fuel-staging and air-staging to reduce nitrogen emission in cfb

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FUEL-STAGING AND AIR-STAGING TO REDUCE NITROGEN EMISSION IN CFB COMBUSTION OF BARK AND COAL Heidi Saastamoinen, and Timo J. Leino Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00850 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 17, 2019

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Energy & Fuels

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FUEL-STAGING AND AIR-STAGING TO REDUCE NITROGEN EMISSION

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IN CFB COMBUSTION OF BARK AND COAL

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Heidi Saastamoinena,*, Timo Leinoa a

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VTT Technical Research Centre of Finland, P.O. Box 1603, FI-40101, Jyväskylä, Finland *

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Tel.: +358 44 568 9860 E-mail address: [email protected]

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Abstract: Nitrogen oxide (NO) and nitrous oxide (N2O) formation in circulating fluidized bed

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combustion can be controlled by air-staging and fuel-staging. An extensive test campaign was

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carried out with a pilot scale CFB test rig to observe the possibilities of the methods in spruce

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bark and bituminous coal combustion as well as in co-combustion. The fuel-staging with liquid

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petroleum gas (LPG) was done alternately from three locations with three intensities. The air-

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staging was studied alone as well as during the fuel-staging experiment. The experimental

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trends for NO and N2O emission formation during the fuel-staging and air-staging are presented

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in this study. It was observed that air-staging and fuel-staging can have opposing effects on

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nitrogen oxide emission formation and, thus, when used together, a clear understanding of the

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fuel behaviour and conditions, as well as NOx chemistry in the combustor, is needed. Under the

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tested conditions, it was observed that if the air-staging is effective, fuel-staging does not bring

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further benefits in NO reduction. Instead, LPG feed can increase the emission in the lack of

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oxygen. However, if it is not possible to carry out air-staging, fuel-staging can be used in

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generating oxygen-lean reducing zones for NO. N2O concentration was further reduced with

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LPG also in the tests with effective air-staging.

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Keywords: CFB, combustion, fuel-staging, air-staging, LPG, coal, bark, biomass

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1

INTRODUCTION

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NO, NO2, and N2O are the main nitrogen pollutants emitted from combustion [1,2]. Direct formation of NO 2

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in the fluidized bed combustion is usually insignificant. Instead, NO2 formation is found in the conditions

ACS Paragon Plus Environment

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where rapid cooling takes place [3]. The formed NO is unstable in atmosphere and oxidizes readily to NO2,

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which causes acidic fallout by reacting with atmospheric moisture. N2O is a greenhouse gas with an

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increasing atmospheric concentration [4] and global warming potential 265 times that of CO2 [5]. Power

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plants in operation may have problems meeting the policies [6] implemented to reduce the emissions of

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nitrogen oxides (NO and NO 2). So far, regulations do not limit N2O emissions.

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The methods for NOx and N2O emission reduction can be divided into primary and secondary techniques, of

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which the first one is less expensive. The primary techniques are used to reduce emissions already in the

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furnace by affecting the combustion conditions in order to produce reducing zones. These techniques,

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unfortunately, usually do not reduce all the harmful nitrogen species. The primary techniques include: (1)

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low excess air, (2) air-staging, (3) flue gas recirculation, (4) reduced air preheat and (5) fuel-

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staging/reburning. The secondary techniques are referred to as end-of-pipe techniques, which are used to

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remove nitrogen oxides from flue gas. These techniques are commonly divided into: (1) selective catalytic

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reduction (SCR) and (2) selective non-catalytic reduction (SNCR) [20].

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Fuel nitrogen is the main cause of nitrogen oxide formation in fluidized bed combustion. Nitrogen emissions

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also form from atmospheric nitrogen (prompt-NO and thermal-NO). However, the formation of prompt-NO

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as result of reactions between N2 and hydrogen radicals from the fuel can be estimated as small [7,8]. The

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combustion temperature in fluidized bed combustion is far too low for thermal-NO to form (800–1000 °C

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instead of ≥1300 °C), since thermal dissociation of N2 to N-radicals is needed to start NO formation [9].

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Lower combustion temperature increases the NO reduction especially when the oxygen concentration is low

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[10]. Relatively low combustion temperature, however, enhances the formation of N 2O especially in

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circulating fluidized beds where almost isothermal conditions (typically 850-900 °C with full boiler load)

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exist in the riser [11].

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Bark with high volatile content releases nitrogen mainly to light gaseous species, promoting the

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homogeneous mechanism in nitrogen oxides formation [12]. Although, the reaction routes of volatile

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nitrogen species are complex [11,21], there is conformity in the NO formation. Released nitrogen


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intermediates (HCN and NH3) react with available oxygen forming NO or in the oxygen lean zone N2;

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Formed NO can also reduce to N2 [3,14]. Lower O2 content in the combustor generally results in less NOx

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and N2O [7].

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Gustavsson and Leckner [17,18] state that the N2O formation is highly dependent on the conditions in the

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furnace. Higher temperature and lower level of excess air decrease emissions. In addition, the solid fuel, fuel

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characteristics and additives affect the formation of nitrogen emission. Fuels with high content of fixed

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carbon such as bituminous coal yield relatively high emissions of N2O (100-150 ppm), whereas the N2O

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emission is lower for lignite and peat and negligible for wood, all with lower content of fixed carbon

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compared to coal [17,19–21]. The formation is commonly explained by oxidation of char-nitrogen, reaction

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of NO with char-nitrogen or by catalytic reactions of nitrogen species on the available particle surfaces

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[22,23]. The high volatile content of fuel also increases the oxygen consumption and produces oxygen-lean

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zones that decrease the N2O formation and increases its reduction [23]. In a previous study with VTT’s pilot

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CFB combustor [24], it was found reasonable to assume that the heterogeneous reaction (1) of NO with char

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nitrogen reduces NO and increases N2O formation when char inventory increases. char  N NO  N 2O

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(1)

This is in agreement with the literature [25,26] of fluidized bed combustion of coal and char.

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Kinetics of reburning are complex and include various hydrocarbon reaction mechanisms [13,27,28]. During

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the reburning process, NO can be homogeneously reduced to N2 either with CHi,-radicals (i=0,1,2) and

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ammonia (NHi, i=0,1,2) radicals (if the reburn fuel contains nitrogen) or with CO. Also, Kilpinen et al. [27]

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conclude that NO reaction to HCN and with CO and H radicals are effective in reburning with methane. If

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active fuel-bound carbon sites are available, they can also reduce NO to molecular nitrogen [29]. The main

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homogeneous NO reduction routes related to reburning in the literature [28–30] can be simplified to:

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NO  

HCNO NCO   HCN     N2 H 2CN NCO  NH

NO  CO  CO2 

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(2)

(3)

1 N2 2

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The reburning method has been used for nitrogen emission reduction in pulverized fuel combustion for

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various fuels [16,17,31–35]. However, studies in fluidized bed combustion conditions are rare. Sirisomboom

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and Kuprianov [36] studied fuel-staging in biomass-biomass co-combustion and the gas reburning in CFB

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integrated with gasification has been studied [37]. In this study, the possibilities of the secondary fuel

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injection to the furnace above the primary zone in circulating fluidized bed combustion and co-combustion of

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biomass and coal is studied by feeding liquid petroleum gas (LPG) in a pilot-scale combustor.

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2

EXPERIMENTAL PROCEDURE

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The effect of air-staging and fuel-staging on nitrogen emission formation in the circulating fluidized bed

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combustion of coal, bark and mixtures of them was studied with VTT’s 50kW pilot combustor. The

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schematic diagram of the combustor can be seen in Figure 1. The riser has a cylindrical shape with a

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diameter of 0.17m and length 8.3m. The walls are constructed with high-temperature and corrosion-resistant

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steel. Fed air is electrically heated and the furnace zone has been equipped with several electric heating zones

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for stabilization, and with several ports for solid material sampling.

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Primary air is fed through an air grid. The secondary air is fed 1.3 metres and tertiary air 1 4.7 metres above

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the grid. Tertiary air 2 is fed just before the cyclone at 8.3 metres above the grid. Primary fuels can be fed

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through two different lines. Additives can be fed through a separate line. Limestone or other additives were

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not used during the experiments. The particle size of natural sand used as the bed material was in the range

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0.1-0.5 mm (mean diameter 0.2 mm). The combustor is equipped with five gas injection lines that can be

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seen in Figure 1 (LPG1-LPG5). LPG1 is interconnected with the primary air feed in order to mix the

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additional gas with primary air. LPG2 is located at 3.0, LPG3 at 6.2 and LPG4 at 7.9 metres above the grid.


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The main flue gas compounds (including O 2, CO2, CO, NO, N2O, NO2, and SO2) were measured with an

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FTIR gas analyser and with an O2-analyser from 2.0 m above the grid and from the flue gas. The sampling

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point at 2.0 metres is above the secondary air feed. The temperatures were measured with thermocouples

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along the CFB riser. All input flow rates are also measured as Nl/min. Laws describing ideal gases are used

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to correct flow rate according to reactor temperatures. Total flue gas flow rate, which includes combustion

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gases, is calculated based on measured flue gas mass flow, air feed and O2 content. The residence time of the

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flue gas is calculated using the flow rates, reactor dimensions and feeding points.

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Figure 1. Schematic diagram of VTT’s 50 kW CFB combustor including feeding ports for air and LPG.

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The combustion tests were conducted with 100% bark, 100% bituminous coal, as well as with two of their

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blends. The first blend consisted of 80% bark and 20% coal, and the second 27% bark and 73% coal on

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energy basis (dry fuel). Fuel compositions are shown in

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Table 1. LPG was injected into combustor as a reburn fuel. The used LPG contains ~95 vol-% propane

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(C3H8) and ~5 vol-% butane (C4H10).

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Table 1. Fuel composition.

Fuel Moisture (wt,%) Ash, dry basis (815 °C) (wt,%) Volatile content (dry basis, wt,%) Heating value, dry basis (kJ/kg) Element analysis, dry basis C-content (wt, %) H-content (wt, %) N-content (wt, %) S-content (wt, %) Cl-content (wt, %)

Finnish spruce bark 12.0 2.6 75.0 18.86

Russian bituminous coal 9.2 16.2 32.8 27.44

50.4 5.9 0.36 0.02 0.012

68.3 4.6 2.22 0.36 0.005

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The total air volume flow (Nl/min) was kept constant, but the feed varied between primary, secondary,

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tertiary air 1, and tertiary air 2. The air-staging steps can be seen in Table 2. Air-staging was done with and

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without the LPG feed. The gas injections with 5 kW load from three different locations (LPG2, LPG3, and

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LPG4) were tested from one location at a time as well as with three different LPG loads (2kW, 4kW and

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5kW) from the injection point LPG2. The solid fuel feed, 45-50 kW depending on the fuel, was not changed

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when doing air- or fuel staging to keep the bottom bed conditions comparable.

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Table 2. Air-staging steps. test # 0 1 2

primary air % tot 49 49 49

secondary air % tot 50 43 32


tertiary air 1 % tot 0 7 18

tertiary air 2 % tot 1 1 1

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Energy & Fuels

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49 49 49 49 49 49

21 16 16 21 32 43

29 34 0 0 0 0

1 1 34 30 19 8

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3

RESULTS AND DISCUSSION

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3.1

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The further from the grid the combustion air was fed, the longer the flue gas stayed in the combustor due to

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decreased flue gas velocity. From the initial air feed (49/50/0/1, prim./sec./tert.1/tert.2 vol-% shares

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respectively) to the staging, in which 33% air was fed into the tertiary air 1 (49/16/33/1), the increase in flue

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gas residence time in the reactor was 0.5-1 seconds. If part of the combustion air was fed into the tertiary air

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2 before the cyclone (maximum case 49/16/0/34), the residence time increased up to 1.5 seconds. This means

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that when air feed to the secondary air is decreased, the flue gas residence time in the oxygen lean zone

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becomes longer both due to lower oxygen concentration in the combustor and due to decreased flue gas

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velocity. LPG feed increased the total flow rate less than 0.5 %. Therefore, its effect to residence time can be

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considered negligible.

Effect of air-staging on the residence time and temperature profile in the combustor

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During the combustion of either 100% bark or mixture containing 80% bark, the increase in air feed

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increased temperature at the feeding height (Figures 2 and 3). This clearly shows that volatiles burn

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immediately if oxygen is available. In all tests for bark and mixtures containing bark, a peak in temperature

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can be seen at the secondary air entry where main part of the volatiles burn. However, during the combustion

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tests for 100% coal both the secondary and the tertiary air 1 feed cools down the temperature in the

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combustor locally. The more the air feed was spread, the lower the gradient appeared in temperature at the

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secondary entry for bark whereas for coal, the decrease in secondary air feed seems to smoothen the

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temperature differences. It can be observed clearly that combustion at the secondary air entry decreases and


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combustion in the upper combustor increases when the share of coal in fuel mixture is increased. When more

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than 20% of the total air was tertiary air 2, part of the combustion occurred after that level.

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Figure 2. Temperature profile in the combustor without the gas feed for 100% bark (left) and 100% coal (right).

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Figure 3. Temperature profile in the combustor during the gas feed tests for 100% bark.

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LPG injection increased the mean temperature (25-50 °C) in the combustor since LPG combustion heats up

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the bed material. It can be seen clearly that LPG burns in oxygen-rich conditions. However, if the combustor

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is oxygen-lean, combustion occurs closer to or in the cyclone, or in other words due to tertiary air 2. Neither

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air staging nor LPG feed affect greatly the density profile in the combustor which was close to constant

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throughout the test set.


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Energy & Fuels

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3.2

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The blending of fuels affects significantly nitrogen emission formation. The increase of coal share in the fuel

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decreases the nitrogen conversion to NO and increases its conversion to N2O, as can be seen in Figure 4. If

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fuel contains mainly or only bark, the nitrogen conversion to N2O is low. For 100% coal, the measured N2O

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concentration is clearly the highest. In the experiments where 73% share of coal was used, nitrogen

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conversion to N2O in the stack is in the same range as to NO. When the share of coal increases, the amount

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of char increases in the combustor and the heterogeneous reaction of NO with char nitrogen reduces NO and

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increases N2O formation [24–26].

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Figure 4. NO and N2O formation with different fuel mixtures (bark/coal) when no LPG is fed and air feed is to primary and secondary air (49/50/0/1).

Nitrogen emission formation during fuel-staging and air-staging

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It was found reasonable to study the test with all bark (100%) and high share of bark (80%) in parallel due to

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similarities in the results. This is done in chapter 3.2.1. Accordingly, in chapter 3.2.2, also the tests with all

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coal (100%) and high share of coal (73%) are analysed in parallel.

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3.2.1

Bark as main fuel

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Carbon monoxide (CO) and char can reduce NO during combustion [27,29,38,39]. In addition, the low

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excess air causing high CO concentrations reduces NO formation [40]. CO concentration in the stack is low

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during combustion tests with 100% and 80% bark (under 90 mg/Nm3). The tertiary air reacts with the rest of


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the CO if the combustion in the riser is incomplete, and the cyclone is an efficient mixer that aids the

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combustion of CO. However, the measured CO concentration in the stack is the highest when the NO

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concentration is the lowest. Considerably high CO concentrations above the secondary air entry are measured

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when the secondary air feed is small, as can be seen from Figure 5.

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Figure 5. Mixture containing 80% bark, NO concentration in stack and CO concentration above the secondary air entry versus oxygen concentration after secondary air feed.

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In this study, the largest reduction in NO during the air-staging tests for pure bark (100%) and for the mixture

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(80% bark, 20% coal) is found when 21% of the total feed is to the secondary air and 49% to primary air and

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the rest of the air is fed as tertiary airs 1 and 2. Decrease in secondary air feed affects the flue gas velocity in

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the combustor making the residence time of reacting nitrogen in reducing conditions longer. When the last

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air feed location was tertiary air 2 instead of tertiary air 1, even more NO is reduced due to extended oxygen-

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lean zone in the reactor. In the lack of oxygen, nitrogen precursors reduce to atmospheric N2 or NO formed in

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the dense bed reduces in reaction with CO and/or char-N.

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The N2O formation is insignificant when char inventory is small [41], which was shown in this study as well.

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Biomass product gas is also found to be an effective N2O reductant in fluidized bed conditions [42]. Lu et al.


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[43] conducted experiments with natural gas in a circulating fluidized bed combustor. They found, that the

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gas combustion itself produced a negligible amount of N2O. However, NO formed with the prompt-NO

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mechanism, which was explained by local hot spots due to uneven burning of gas. In this study, the

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temperature in the combustor was electrically controlled and such hot spots were not present. Anyhow, the

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above-mentioned mechanism can affect a commercial-scale combustor.

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The increase in LPG feed reduces NO almost in a linear fashion for bark as can be seen in Figure 6, in which

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the change in flue gas NO concentration due to LPG feed compared to corresponding air feed without the

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LPG feed is illustrated. With 5 kW LPG feed to the lowest feeding point (LPG2) above the secondary air

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entry, 22% less NO forms than without fuel-staging. However, it must be pointed out that there is no

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reduction with the most reducing air feeds (49/21/29/1). It seems that if the air-staging is effective, fuel-

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staging is not needed and if used it may increase NO emission. The results are similar with the mixture that

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contains mainly bark (80%).

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Figure 6. Bark 100%, the change in flue gas NO concentration due to LPG with three intensities compared to no LPG feed at different air-staging combinations (prim/sec/tert1/tert2).

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In the study by Pels et al. [44] it was observed that during the volatile combustion intermediate NCO, which

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originates from HCN, reacts to NO at higher temperature whereas lower temperatures favoured N2O


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formation. Loeffler et al. [45] also observed that above 800 °C, the conversion to NO increases if radical

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level in the combustor is high. However, the effect in lower temperatures was opposite, which was explained

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by reaction (4).

NO  HO2  NO2  OH

(4)

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Therefore, the temperature increase and high radical level due to LPG feed would expect to lead to higher

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NO formation during the combustion of bark with high volatile content. However, more NO is reduced when

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the intensity of LPG feed is increased. Both of the above-mentioned studies were done in laboratory scale

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equipment in which oxygen concentration can be controlled. In this study, the LPG feed significantly affects

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the oxygen content above the feeding height. Especially, if part of the combustion air is fed into the tertiary

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air 2 feed, the lower the location of the LPG feed is in the combustor, the more NO is reduced (Figure 7).

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Maximum reduction (31%) in NO concentration in the stack is obtained with the lowest air feed (49/32/0/19)

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and the lowest LPG feed (LPG2). The lower is the increase in reburn fuel in the combustor, the longer is the

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residence time of nitrogen compounds in oxygen lean zone if tertiary air 1 is not available. Therefore, it is

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assumed that the decrease in oxygen concentration due to LPG feed plays a greater role in NO reduction than

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temperature, which, anyhow, accelerates the reactions of the radical pool with NO.

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Figure 7. 100% bark, the change in flue gas NO concentration due to LPG feed from three locations compared to no LPG feed at different air-staging combinations (prim/sec/tert1/tert2)


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Energy & Fuels

3.2.2

Coal as main fuel

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During bituminous coal combustion, the total nitrogen emission in the stack decreases along with the reduced

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secondary air feed (see Figure 8). According to an earlier study [24] in which bituminous coal was

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combusted with VTT’s pilot combustor, NO concentration above dense bed can be correlated with the

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average bed temperature and oxygen concentration. In this study, the conditions in the dense bed were kept

246

constant to compare the effects of fuel-staging and air-staging. It is assumed that most of the heterogeneous

247

nitrogen reactions on the char surface, such as NO reduction to N2O (1), have already occurred in the dense

248

bed. N2O concentration seems to decrease further in the riser in oxygen-deficient conditions, but NO

249

concentration stays quite stable despite the changes in air feed.

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Figure 4. 100% coal, NO and N2O concentration in stack and CO concentration above the secondary air entry versus oxygen concentration after secondary air feed.

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During the coal (100%) combustion tests, LPG feed reduces both NO and N 2O emission, as can be seen in

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Figure 9. For pure coal, the test matrix included test with all air feed variations only with the LPG feed

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intensity of 5 kW. NO reduction with LPG is low when tertiary air 1 is not available and the oxygen

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concentration remains low in the riser. In general, more nitrogen emission is reduced with LPG when there is

258

more oxygen available at the secondary air entry, which is reverse to tests without LPG, in which the

259

decrease in secondary air feed decreases the total nitrogen emission.


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Figure 5. 100% Coal. The change in flue gas NO and N2O concentration due to LPG feed compared to no LPG feed at different air-staging combinations (prim/sec/tert1/tert2).

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The LPG feed reduces NO during the tests for 73% coal except if the secondary air feed is small and part of

265

the combustion air is fed at the tertiary air 2 instead of feeding it at the tertiary air 1 feeding point

266

(49/21/0/30), see Figure 10. Again, it seems that if the air-staging is effective, fuel-staging is not needed.

267

Approximately 34% reduction in NO can be achieved with 5kW LPG feed to the lowest feed point (LPG2)

268

compared to a case in which no LPG is fed. N2O is reduced with LPG in all tests. The reducing effect of LPG

269

is highest in the no-staging test (49/50/0/1) and decreases when the staging is increased. It is also remarkable

270

that LPG reduces more NO and N2O if secondary air feed is high, but if LPG is not available, the reduction

271

increases in the lack of oxygen above the dense bed. Clear conclusions regarding the intensity of LPG feed

272

on NO reduction cannot be made with available measurements.

273 274 275

Figure 10. Mixture containing 73% coal and 27% bark, the change in flue gas NO concentration due to LPG feed with three intensities compared to no LPG feed at different air-staging combinations


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(prim/sec/tert1/tert2).

277

N2O concentration decreases along with the increase in the intensity of LPG feed, as can be seen in Figure

278

11. High temperature and low air-ratio is found to lower N2O emissions [46,47]. According to the

279

measurements in this study, the increase in LPG feed increases the temperature and lowers the oxygen

280

concentration in the riser. In addition, LPG feed increases N2O reducing radicals in the reactor. Gustavsson

281

and Leckner [17] list the main gas-phase reactions that decompose N2O. They include decomposition via H

282

and OH radicals (5 and 6) and thermal decomposition (7).

N2O  H  N2  OH

(5)

N2O  OH  N2  HO2

(6)

N 2O  N 2  O

(7)

283

They found that the N2O reduction is strongly dependent on temperature and there were no differences

284

between the fuels used for the reduction, meaning that differences between released radicals did not affect

285

the reduction. N2O emission approached zero if the temperature increases above 900 °C. In this study, LPG

286

feed clearly increases temperature throughout the combustor and air feed slightly decreases the temperature.

287 288 289 290 291

Figure 6. Mixture containing 73% coal and 27% bark, the change in flue gas N2O concentration due to LPG feed with three intensities compared to no LPG feed at different air-staging combinations (prim/sec/tert1/test2).

4

CONCLUSIONS

292

The effect of air-staging and fuel-staging on nitrogen emission formation in circulating fluidized bed

293

combustion of coal, bark and their mixtures was studied. LPG was used in fuel-staging. It was observed that


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Page 16 of 21

294

air-staging and fuel-staging can have opposing effects on nitrogen emission formation and, thus, when used

295

together, a clear understanding of the conditions in the combustor is needed. The following conclusions can

296

be made on the study:

297

a) The decrease in secondary air feed decreased oxygen concentration and flue gas velocity in the

298

combustor, which led to lower NO emission due to increased flue gas residence time in the reducing

299

conditions and increased CO concentration in the riser. The largest reduction during the air-staging

300

experiments was obtained when 21% of the total feed was to the secondary air and 49% to primary

301

air and the rest to tertiary air 1 or 2. This applied to all used fuels. If the rest of the combustion air

302

was fed to the tertiary air 2 instead of tertiary air 1, meaning that the flue gas residence time in the

303

reducing zone further increased, the total emission reduction was larger.

304 305

b) The increase in LPG feed reduced NO during bark combustion; a maximum of 31% of NO reduction

306

from flue gas was obtained with a 5kW LPG feed. Such a reduction was not, however, clearly

307

observed with the mixture of 27% bark and 73% coal.

308 309

c) N2O formation was insignificant during bark combustion, but increased with the increase in coal

310

share in the fuel mixture. LPG was found to efficiently reduce N2O when it was formed. The increase

311

in LPG feed increased N2O reduction during the experiments with 27% bark and 73% coal;

312

maximum of 30% N2O reduction from flue gas was obtained with 5kW LPG feed.

313 314

d) If the air-staging was effective, further NO reduction did not occur with fuel-staging. Instead, the

315

LPG feed can increase emissions. However, N2O concentration in the stack was further reduced with

316

LPG, since LPG feed increases the temperature and, thus, speeds up the decomposition of N2O, and

317

increases the N2O reducing radical pool in the combustor.

318 319

e) The decrease in oxygen concentration due to staging with LPG played a bigger role in NO reduction

320

than increase of the temperature due to LPG combustion in pilot CFB combustor, in which the

321

temperature stayed well below Prompt-NO formation temperature.

322


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f) In the experiments where the fuel-staging reduced the NO in the first place, the lower the LPG feed

324

was located in the combustor, the more NO was reduced. The location of the LPG feed did not

325

clearly affect N2O reduction.

326 327

Unfortunately, the LPG addition did not clearly benefit NO reduction in relation to pure air-staging.

328

However, it is clear that both techniques can have beneficial effects on nitrogen emission reduction. In

329

commercial scale, also lower cost gases such as raw gas from anaerobic digestion, landfill gas or product gas

330

from gasification could be used for fuel-staging. Solid fuels can be used if their combustion behaviour is

331

known so that oxygen-lean NO reducing zone is produced to desired location. To obtain the benefits of the

332

joint use of fuel-staging and air-staging in nitrogen emission reduction during the CFB combustion,

333

validation of sophisticated flow models with nitrogen compound measurements of along the combustor riser

334

would be needed.

335

NOTATION

336

CFB

Circulating fluidized bed

337

LPG

Liquid petroleum gas

338

ACKNOWLEDGEMENTS

339

This work has been done within the project “Biofficiency - Highly efficient biomass CHP plants by handling

340

ash related problems” carried out in European Union’s Horizon 2020 research and innovation programme

341

under grant agreement 727616.

342

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fuel-staging and air-staging to reduce nitrogen emission in cfb

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