Scholarly article on topic 'Ignition and NO Emissions of Coal and Biomass Blends under Different Oxy-fuel Atmospheres'

Ignition and NO Emissions of Coal and Biomass Blends under Different Oxy-fuel Atmospheres Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — J. Riaza, L. Álvarez, M.V. Gil, C. Pevida, J.J. Pis, et al.

Abstract The effect of co-firing coal and biomass on the ignition behaviour and NO emissions was evaluated under both air and O2/CO2 (21-35% O2) atmospheres. The results showed a worsening of the ignition properties in the 21%O2/79%CO2 atmosphere in comparison with air. Furthermore, in order to obtain similar or better ignition properties, the oxygen concentration in the O2/CO2 mixture must be 30% or higher. A decrease of the ignition temperature was observed with the addition of biomass in air and oxy-fuel conditions. The results also indicate that NO emissions in the 21%O2/79%CO2 atmosphere were lower than under air-firing conditions, although they increased in the 30%O2/70%CO2 and 35%O2/65%CO2 atmospheres. The addition of biomass resulted in lower NO emissions in all cases.

Academic research paper on topic "Ignition and NO Emissions of Coal and Biomass Blends under Different Oxy-fuel Atmospheres"

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Energy Procedía 37 (2013) 1405 - 1412

GHGT-11

Ignition and NO emissions of coal and biomass blends under different oxy-fuel atmospheres

J. Riaza, L. Álvarez, M.V. Gil, C. Pevida, J.J. Pis, F. Rubiera *

Instituto Nacional del Carbón, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain

Abstract

The effect of co-firing coal and biomass on the ignition behaviour and NO emissions was evaluated under both air and O2/CO2 (21-35% O2) atmospheres. The results showed a worsening of the ignition properties in the 21%O2/79%CO2 atmosphere in comparison with air. Furthermore, in order to obtain similar or better ignition properties, the oxygen concentration in the O2/CO2 mixture must be 30% or higher. A decrease of the ignition temperature was observed with the addition of biomass in air and oxy-fuel conditions. The results also indicate that NO emissions in the 21%O2/79%CO2 atmosphere were lower than under air-firing conditions, although they increased in the 30%O2/70%CO2 and 35%O2/65%CO2 atmospheres. The addition of biomass resulted in lower NO emissions in all cases.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: oxy-combustion, coal, biomass, ignition behaviour, NO emissions, entrained flow reactor.

1. Introduction

The use of coal in power plants generates a large amount of CO2 which is the chief contributor of global climate change. A diverse power portfolio including Carbon Capture and Storage (CCS) technologies and renewable energies is needed to reduce atmospheric CO2 to below 1990 levels [1]. During oxy-coal combustion, coal is burnt in a mixture of oxygen and recycled flue gas (mainly CO2 and H2O), to yield a rich CO2 stream, which after purification and compression is ready for sequestration [2]. Biomass is a renewable fuel which can be used to reduce CO2 emissions, as it is considered carbon neutral since biomass consumes CO2 as it grows. The combination of oxy-coal combustion with biomass co-firing can afford a method of disposal of CO2. Furthermore, it can also be a method for increasing CO2 capture efficiency that could reduce cost and efficiency penalties [3].

* Corresponding author. Tel.: +34 985 118 975; fax: +34 985 297 662 E-mail address: frubiera@incar.csic.es (F. Rubiera)

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.06.016

The ignition of solid fuel particles is an important preliminary step in the overall combustion process due to its influence on flame stability and pollutant formation. The ignition temperature is not an inherent property of the fuels, as it is dependent on the operating conditions (temperature, heating rate, surrounding atmosphere, etc), and blending fuels of different nature may also affect the ignition properties [4]. In oxy-fuel combustion of pulverised coal, poor ignition quality has been often noted during pilot-scale trials when operating with substantial gas recirculation [5]. During oxy-fuel combustion a significant reduction of NO can be achieved in relation to air-firing, partly due to the suppression of thermal NO formation and, also, because recycled NO is reduced to N2 after being introduced into the flame zone [4]. Co-firing coal and biomass in air-firing conditions was found to enhance ignition properties and NO emissions reduction [6]. The objective of this work has been to evaluate the effect of biomass blending with coals of different rank on their ignition behavior and NO formation under oxy-firing conditions.

2. Experimental

2.1. Materials

Two coals of different rank were used in this work: a semianthracite from Spain (HVN) and a South African high-volatile bituminous coal (SAB). A biomass, olive residue (OW), was also employed. This biomass is the wet solid residue that remains after the process of pressing and extraction of the olive oil. The coal and biomass were ground and sieved to obtain a particle size fraction of 75-150 pm. The proximate and ultimate analyses together with the high heating values of the samples are given in Table 1.

Table 1. Proximate and ultimate analyses and high heating values of the fuel samples

Proximate analysis (wt.%, db) Ultimate analysis (wt.%, daf) High heating value

Ash V.M. F.C. C H N S O (MJ kg-1, db)

HVN 10.7 9.2 80.1 91.7 3.5 1.9 1.6 1.3 31.8

SAB 15.0 29.9 55.1 81.5 5.0 2.1 0.9 10.5 27.8

OW 7.6 71.9 20.5 54.3 6.6 1.9 0.2 37.0 19.9

2.2. Experimental device

An entrained flow reactor (EFR), which has been described in detail elsewhere [7], was employed to carry out the ignition and combustion experiments. Briefly, the reactor has a reaction zone 140 cm length and an internal diameter of 40 mm; the reactor is electrically heated and is capable of reaching a maximum temperature of 1100 °C. The gases are preheated to the oven temperature before being introduced into the reactor through flow straighteners. Fuel particles were introduced through a cooled injector before entering the EFR reaction zone. Reaction products were quenched by aspiration into a stream of nitrogen using a water-cooled probe. This probe was inserted into the reaction chamber from below. Particles were removed by means of a cyclone and a filter. The concentration of the exhaust gases was measured using a battery of analysers (O2, CO, CO2, SO2 and NO).

Two types of experiments were carried out: ignition and combustion tests. For both types of experiments, air (21%O2/79%N2) was taken as a reference and three binary mixtures of O2/CO2 were compared (21%O2/79%CO2, 30%O2/70%CO2 and 35%O2/65%CO2). During the ignition experiments, the reactor was heated at 15 °C min-1 from 400 to 800 °C, the oxygen excess was set at a value of 25%. The criterion for determining the ignition temperature was based on the first derivative curves of the gases composition. The ignition temperature was taken as the temperature where the first derivative curve, normalised with respect to the maximum derivative value, reached a value of 10 % [7, 8]. The combustion experiments were carried out at a reaction temperature of 1000 °C employing a particle residence time of 2.5 s and different oxygen excess percentages.

3. Ignition results

3.1. Effect of the oxy-fuel atmosphere

Ignition tests were conducted in both air and oxy-fuel environments. As an example, in Figure 1 it can be seen the variation in the concentration of different gases (CO, CO2, NO and O2) during the ignition tests of coal SAB in air and oxy-firing conditions.

21%O2/79%CO2

35%O2/65%CO2

-Ù—CO2

-©-CC

-B— NO " y

300C 250C

1500 r

1000 :

400 420 440 460 480 500 520 540 Temperature (°C

Fig. 1. Derivative curves of the CO2 concentration during SAB ignition

Temperature (°C

Temperature (°C)

Faundez et al. [8] stated that ignition is characterised by a rapid decrease in CO production, a significant O2 consumption, and an increase in the production of CO2 and NO. From Figure 1 it can be observed that, for coal SAB, this happened around 530-550 °C in the case of ignition in air, and in the oxy-fuel atmospheres ranged between 500-570 °C depending on the oxygen content. The exact values were derived from their CO2 and NO derivative curves. As an example in Figure 2, the CO2 derivative curves of coals HVN and SAB, under air and oxy-firing conditions, are shown.

Fig. 2. Derivative curves of the CO2 concentration during HVN and SAB ignition

Table 2 shows the values of the ignition temperatures for HVN and SAB obtained from their corresponding CO2 and NO derivative curves.

Table 2. Ignition temperatures (in °C) for coals HVN, SAB and blends HVN-OW and SAB-OW under air and oxy-fuel conditions

Blends 21%O2/79%N2 21%O2/79%CO2 30%O2/70%CO2 35%O2/65%CO2

HVN 700 723 669 642

90HVN-10OW 636 662 615 567

80HVN-20OW 574 612 551 503

SAB 543 565 524 498

90SAB-10OW 510 532 491 455

80SAB-20OW 461 478 444 425

625 650

675 700

- 21%O2/79%N2

- 21%O2/79%CO2

i0%02/70%002

- 35%02/65%C02

725 750 775

525 505 565

As can be observed in Table 2 higher ignition temperatures were required when N2 (21%O2/79%N2) was replaced by CO2 (21%O2/79%CO2), i.e., an ignition delay took place. Also, it can be observed that for oxygen concentrations up to 30%, the ignition temperature was lower than that obtained in air. Other authors [9-14] have also performed ignition experiments for different types of fuel in air and oxy-firing conditions using a wide range of furnaces; their principal findings are summarised in Table 3.

In all cases an ignition delay took place when replacing N2 by CO2 for the same oxygen concentration. Khatami et al. [9, 10] have attributed the longer ignition delay partly to the effect of the volumetric heat capacity of the gas mixtures. The temperature rise during ignition is inversely proportional to the heat capacity of the surrounding gases, and since CO2 capacity is higher than N2, a reduction in gas temperature will be obtained. However, the heat capacity and the temperature of the surrounding gases are not the only factors that affect ignition; the oxygen concentration, the heating and devolatilisation rates also have a great influence [4]. Shaddix et al. [13] observed that the ignition of the volatiles may be affected since CO2 decreases the rate of devolatilisation due to the lower mass diffusivity of the volatiles in the CO2 mixture. Other authors [9-11] have observed that when increasing the oxygen concentration, the ignition delay in O2/CO2 atmospheres, in comparison with air, became smaller due to the increase of the oxygen mass flow to the surface of the particle. Similar ignition properties as in air-firing conditions could be achieved in oxy-fuel environments with oxygen concentrations in the range of 27-30%.

J. Riaza et al. / Energy Procedia 37 (2013) 1405 -Table 3. Ignition of coal and biomass blends in air and different oxy-firing conditions

Authors

Furnace used

Experimental findings

Khatami [9, 10] Drop tube furnace

Similar ignition temperatures in air and in 27%O2/73%CO2 (lignite and bituminous coals)

Ignition delay in O2/CO2 atmospheres in comparison with O2/N2 atmospheres for different oxygen concentrations (20-100% O2) for lignite, bituminous and sub-bituminous coals

Arias [11]

Entrained flow reactor

Similar ignition temperatures in air and in 30%O2/70%CO2 for a high-volatile bituminous coal and a semianthracite, and for blends of the high-volatile bituminous with biomass

Qiao [12]

Wire mesh

Higher ignition temperatures when replacing N2 by CO2 for 21% oxygen content. Decrease on ignition temperatures in O2/CO2 atmospheres with higher oxygen content. Similar ignition temperatures in 60%O2/40%CO2 as in 100% O2. Two coals were employed, a brown coal and a bituminous coal

Ignition delay in O2/CO2 atmospheres in comparison with O2/N2 atmospheres for different oxygen concentrations (11-36% O2) for a bituminous coal

Ignition delay when replacing N2 by CO2 for 20% oxygen content at different furnace temperatures. Two bituminous coals were employed

Similar ignition temperatures in air and in 30%O2/70%CO2 for a high-volatile bituminous coal and a semianthracite, and for the blends of the two coals with biomass

Shaddix [13] Entrained flow

reactor

Liu [14]

Entrained flow reactor

This work

Entrained flow reactor

3.2. Effect oof the addition of biomass

The effect of blending coal and biomass on the ignition behaviour was studied under air and oxy-firing conditions. Blends of the two coals considered, the semianthracite HVN and the high-volatile bituminous coal SAB, with 10 and 20 wt% of the olive waste OW were used. The ignition temperatures are shown in

It can be observed that the addition of the olive residue, OW, caused a significant reduction in the ignition temperature of both coals in all the atmospheres studied. This decrease was proportional to the amount of biomass in the blends and it was more pronounced for the blends HVN-OW. These results suggest that the effect of addition of biomass is more significant in high rank coals. When two fuels are fired as a blend, the ignition properties may be different to those exhibited when they are ignited individually [11]. Faundez et al. [8] have observed that, when blending fuels with different volatile content, the ignition of the higher volatile component of the blend enhances the ignition of the lower component. However, when both fuels have similar volatile content, they compete for the oxygen available. As a result, the enhancement of the ignition properties would be lower when blending low rank coal and biomass, than when blending high rank coal and biomass.

4. NO emissions

Coals HVN and SAB and their blends with biomass were burnt at different levels of excess oxygen for each atmosphere studied. The fuel equivalence ratio, defined as the ratio between the fuel mass flow rate and the stoichiometric value, was used to determine the excess oxygen during combustion. As an example, the NO concentrations (in ppm) of blends SAB-OW under the different atmospheres studied are shown in Fig 3. A decrease in NO concentration was observed as the fuel equivalence ratio increased,

Table 2.

since the lower amount of oxygen available would reduce fuel-N conversion to NO. Also, under fuel-rich condition, the presence of large amounts of hydrocarbons from coal pyrolysis, unburnt char and CO favoured the reduction of NO to HCN, NH3 and molecular nitrogen N2 [14].

■-X SAB

♦ 21%O2/79%N2 "0

□ 21% O2/79%CO2

O 30% 02/70%C02

X35%02/65%C02

90SAB-100W

♦....... ' "Y.

♦ 21%02/79%N2 *"■'-... 0

□ 21% 02/79%C02

O 30% 02/70%C02

*35%02/65%C02

♦ 21%02/79%N2 80SAB-200W

□ 21% 02/79%C02

O 30% 02/70%C02

*35%02/65%C02

......Q... ' *

0,4 0,6 0,8

Fuel equivalence ratio

0,6 0,8 1,C

Fuel equivalence ratio

0,6 0,8 1,0 Fuel equivalence ratio

Fig 3. NO concentrations (in ppm) of blends SAB-OW at different fuel equivalence ratio in air and oxy-fuel conditions

In order to facilitate a comparison of the behaviour of these coals and their blends with biomass, under air and oxy-firing conditions, the NO values were interpolated at a fuel equivalence ratio of 0.8 (which corresponds to a 25% oxygen excess) using the curves shown in Fig. 3. These NO emissions (in ppm) for coals HVN and SAB and their blends with olive residue, OW, are shown in Table 4.

Table 4. NO emissions (in ppm) for blends HVN-OW and SAB-OW under air and oxy-fuel conditions

Blends 21%O2/79%N2 21%O2/79%CO2 3O%O2/7O%CO2 35%O2/65%CO2

HVN 384 36O 578 613

90HVN-10OW 392 346 547 585

8OHVN-2OOW 395 339 548 578

SAB 4OO 359 498 527

9OSAB-IOOW 333 29O 418 422

8OSAB-2OOW 27O 181 27O 354

For both blends, HVN-OW and SAB-OW, the NO concentration obtained under the 21%O2/79%CO2 atmosphere was lower than that achieved in air combustion. The lower NO concentrations in the oxy-fuel combustion atmospheres than in air are partly explained by the suppression of thermal NO formation. However, at the temperature used in this work (1000 °C), the thermal NO formation could make only a minor contribution to the formation of NO. It would appear, therefore, that the lower NO concentration obtained in the 21%O2/79%CO2 atmosphere was probably due to the greater NO reduction to N2 resulting from the higher CO concentrations in oxy-firing systems [15]. The NO concentrations measured in this work showed a marked increase as the oxygen concentration in the O2/CO2 mixture increased to a value of 30 or 35%. This is due to an enhancement of the combustion rate and therefore of fuel-N to NO conversion [16]. Other authors [7, 17] have reported the NO concentrations in terms of NO per unit of energy supplied or NO per unit of fuel burnt, and they have found small differences in NO emissions under oxy-firing conditions for different O2 concentrations.

For the SAB coal the NO concentration in air and oxy-firing conditions decreased after the addition of biomass, this decrease became greater as the biomass concentration increased. For the HVN semianthracite the same tendency was observed but the decrease in NO concentration values was smaller. Since HVN and the biomass used in the present work have similar nitrogen contents, the lower NO emissions during co-firing experiments cannot be explained by the dilution of N in the mixed fuel. However, in the case of coal SAB the dilution of N might have a greater effect since the nitrogen content of OW is slightly lower than that of SAB. Besides these differences in nitrogen content, most of the biomass is released as volatiles (about 75% at temperatures above 800 °C) and the fuel-N is predominantly liberated as NH3, which may be oxidised to NO but also act as a reducing agent in further reactions with NO to form N2 [18]. On the other hand, coals released less volatiles and the fuel-N is mainly evolved as HCN, which has a lower potential to reduce NO to N2.

5. Conclusions

The goal of this study was to evaluate the effect of blending biomass with coals of different rank on the ignition characteristics and NO emissions in air and oxy-firing conditions. The most important conclusions of this work are as follows:

(a) A significant ignition delay was observed when N2 was replaced by CO2 for an oxygen concentration of 21% for both individual coals and coal and biomass blends, due to differences in the specific molar heats of N2 and CO2. However, when the oxygen concentration was increased to 30% in the O2/CO2 mixture, the ignition properties were similar to those obtained in air-firing conditions; this is due to an increase in the mass flow of oxygen to the surface of the fuel particles.

(b) Co-firing coal and biomass resulted in a decrease of the ignition temperature in both air and oxy-firing conditions. However, this improvement was more significant for the case of the semianthracite due to its lower volatiles content in comparison with the biomass. The ignition properties of the high-volatile bituminous coal seemed to be less affected by the addition of biomass.

(c) NO emissions were lower in the 21%O2/79%CO2 atmosphere in comparison with those obtained in air-firing conditions due to the additional reduction of NO to N2 by reaction with CO. However, when increasing the oxygen concentration in the oxy-firing atmospheres, the NO emissions also increase due to the higher fuel-N conversion to NO.

(d) The addition of biomass reduced significantly the NO emissions for both the semianthracite and the high-volatile bituminous coal in air and oxy-firing conditions. This decrease was proportional to the concentration of biomass. However, the reduction of NO emissions was less noticeable in the case of the semianthracite.

Acknowledgements

This work was carried out with financial support from the Spanish MICINN (Project PS-120000-2005-2) co-financed by the European Regional Development Fund. L.A. and M.V.G. acknowledge funding from the CSIC JAE program, co-financed by the European Social Fund. J.R. acknowledges funding from the Government of the Principado de Asturias (Severo Ochoa program).

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