5540
Ind. Eng. Chem. Res. 2004, 43, 5540-5547
Control of Pollutants during FBC Combustion of Sewage Sludge M. Helena Lopes,* Ibrahim Gulyurtlu, and Isabel Cabrita Instituto Nacional de Engenharia, Tecnologia e Inovac¸ a˜ o, INETI-DEECA, Estrada Pac¸ o do Lumiar, 22, 1649-038 Lisboa, Portugal
This paper presents a study of pollutant emissions during combustion tests of sewage sludge in a pilot fluidized-bed system. Mono-combustion of sludge and its co-combustion with coal were compared with combustion of coal alone. It was verified that the high N content in the sludge did not lead to an increase in NOx production and that air staging gave rise to very low levels of NO emissions. SO2 diminished when the sludge was introduced, but achieving acceptable levels to meet emission limits still required the use of limestone. The emissions of heavy metals were increased during co-combustion but decreased during sludge mono-combustion, with the exception of Hg, which was retained more efficiently in cyclone ashes during co-combustion. The study of the dependence of the Pb and Cd distributions on the diameter of the fly ash emitted showed that these metals were associated with particles with an average diameter of 2-3 µm and that their removal would only be possible with adequate gas cleaning. 1. Introduction The combustion of sewage sludge has been performed in the U.S., Germany, Japan, and other developed countries, for several decades, using multiple hearth furnaces, rotary kilns, grate systems, and fluidized-bed systems.1 More recently, the utilization of sewage sludge as a substitute for coal in power stations has also been promoted.2,3 As a result of the growing interest in the utilization of residues and the need for the safe destruction of sludge, together with environmental pressures imposed on landfill disposal and agricultural reuse, combustion is foreseen to account for about 40% of European sludge production (about 10 Mt) by 2005.4 Fluidized-bed combustion (FBC) is the technique most widely utilized for the combustion of sewage sludge. In Germany, for example, 70% of the 39 plants performing sludge combustion use stationary FBC systems.1 Sludge can be used with several degrees of moisture, from less than 10% (predried) to about 80% water content (mechanically dewatered), with the use of an auxiliary fuel, such as oil or propane, required in the latter case to sustain combustion. During the combustion process, several pollutants can be released, namely, CO, SO2, NO, NO2, N2O, HCl, HF, unburned hydrocarbons, dioxins and furans, particles, and Hg and other heavy metals, and their emissions depend on the fuel composition, type of combustion system, and final treatment of the flue gases. In the case of FBC, the correct adjustment of operating parameters can contribute to significantly reducing their formation. The production of SO2 depends directly on the S content of the fuels. In fluidized-bed systems, it is possible to reduce SO2 emissions through the use of sorbents such as limestone or dolomite. CaCO3 is calcinated to CaO and reacts with SO2, forming anhydrite, a reaction that is maximized at temperatures between 820 and 840 °C, which lies in the normal range of FBC operation.5 Although the reaction is stoichiometric, higher Ca/S proportions are usually required * To whom correspondence should be addressed. Tel.: (++351) 21 716 51 41. Fax: (++351) 21 716 65 69. E-mail:
[email protected].
because of the pore-blocking effect of CaSO4. For this reason, the particle diameter of the adsorbent is very important. The sulfation of CaO is more effective for small particles; however, a compromise exists between the particle diameter and the residence time of the particles inside the reactor because of elutriation effects. There are indications that an optimum size of limestone corresponds to 0.3-0.4 mm for a Ca/S ratio of 2 and a fluidizing velocity of 1m/s.5 For fuels containing Ca compounds, the required sorbent addition is generally lower; however, this depends on the activity of the Ca compounds present regarding SO2 capture. In FBC systems, NO is essentially produced from N contained in fuels (fuel-N), because combustion temperatures generally below 1000 °C are low enough not to cause NO to be generated from N2 present in air (thermal NO). For coal combustion, the NO obtained is higher when the temperature or excess air level increases, and thus its production can be decreased through adjustment of the temperature, excess air level, and air staging. In the case of sludge, this is more controversial1 and seems to vary with the degree of moisture or the pretreatment applied to the sludge. The N content of sludges is usually higher than that of coal (2-4% N), especially in wet or mechanically dewatered sludges (6-8% N), so higher NO levels would be expected. However, experiments6-8 have shown that, in stationary FBC systems, the conversion of N to NO for predried sludges is similar, and even lower for the case of wet sludge, to that for coals. It was claimed that there could be two reasons for this lower emission of NO: One is the nature of fuel-N, which is mostly released as NH3 in the case of the combustion of wet sludge. NH3 could act as is the case of selective noncatalytic reduction (SNCR). Alternatively, for dried sludges, the decrease is attributed to the catalytic reduction of NO with CO by sludge ash rich in CaO and Fe2O3 that accumulates in the system, forming an “active bed”. Furthermore, it was verified8 that, for dried sludges, the levels attained without air staging, between 900 and 1300 mg/Nm3, were reduced to below 260 mg/Nm3 when the air was staged. Studies carried out in the large-scale FBC of sludge9,10 have revealed that, under well-controlled bed
10.1021/ie049765w CCC: $27.50 © 2004 American Chemical Society Published on Web 07/31/2004
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5541
temperatures, by employing two-stage combustion with recirculation of flue gases, it is possible to maintain the levels of NO below 200 mg/Nm3. Another issue of concern regarding sewage sludge combustion is the possibility of emission of heavy metals. The most volatile metals might be released attached to small fly ash particles, which are difficult to collect, or might even be emitted in gaseous phase, as in the case of Hg.11 Metal volatility is decreased in FBC systems compared to PCC (pulverized coal combustion) systems because of the lower working temperatures; thus, the majority of the metals remains in the ashes captured in the boiler, cyclones, and dedusting system. The possibility of using sorbents in FBC systems could also contribute to the fixing of heavy metals. However, ashes become contaminated and could pose a serious threat in terms of safe environmental disposal or reutilization.12 The behavior of heavy metals depends on the content of Cl and S,13,14 as well as on the operating conditions employed.15 Cl is known to promote volatilization, as some chlorinated heavy metals salts are volatile above 300 °C, especially those of Cd and Pb. The presence of S could delay the volatilization through the formation of sulfates, but the latter are unstable above about 500 °C,16 and consequently, the combustion temperatures play a critical role in the behavior of metals. Hg emissions are of particular concern in the combustion of sewage sludge because sludge contains higher Hg concentrations than coal, but usually low Cl concentrations; in municipal solid waste (MSW) combustion, for example, this contributes to the production of oxidized Hg, which is more easily captured by scrubbers.17 Intensive research is currently underway into the treatment of Hg, to decrease emissions to the new limits (0.05 mg/Nm3) imposed by a recent EU directive.18 In Germany, for example, where the levels were set to 0.03 mg/Nm3, several techniques have been developed, including the use of chemicals to promote Hg oxidation, scrubbers, and filtration with activated carbon or coke as a final treatment step.19 The study undertaken at INETI aimed at comparing the formation of pollutants during co-combustion of a thermally dried sewage sludge with coal with those observed during mono-combustion of the same sludge and of coal separately on a pilot fluidized bed. The emissions were determined without the deployment of flue gas treatment to establish the correlation of their variation with the type of fuels used and the effectiveness of the adjustment of the operating parameters in the fluidized bed to control the primary emission of pollutants. The levels of gaseous pollutants, particulates, and heavy metals were also compared on the basis of specific energy input, to have a more realistic assessment of the environmental problems in terms of energetic valorization of various residues, considering the different heating values of the different fuels. 2. Materials and Methods 2.1. Fluidized-Bed System. The combustion tests were carried out in a pilot-scale fluidized bed a schematic representation of which is given in Figure 1. It is square in cross section with each side being 0.3 m long. The combustor is built of refractory steel, is insulated outside, and is cooled with cooling coils placed both in the bed region and in the riser. Its height is 5.0 m, and it is provided with two high-efficiency cyclones for the
Figure 1. Scheme of the pilot-scale fluidized bed: 1, ventilator; 2, FBC reactor; 3, first cyclone; 4, second cyclone; 5, stack ventilator; 6 fuel screw feeder; 7, bed ash drum; 8, first cyclone ash; 9, second cyclone ash; 10, data acquisition; 11, stack nozzle.
removal of particles from the flue gases, which are then released to the atmosphere through a small stack without flue gas treatment. The primary cooling air is introduced through the windbox, and there are several entries for secondary air along the freeboard. The solid fuels are introduced 0.5 m above the distributor plate. Several gas sampling probes are situated at various heights of the reactor to carry out instantaneous measurements of CO, CO2, NO, SO2, and O2 using heated sampling lines and automatic gas analyzers [nondispersive infrared (NDIR) for the first four gases and paramagnetic for O2]. The temperatures and pressure of the system are continuously measured with thermocouples and water manometers, respectively. For each run, fresh quartz sand with an average size of 0.32 mm is introduced in the bed zone. The ashes accumulate in the bed together with the sand, and the fly ashes that are removed by the cyclones are collected in steel drums at the end of each run for further analysis. The system works under atmospheric pressure and is operated in the bubbling regime, with low fluidizing velocities and without recirculation of the ashes captured in the cyclones. The fuel feeding rate is maintained relatively low, with a thermal input of about 0.1 MW. Each test lasted for about 8 h. The fluidized bed was preheated with propane to a temperature of about 600 °C, and then the supply of solid fuels was started. The system was allowed to stabilize for about 3 h, and subsequently measurements were taken under steadystate conditions. The final measurement of emissions before release of the flue gases to the atmosphere is carried out in the stack at flue gas temperatures of about 120 °C, employing continuous gas analyzers for gaseous pollutants. For HCl, particles, and heavy metals, an isokinetic probe is used, and the sampling generally lasts for at least 1 h.
5542 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 Table 1. Proximate Analysis of Fuels moisture (wt % a.r.) volatile material (wt % d.b.) ash (750 °C) (wt % d.b.) fixed carbon (wt % d.b.) LHV (MJ/kg d.b.)
coal
SS
1.6 37.8 7.5 54.7 31.0
6.6 49.8 42.8 7.4 13.1
2.2. Analytical Techniques. ASTM standards normally used for coal analysis were employed to characterize the main composition and properties of fuels. The chemical analysis was carried out after strong digestion in closed Teflon vessels in a microwave oven (CEM MDS 2000) employing strong acids (HF, HNO3, H3BO3) based on SW 846 method 305220 and analysis by (FAAS) flame atomic absorption spectrometry (Solaar Unican) coupled with a graphite furnace (GFAAS) with Zeeman correction for the lower metal concentrations. Hg in fuels and ashes was measured with an automatic Hg analyzer (Leco AMA 254) in accordance with SW 846 method 7473,20 and stack Hg measurements were carried out in solutions after digestion of the filter ashes and the scrubbing solutions (HNO3/H2O2 and KMnO4/H2SO4) of the sampling train (Anderson Universal Stack Sampler Unit).21 HCl was sampled in accordance with EPA methods21 and measured by capillary electrophoresis (Waters ion electrophoresis) in accordance with method SW 846 EPA 6500.20 The loss on ignition (LOI) was evaluated by the weight loss after combustion of the ashes at 750 °C. For the study of particulate size of emitted fly ashes and their metal concentrations, a cascade impactor (Mark III) that was inserted in the flue gas duct was used. The particles were collected in quartz fiber filters and analyzed after digestion by GFAAS. Because of low concentrations present in these fractions and high filter blanks of most metals, only Pb and Cd were evaluated. The size analysis of fine ashes was performed by laser diffraction (Malvern Series 2600 Droplet and Particle Sizer), and for the larger ashes, dry sieving was performed.
2.3. Fuel Description. The compositions of the fuels are presented in Tables 1 and 2 (together with uncertainties of the chemical analysis). Sewage sludge, commercialized as Biogran, was obtained in the U.K. It is an odorless, dry, and granular material. It is similar to most sewage sludge, containing a high level of volatile matter with significant heating value and a substantial ash content, because of the presence of high quantities of lithophilic elements Fe, Ca, Al, and Si. The coal, a bituminous type from the U.S., contains high levels of volatile matter and fixed carbon and a low ash content. Both the coal and the sludge have low Cl contents. S in coal is twice that in the sludge, but the concentrations of N and P are higher in the sludge. The concentrations of heavy metals in the coal are quite low. The sludge contains significant levels of Mn, Cr, Pb, Cu, Zn, Cd, Ni, Co, and Hg. 2.4. Description of Test Runs. The conditions that were used in the tests are presented in Table 3. Three sets of combustion tests were performed: (a) combustion of coal alone (Coal1 and Coal2), (b) co-combustion of sludge with coal without limestone (Mix1 and Mix2) and with limestone (Mix3 and Mix4) [for the co-combustion tests, a proportion of 65% coal and 35% sludge was chosen to keep a high calorific value (about 25 MJ/kg) and a moderate level of ashes (about 20%)], (c) monocombustion of sludge (SS1 and SS2). Coal was previously sieved to a size range between 0.5 and 4 mm, and the sludge was fed as dry granulates ranging from 2 to 5 mm in size. The air was staged between the bed zone level and the riser (0.1 m above the bed bottom) to control the formation of NO. The global excess air was kept at about 50%. The temperature was in the range 750-850 °C at the top of the bed and it decreased along the riser to 200-340 °C at the exit, as can be seen in Figure 2. This was due to the low fuel feeding rates used and the fact that air was not preheated; hence, the thermal load was not sufficient to make up the heat losses along the freeboard height.
Table 2. Composition of Fuels content (wt % d.b.)
content (wt % d.b.)
component
coal
SS
component
coal
SS
C H N Cl S P
79.1 5.0 1.8 0.06 2.15 0.51
30.9 3.8 3.7 0.07 0.96 3.11
Ca Fe Al K Na Mg
0.20 ((0.02) 0.65 ((0.1) 1.17 ((0.1) 0.04 ((0.01) 0.03 ((0.01) 0.02 ((0.005)
5.3 ((0.2) 1.7 ((0.1) 3.0 ((0.1) 0.6 ((0.03) 0.2 ((0.04) 0.5 ((0.04)
component
coal
SS
component
coal
SS
Mn Co Ni Cr Pb
12.3 ((6.5) 1.8 ((0.6) 5.1 ((2.5) 12.2 ((4.8) 2.0 ((0.05)
394 ((12) 5.3 ((0.6) 43 ((9.3) 210 ((19) 365 ((17)
Cu Cd Zn Hg As
5.0 ((2.0) 0.17 ((0.05) 22.8 ((9.5) 0.08 ((0.01)
