per cent O2 to be achieved.
• Coal reburn injectors.
New coal reburn injectors, ten in total, were installed in the front and rear walls of the furnace above the burners. The general arrangement of these is presented in Figure 4. Each injector comprises a central circular primary duct for reburn coal and transport medium, surrounded by an outer nozzle of square cross-section for the additional FGR required for mixing.
• OFA injectors.Ten OFA injectors were installed (five in each side wall, the presence of gas tempering inlets in the front and rear walls of the furnace prevented them from being located in the preferred position directly above the reburn injectors).
The design is similar to that used in conventional air staging applications, with each injector comprising a central unswirled core surrounded by a swirled annulus.
• FGR system.The existing FGR system was upgraded by replacing one of the three original fans with a new fan having a higher throughput and delivery pressure so as to ensure adequate margin for the additional duty imposed by reburn.
• Mills. The mills originally supplied to Vado Ligure were specified on the basis of high quality world traded coals (ie high calorific value), and the plant was limited in its capability to operate with lower quality (ie high ash, lower calorific value) coals with regard to maintaining full output and spare mill capacity. A number of higher capacity MPS 75N mills were available to ENEL, and it was decided to make use of these along with rotary classifiers in order to achieve the good fineness and throughput required to achieve acceptable CIA and plant flexibility.
The main elements of the reburn modification are summarised in Figure 5.
NOx emissions, determined during a series of parametric tests, are presented in Figure 6. The data show that under baseline conditions the performance of the low NOx burners was in line with expectations, typically NOx levels of around 620 mg/Nm3@6 per cent O2 were observed.
Reburning was found to reduce the NOx emissions to between 300 and 370 mg/Nm3 @6%O2 (ie a reduction of 40 to 52 per cent) for reburn zone stoichiometries of between 0.8 and 0.9. This performance was significantly better than that initially anticipated.
Figure 7 shows typical operating data. These show that NOx emissions are insensitive to reburn coal heat input (coal flow to mill E, the reburn mill) and to main burner firing pattern (B mill is taken out of service temporarily with no appreciable effect on NOx). Finally it should be noted that deep furnace air staging (ie with reburn out of service but with overfire air still operating) achieves NOx levels that are comparable with coal-over-coal reburning.
Unburned loss data are presented in Figure 8. The introduction of coal reburn leads only to a slight worsening of burnout – typically carbon in ash levels were increased from around 6 per cent CIA to 8 per cent CIA, with poorer burnout exhibited at lower stoichiometry. However, the unburned loss under air staging conditions was considerably worse (CIA levels of up to 15 per cent were not uncommon), typically double that of the baseline performance. The poor burnout performance of the air staging conditions arises from the deep staging of the whole of the primary zone, whereas for reburn the primary zone is operated with an oxidising stoichiometry.
The NOx reductions achieved by coal reburn are therefore obtained with a comparatively benign furnace environment (less risk of fireside corrosion) and only marginally poorer CIA. However, air staging technology can still be effective in situations where fireside corrosion is less of an issue (ie low sulphur, low chlorine coals) and where burnout does not significantly impact on the power plant economics (ie reactive coals giving low CIA in all combustion modes, or no end use for the flyash arising).
CO levels were found to be low for all modes of operation (low NOx burners alone, air staging and coal reburn). The typical levels were in the range 10 to 20 mg/Nm3.
Detailed boiler thermal performance analysis was undertaken using OEM design codes for a number of selected test conditions. The balance of the heat pick-up was only marginally affected – furnace absorption was slightly reduced, but the convective pass was able to compensate for this, and the gas temperatures leaving the boiler were unchanged. Furnace exit gas temperature (FEGT) with coal reburn was similar to baseline; typical results are summarised below and show that for the tests analysed the FEGT was lower with coal reburn (due to the impact of dilution by the FGR), but that the differences were well within the normal variability in FEGT that arises from furnace ash deposition:
Baseline (high excess air) 1254
Baseline (low excess air) 1266
Air staging 1245
Coal reburn 1248
Overall boiler operation was found to be largely unaffected by the introduction of coal reburn.
The plant gross thermal efficiency was found to be slightly worse under air staging and coal reburn conditions as a result of the increased CIA. However, the effects were small, as shown in these typical results:
efficiency (% GCV)
Baseline (high excess air) 89.08
Baseline (low excess air) 89.30
Air staging 88.45
Coal reburn 88.86
In summary the coal-over-coal reburn process has been successfully retrofitted to the compact furnace of Vado Ligure power station. NOx reductions have exceeded all expectations, burnout has been slightly increased but retained within acceptable limits, and boiler operation/thermal efficiency has not been adversely affected. The reburn process operates routinely as required by commercial/environmental drivers.
The following conclusions can be drawn from the work at Vado Ligure:
• NOx emissions of 300 to 370 mg/Nm3@ 6 per cent O2 have been achieved, giving a reduction of about 40 to 50 per cent from a baseline emission of 620 mg/Nm3@6 per cent O2. The performance of coal reburn was found to be considerably better than the original expectation of 425 mg/Nm3.
• Burnout was only marginally worsened, CIA increased from typically 6 per cent to 8 per cent.
• Furnace air staging can achieve similar NOx reductions, but at the expense of considerably increased CIA (typically 15 per cent at Vado Ligure), and greater risk of waterwall corrosion, due to the need to operate the main combustion zone under deeply staged conditions. For reburn the main combustion zone is oxidising. In certain circumstances (ie reactive coals containing low levels of sulphur and chlorine) furnace air staging can be an effective technology.
• Plant thermal performance is not significantly affected by the introduction of coal reburn, FEGT is similar to baseline, and is within any variability arising from furnace ash deposition effects.
• Thermal efficiency is only marginally worsened by coal reburn, as a result of the slight increase in CIA.