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Coal char combustion in O2/N2 and O2/CO2 conditions in a drop tube reactor: an optical study

Milena Rodríguez Avila¹, Markus Honkanen¹, Risto Raiko¹, Antti Oksanen¹

1. Tampere University of Technology

Department of Energy and Process Engineering,
Tampere University of Technology,
P.O. Box 589, FIN-33101,
Tampere, Finland

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This article presents how the combustion of coal char was studied optically in a specially designed drop tube reactor at 1123 K under varying oxygen concentrations and residence times. The char particles were produced in a drop tube reactor (at 1123 K) with nitrogen flow from pulverized coal that was sieved to a size fraction of 100–125 µm. The oxygen concentrations were set to 3, 12, and 30 vol-% in N2, and 30 vol-% in CO2. The drop tube reactor was equipped with movable feeding and collecting probes, and the sample particles were quenched in nitrogen flow. A two-color pyrometer was used to measure the temperature, size, and velocity of the particles, and a charge-coupled device camera was used to measure particle size and velocity. The results of the experiments show that an increase in the oxygen concentration causes an increase in the char surface temperature and a decrease in the reaction time. Carbon dioxide in turn reduces the surface temperature of the particles significantly. By replacing N2 with CO2 at the same O2 concentration from the atmosphere inside the reactor, the average particle surface temperature shows a decrease of approximately 300 K. This result is notable for boiler design in the future because it shows that the combustion temperature inside the boiler can be moderated.

drop tube reactor, oxygen, carbon dioxide, particle temperature, particle size, combustion, coal char

Numerical studies of the integration of a Trapped Vortex Combustor into traditional combustion chambers

1. DMA, University of Rome “La Sapienza”

2. Lehrstuhl für Energiesysteme, Technische Universität München

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Combustion technology based on premixing reactants with combustion products has demonstrated that efficiency and emissions may be improved for some industrial ap-plications, notably furnace burners.

Work in progress in the US and EU started with applications to gas turbines (GT); the main advantage in this case is lower emissions, especially NOX, and better temperature uniformity at the combustor exit, possibly leading to better pattern factor. For sta-tionary combustion, for example furnaces, the Trapped Vortex Combustor (TVC) may be considered a very promising technology, again to reduce emissions and especially to ensure temperature is uniform in the exhaust products. This last is a key feature in certain types of heat treatment, for instance in steel rolling mills.

The TVC concept, as conceived for GTs, was introduced in 1995 by Katta and Roquemore [2][3][4] and was based on fluid dynamic studies by Little and Whipkey [5] on drag reduction on bluff bodies. It was configured to mix air, fuel and hot prod-ucts at turbulent scales fine enough that the combustion mode could become in fact “flameless”, or close to “flameless”. Since the flameless strategy requires recirculation of hot combustion products within the combustion chamber, it seems reasonable and feasible to achieve a (mostly) flameless combustion at high flow rate regimes by means of trapping toroidal vortices in suitable cavities. As already known, a vortex ensures a high recirculation factor, Kv, between hot combustion products and reactants, and, ultimately, flame stability. The flameless regime is considered achievable if Kv > 3.5 - 4 (this means up to 75 – 80 % of the gas in the combustion chamber is made of hot combustion products). In a TVC, Kv is about 18-22 (95 % of recirculation means Kv = 20). If this situation is realized, the immediate advantage will be a much lower pressure drop in GT applications, a reduced or a totally suppressed need for flame anchoring, and lowered NOx emissions.

This paper focuses on numerical studies performed on novel GT-derived TVC geome-tries which can successfully deal with part, at least, of the requirements mentioned above. The ultimate goal is to design a TVC that can “easily” replace a traditional pilot flame-based GT combustor as well as industrial burners. The geometries presented here are particular, in that they are designed to fit inside a pre-existing combustor chamber and were arrived at using the same reference volume. The aim, in this case, was to provide a novel class of TVC capable of operating at up to 30 atm, using liquid or gaseous fuels; however, results can be generalised to those burners where exhaust temperature uniformity is important.

This work is part of the work performed with AVIO of Italy for the EU Project “TLC” (“Toward Lean Combustion”), started in March 2005 and where these authors were team members.

Yinghai Wu¹, Lufei Jia¹, Edward J. Anthony¹, Marianna Nobili², Antonio Telesca², Fabio Montagnaro³

1. CanmetENERGY

1 Haanel Drive, Ottawa, Ontario, K1A1M1, Canada

2. Department of Environmental Engineering and Physics, University of Basilicata

Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy

3. Department of Chemistry, University of Naples "Federico II"

Monte Sant’Angelo, 80126 Naples, Italy

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Ashes produced from fluidized bed combustors (FBC) burning high-sulphur fuels often contain 20 – 30 % unreacted CaO because of the limestone added to remove SO2 in situ. This paper presents the results from experiments into reactivating partially sulphated FBC ash (both bed ash and fly ash) with liquid water, steam and sodium carbonate. The water- or steam-hydrated ashes were subsequently re-sulphated in a thermogravimetric analyzer (TGA) with simulated flue gas. The TGA results show that, while liquid water and steam successfully hydrate and reactivate the unreacted CaO in the bed ash, the treated ashes sulphated to widely different extents. Attempts to reactivate fly ash with hydration failed, although fly ash by itself is extremely reactive. A pilot-scale mini-circulating FBC (CFBC) was also used to evaluate the results of reactivation on the bed ash by hydrating with liquid water and admixtures of inorganic salt (Na2CO3) in the form of either powder or solution. When the treated ash was re-injected into the combustor with the fuel, the effect on SO2 removal efficiency was negligible if Na2CO3 was added as powder. Doping with aqueous solution resulted in enhanced SO2 removal; however, the extent was lower than the level achieved if only water hydration was employed. Increasing the amount of water (from 10 % to 30 %) to reactivate the ash did not improve the sulphur capture capacity in the mini-CFBC. Overall, this study suggests that the most practical way for re-use of the partially sulphated bed ash as a sulphur sorbent is reactivation by water. A proposal for utilization of the fly ash in an economically reasonable way is also discussed.

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