ARTICLE pubs.acs.org/EF
Study on NO Reduction and Its Heterogeneous Mechanism through Biomass Reburning in an Entrained Flow Reactor Ping Lu,†,* Yongqiao Wang,† Zhen Huang,‡ Fei Lu,† and Yongsheng Liu† † ‡
School of Energy & Mechanical Engineering, Nanjing Normal University, Nanjing 210042, China Nanjing Longrow Environmental Engineering Co. Ltd, Nanjing 210029, China ABSTRACT: The effects of biomass types (including cotton stalk, wheat straw, rice husk, and rice straw), the stoichiometric ratio in the reburning-zone (SR2), the reaction temperature in the reburning-zone (t2), the particle sizes of biomass reburning fuels (dp), and the reburning fuel fraction (Rff) on NO reduction efficiency during biomass reburning were investigated systematically in an entrained flow reactor. The NO heterogeneous reduction mechanism resulting from the reburning of wheat straw and its char was analyzed. The results indicated that cotton stalk has the best performance of NO reduction, wheat straw is in second place, and rice husk and rice straw are less effective. In the range of t2 = 9001100 °C NO reduction efficiency increases when the reburning-zone reaction temperature is increased at the same SR2. NO reduction efficiency increases insignificantly with a decrease in the particle size of the biomass with dp < 425 μm. NO reduction efficiency follows a pattern of first increasing and then decreasing with the decreasing of the SR2 or the increasing of the Rff. The higher NO reduction efficiency (more than 50%) can be achieved in the range of SR2 = 0.70.8 or Rff = 2025% during reburning with the four types of biomass. The contribution of NO heterogeneous reduction by wheat straw char to the total NO reduction was in the higher range of 5968%, whereas the Rff was in the range of 1026%.
’ INTRODUCTION Nitric oxides (NOx), particularly nitric oxide (NO) and nitrogen dioxide (NO2), are some of the toxic pollutants emitted from coal-fired power plants, resulting in acid rain, high ground-level ozone concentrations, and elevated fine particulate levels.1,2 Reburning, as a combustion modification technology, is considered as one of the most promising cost-effective NOx emission control technologies. The reburning technology requires creating three distinct reaction zones along the height of a coal-fired furnace, that is, the primary-zone, the reburning-zone, and the burnoutzone. Reburning fuels with a 1025% heat input of the total heat input are injected into the reburning-zone to establish a fuel-rich zone, in which NOx produced in the primary-zone is reduced to N2. Subsequently, additional air is introduced to complete the combustion of the unreacted fuels in the burnout-zone. Many studies on reburning have been carried out in bench and largescale combustors using a variety of fossil fuels, such as natural gas, pulverized coal, and biomass, and so forth. These studies revealed that typical NOx reductions can be up to about 5070%.28 Biomass as a reburning fuel may be a good alternative compared with conventional fossil fuels mainly because: (1) biomass fuels present lower levels of sulfur, nitrogen, and toxic metals, (2) they are CO2 neutral, and (3) the reburning fuels usually represent only around 1025% of the total heat input so that relatively small quantities of biomass are required. Some researchers, including Maly,2 Harding,9 Ballester,10 and Casaca,11 have studied the effects of biomass types, the particle size distribution of reburning fuels, and process conditions (e.g., the reburning fuel fraction and the flue gas residence time in the reburning-zone) on NOx reduction. As is well-known, NO reduction reactions with solid fuels (e.g., coal, biomass) include a homogeneous mechanism involving NO reacting with volatiles r 2011 American Chemical Society
and a heterogeneous mechanism involving NO reacting with chars. Chen,12,13 Liu,14 and Lu15 studied the char contribution to the total NO reduction during coal reburning. Results indicated that coal types have an important influence on NO reduction and char heterogeneous reduction, namely that the char contribution to the total NO reduction increases with the increasing of the volatile matter content in the coal, and heterogeneous mechanisms are more important than homogeneous mechanisms for lignites. However, little information is available on the contribution of heterogeneous reduction during biomass reburning. The primary objectives of this study were to discover the optimal parameters during reburning with different biomass types. The effects of biomass types, the reaction temperature in the reburning-zone (t2), the reburning fuel fraction (Rff), the particle size of biomass (dp), and the air: fuel stoichiometric ratio in the reburning-zone (SR2) on NO reduction were systematically carried out in an entrained flow reactor. The heterogeneous mechanism resulting from biomass reburning was analyzed.
’ EXPERIMENTAL SECTION The entrained flow reactor (EFR) was constructed from an alumina tube (50 mm ID 2.0 m height) as shown in Figure 1. It consisted of a primary-zone, a reburning-zone with an electrical heating system, a feeding system, a temperature measuring and indicating system, and a flue-gas sampling and analysis system. A screw feeder particle delivery system was used to supply the reburning biomass pneumatically (using nitrogen as the carrier gas) through a water-cooled injector. An acetylene-fired nozzle-mix burner was used to simulate the primary-zone Received: February 18, 2011 Revised: June 16, 2011 Published: June 16, 2011 2956
dx.doi.org/10.1021/ef2002553 | Energy Fuels 2011, 25, 2956–2962
Energy & Fuels
ARTICLE
Figure 1. Schematic diagram of the entrained flow reactor: (1) C2H2 cylinder, (2) air compressor, (3) air filter, (4) valve, (5) flow rate meter, (6) pressure gauge, (7) air container, (8) electrical cabinet, (9) thermocouples, (10) burner, (11) primary-zone, (12) N2 injector, (13) screw feeder, (14) water cooled injector, (15) quenched water, (16) induced draft fan, (17) flue gas cleaner, (18) NO cylinder, (19) N2 cylinder, (20) flue gas analyzer, (21) water-cooled probe, (22) computer, (23) reburning-zone, (24) heating elements.
Table 1. Analyses of the Tested Samples in the Experiments Proximate analysis (air-dry or dry and ash free, wt%)
Ultimate analysis (air-dry, wt%)
LHV (kJ/kg)
sample
Mad
Aad
Vad
Vdaf
C
H
O
N
S
Q net,ad
rice straw wheat straw
11.32 11.61
12.88 6.80
60.52 65.92
82.45 90.65
36.08 40.26
5.64 6.07
33.09 34.38
0.77 0.36
0.22 0.52
12.70 13.67
cotton stalk
13.09
4.49
63.72
93.42
40.97
6.06
33.77
1.09
0.53
13.79
rice husk
12.01
16.07
56.73
77.93
36.05
5.50
30.05
0.32
0
13.96
wheat straw char
3.21
32.54
4.43
11.98
55.66
1.04
5.42
1.59
0.54
23.21
of the reburning process. NO was added downstream of the precombustor to simulate primary-zone combustion products, in which NO concentration was kept constant at 800 ppmv. The air: fuel stoichiometric ratios in the primary-zone (SR1) were kept at 1.1 in all the experiments. The air: fuel stoichiometric ratios in the reburning-zone (SR2) were determined by the feed rate of the biomass and primary-zone conditions. Reaction temperature in the reburning-zone (t2) was controlled in the range of 9001100 °C. The total plug-flow flue gas residence time (τR) in the reburning-zone was estimated at 800 ms based on the flue gas flow rate and the reaction temperature. Gas and solid samples were taken from the sampling ports along the EFR using a stainless steel water-cooled probe (5 mm ID). All concentrations of NOx, CO, CO2, and O2 were measured by an online analyzer on a dry basis. Reburning fuels tested in the experiments included rice straw, wheat straw, cotton stalk, rice husk, and wheat straw chars. The proximate analysis, ultimate analysis, and lower heating value (LHV) of the samples are shown in Table 1. The wheat straw char was prepared in a heated fixed bed reactor and its preparation conditions under a rapid pyrolysis process are listed as following: the flow rate of the carried gas (N2) was 5 L/min, the pyrolysis time is 10 min, and the pyrolysis temperature was controlled at 400, 600, 800, and 900 °C, respectively. In the experiments,
all of the samples were ground and sieved to two parts: coarse particles (180425 μm) and fine particles (0180 μm). The reported NO reduction efficiency was calculated as: ηNO ¼ ½1 jðNOÞout =jðNOÞin 100%
ð1Þ
Where ηNO is NO reduction or reburning efficiency, %; j(NO)in is NO concentration at the inlet of the reburning-zone, ppmv; j(NO)out is NO concentration at the outlet of the EFR, ppmv. NO2 concentration were found to be
