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This paper reports on a study that contributed to the process design of an industrial-scale energy-from-waste. (EfW) combustion plant that processes o...
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Ind. Eng. Chem. Res. 2007, 46, 2648-2656

Study of the Process Design and Flue Gas Treatment of an Industrial-Scale Energy-from-Waste Combustion Plant Liban Yassin, Paola Lettieri,* and Stefaan J. R. Simons Centre for CO2 Technology, Department of Chemical Engineering, UniVersity College London, Torrington Place, WC1E 7JE, London, United Kingdom

Antonino Germana` Germana` & Partners Consulting Engineers, Rome 00154, Italy

This paper reports on a study that contributed to the process design of an industrial-scale energy-from-waste (EfW) combustion plant that processes over 250 000 tonnes of waste and produces 34 MWe of electric energy. The study also investigates the effect of replacing the conventional calcium hydroxide sorbent with sodium bicarbonate in the removal of acidic pollutants from the flue gas. Hydrated lime is widely used to remove acidic pollutants such as HCl, SO2, and HF from flue gas streams in municipal solid waste (MSW) combustion. However, it is corrosive, it is required in excess, and it requires low operating temperatures for the removal to be effective. Mass and energy balances of the process were performed using the proximate and ultimate analyses of the waste and a mathematical model based on the unreacted-core model has been applied to compare the efficiency of using both sorbents. The economic feasibility of using sodium bicarbonate instead of hydrated lime in the plant has also been investigated. This study showed that although sodium bicarbonate is a more expensive sorbent, it is more efficient, and it is economically a more attractive option for the removal of acidic pollutants than hydrated lime. 1. Introduction The total quantity of waste has increased significantly in the European Union (EU) countries and the rest of the world, raising the critical question of its safe treatment and disposal.1 Although landfilling is the predominant treatment option, it is a missed and wasted opportunity. Value should be recovered from waste, whether as materials through recycling and composting or as energy or fuel through efficient thermal and biological processes.2 The residual wastes that remain after their reduction, reuse, and recycling can be sent directly or as a refuse-derived fuel (RDF) to dedicated facilities, such as incinerators, or other energy-from-waste (EfW) plants that incorporate advanced thermal treatment processes (ATTs), such as gasification and pyrolysis. It can also be co-combusted with other fuels, such as coal, in power generation, cement production, or other large thermal processes. Energy is then recovered as heat, which can be used for district and industrial heating and/or power, which can be sold to the national grid. Hence, EfW materials, when used as an energy source, can reduce our reliance on landfills while increasing the contribution of electricity from renewable sources. It is estimated that by 2020, 17% of all electricity used in the United Kingdom could, theoretically, come from waste. A more realistic and significant contribution would be 10%, compared to 0.5% currently observed.3 By displacing fossil fuels, waste as an energy source can also fill the United Kingdom’s fuel gap that has been created by diminishing existing energy sources coupled with high prices for oil and gas. In February 2006, the United Kingdom’s Department for Environment, Food and Rural Affairs (Defra) published a consultation paper on the review of England’s Waste Strategy, proposing a target for EfW materials as 25% of the municipal solid waste (MSW) generated by 2020, which * To whom correspondence should be addressed. Tel.: +44 (0) 20 7679 7867. Fax: +44 (0) 20 7383 2348. E-mail: [email protected].

corresponds to ∼700 MWe of electric capacity, compared to 9% currently observed, which corresponds only to 260 MWe of electricity.4 This has led some local authorities in the United Kingdom to embrace the role of EfW materials. Three new EfW plants in Hampshire accounted for 46% and 35% of its MSW and recycling capacities in 2004-2005, and a new EfW plant to treat 500 000 tonnes per year is under construction in Kent; in Sheffield, the existing EfW plant with a capacity of 135 000 tonnes is being replaced by a new plant with a capacity of 225 000 tonnes,5,6 which has approximately the same size as the plant investigated in this paper. Thermal treatment processes of waste result in flue gases that contain a variety of pollutants, such as acidic gases, heavy metals, volatile organic compounds (VOCs), and dioxins/furans. These pollutants can be effectively reduced to below the acceptable and regulated emission levels set by the EU Waste Incineration Directive by a range of physical and chemical processes. The capture and removal of the flue gases are generally based around the following basic steps: (1) addition of ammonia to the combustion chamber, (2) cooling of the flue gas, (3) acid neutralization, (4) addition of activated carbon, and (5) filtration. The removal of acidic pollutants such as HCl, HF, and SO2 can occur in a Venturi reactor by means of adding a neutralizing sorbent such as hydrated lime (Ca(OH)2) or sodium bicarbonate (NaHCO3). Hydrated lime is widely used in all major air pollution control systems, because it is readily available and is much less expensive than NaHCO3. However, it is corrosive and must operate at low temperatures. The use of hydrated lime to remove acidic pollutants has also been reported to have relatively low conversion efficiency, mainly because of the short residence time encountered; therefore, the sorbent is required to be present in excess (see Yan et al.7). Moreover, such high usage of the sorbent results in the generation of more fly ash as waste. NaHCO3, on the other hand, is easier to handle and has

10.1021/ie060929d CCC: $37.00 © 2007 American Chemical Society Published on Web 03/20/2007

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2649

Figure 1. Schematic diagram of the combustion plant.

higher removal efficiency at a wide range of temperatures, and lower quantities of the sorbent are required. Liuzzo et al.8 also reported that NaHCO3 can partially reduce the amount of nitrogen oxides (NOx) in the flue gas through reaction with nitrogen dioxide (NO2) and the formation of sodium nitrate (NaNO3), which is a solid salt. The superior performance of NaHCO3 has not been fully explained in the literature; nevertheless, its reactivity can be attributed to the physical nature and chemical behavior of the sorbent. The reactions of the sorbent with the acidic gases involve a thermal activation stage, where NaHCO3 converts rapidly to sodium carbonate (Na2CO3), when brought into contact with the hot flue gases. Na2CO3 has a high specific surface area and porosity, which may explain its superior performance, when compared to hydrated lime. The reactions between calcium-based sorbents such as hydrated lime and limestone (CaCO3) with HCl and SO2 have been extensively investigated in the literature, with experimental results at the laboratory scale covering a wide range of operating conditions and systems.9-12 However, there are limited studies on the capture of acidic gases by NaHCO3. Karlsson et al.13 studied the reaction kinetics of HCl and Ca(OH)2 in the temperature range of 140-400 °C and proposed a first-order reaction, with respect to both reactants. HCl sorption studies using calcium- and sodium-based sorbents were also performed by Duo et al.14 Simulated gases with a composition similar to “air-blown” and “oxygen-blown” integrated gasification combined cycle (IGCC) fuel gases were used at 300-600 °C and Na-sorbents were considered to be more reliable for HCl removal. Verdone and De Filippis15 reported that, from a thermodynamic viewpoint, the use of sodium-based sorbents is preferred for removing both HCl and SO2 at low and high temperatures. In practice, the reactions of HCl and SO2 with the sorbents never occur alone. Other species such as CO2 and NOx will be present. Limited studies have been performed on the simultaneous absorption of HCl and SO2 by hydrated lime. Experimental work that was performed by Matsukata et al.,16 Chisholm

et al.,17 and, more recently, Chin et al.18 has shown that increasing the HCl concentration increases the SO2 absorption rates. Chin et al.18 investigated the behavior of the lime-HCl reaction in the presence of SO2, CO2, O2, and moisture. The reactions were performed in a thermogravimetric analyzer, and the reaction products were identified and quantified using X-ray diffraction (XRD). Reactions between lime and SO2 were observed to be slower than those between lime and HCl in a single-gas system. This behavior was attributed to the physical properties of the products formed, such as the solubility in water. Product crystals formed by lime reaction with HCl are more soluble than those produced with SO2. This paper reports on a study that investigates the process design of an industrial-scale EfW combustion plant that processes more than 250 000 tonnes of urban waste per year with energy recovery. The plant design was conducted at Germana` & Partners Consulting Engineers in Rome, Italy. This work also investigated the effect of replacing the conventional Ca(OH)2 sorbent with NaHCO3 in the removal of acidic pollutants from the flue gas. Mass and energy balances of the process were conducted using the proximate and ultimate analyses of the waste, and a simplified mathematical model has been applied to evaluate the efficiency of using both sorbents. The model simulates the reactions between the sorbents and the acidic gases in the Venturi reactor. Input design conditions and parameters of the plant have been incorporated into the model to predict the effect of the different controlling steps of the reactions on the conversion rate. The model also predicts the time required to neutralize and remove these pollutants and the number of recycle stages required for the removal process. The economic feasibility of using NaHCO3 instead of lime in the plant has also been determined. The work presented here is currently being developed to investigate the efficiency of energy recovery and flue gas treatment systems that use fluidized-bed combustion and gasification technologies, compared to traditional mass-burn incinerators. Overall, this study is part of the research program of EPSRC’s Sustainable Urban Environment (SUE) Waste Management

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Table 1. Conversion-Time Equations for the Sorbent-Gas Reactions reaction

expression Fixed-Size Particles for Ca(OH)2 Reactions

aFBR 3bMBkgCAg

diffusion through gas film

t ) xB θ

diffusion through ash layer

t ) 1 - 3(1 - xB)2/3 + 2(1 - xB) θ

chemical reaction

rc t ) 1 - ) 1 - (1 - xB)1/3 θ R

where

θ)

where

where

θ)

θ)

aFBR2 6bMBDeCAg

aFBR bMBksCAg

Variable-Size Particles for NaHCO3 Reactions diffusion through gas film

t ) 1 - (1 - xB)2/3 θ

where

θ)

yFBR2 2bMBDeCAg

chemical reaction

t ) 1 - (1 - xB)1/3 θ

where

θ)

aFBR bMBksCAg

Consortium. The work investigates the appropriate scales and technologies for energy recovery from waste by combustion and gasification to identify the most energy-efficient process designs based on energy conversion efficiencies, environmental impact, and economics. 2. Energy from Waste 2.1. The Industrial Case Study. The plant under investigation uses two stoker grate combustors that have been fitted with selective noncatalytic reduction (SNCR) systems in two process lines. Each process line is designed to treat 17.24 tonnes of MSW per hour and has a combined net power generation of 34 MWe, using a steam turbine. A schematic diagram of the waste treatment process is shown in Figure 1, in which the combustion units and the flue gas treatment systems are illustrated. The flue gas cleaning system in each process line of the plant consists primarily of a conditioning tower, a dry Venturi reactor, and fabric filters. Activated carbon is injected in the Venturi reactor to capture dioxins/furans and residual mercury from the flue gas, while hydrated lime neutralizes acidic gases such as HCl, SO2, and HF. A recycle loop is incorporated in the plant design to ensure maximum sorbent utilization by sending partially reacted material that has been collected by the bag filters to the boiler, as shown in Figure 1. This is a novel feature of the plant, which increases the thermal efficiency of the system as the excess air and oxygen content are significantly reduced. This reduction in excess air and oxygen content, in turn, reduces the formation of dioxins/furans. The plant is an example of a state-of-the-art combustion plant that has efficient energy recovery and gas cleanup systems while reducing waste to inert residues. Emissions from these plants pose no greater threat to the environment and/or human health than any other waste management activity, given appropriate management and regulation.6 2.2. The Model. The reaction between the sorbent and the flue gas is noncatalytic and heterogeneous. Hence, a simplified version of the unreacted-core model in the form of eq 1 was used.19

aA (gas) + bB (solid) f product (solid)

(1)

It is assumed that (i) the solid particle is spherical; (ii) the reactions are irreversible and first order, relative to A; and (iii)

isothermal conditions are maintained. The neutralization reactions for both NaHCO3 and Ca(OH)2 sorbents are as follows: For sodium bicarbonate (NaHCO3):

2NaHCO3 f Na2CO3 + CO2 + H2O

(2)

Na2CO3 + 2HCl f 2NaCl + CO2 + H2O

(3)

1 Na2CO3 + SO2 + O2 f Na2SO4 + CO2 2

(4)

For calcium hydroxide (Ca(OH)2):

Ca(OH)2 + 2HCl f CaCl2 + 2H2O

(5)

1 Ca(OH)2 + SO2 + O2 f CaSO4 + H2O 2

(6)

Two different cases for the reactions of Ca(OH)2 and NaHCO3 are considered. For the Ca(OH)2 reactions, it is assumed that the continuous formation of solid product and inert material, without flaking off the sorbent particles, would maintain a constant particle size. Hence, the reactions are modeled assuming fixed-size particles. For the NaHCO3 reactions, the particle size changes as the reaction progresses, because of the formation of gaseous products that flake off the solids. Hence, the reactions are modeled assuming variable-size particles. This is due to the physical and chemical nature of NaHCO3, as highlighted earlier. NaHCO3 decomposes to Na2CO3 when it is injected into the Venturi reactor and comes into contact with the hot flue gases. Na2CO3 reacts with the acidic gases and decomposes further, producing H2O and CO2 gases into the surrounding atmosphere. This creates a network of void spaces throughout the particle, which exposes fresh reactive sites and allows the acidic gases to diffuse through them. This, coupled with attrition between the sorbent particles, imposes a stress in the ash layer, which then detaches and flakes off the particle. For the fixed-size particles, three process steps are identified, which may control the overall reaction rate: diffusion through the gas film, diffusion through the ash layer, and chemical reaction. When no ash layer covers the unreacted core as the reaction progresses, the particle continues to shrink with time. Here, only two process steps may control the overall reaction rate. These are gas-film diffusion and chemical reaction. The rate equations used for the gas-sorbent reactions are summarized in Table 1. The term θ denotes the time (in seconds) required

Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2651 Table 2. Plant Design Conditions and Model Parameters property Flue Gas Properties temperature flow rate density viscosity

150 °C 44.58 m3/s 0.87 kg/m3 0.00002276 Pa s

Operating Parameters inlet SO2 flow rate 1.01 kmol/h inlet HCl flow rate 3.30 kmol/h particle diameter 120 µm Model Parameters ks,SO2 Ca(OH)2 Na2CO3 ks,SO2 Ca(OH)2 Na2CO3 De,SO2 Ca(OH)2 Na2CO3 kg,HCl Ca(OH)2 Na2CO3 ks,HCl Ca(OH)2 Na2CO3 De,HCl Ca(OH)2 Na2CO3

Table 3. Waste Composition

value

0.364 m/s 0.364 m/s 0.461 m/s 1.272 m/s 0.0000029 m2/s 0.000008 m2/s 0.476 m/s 0.476 m/s 1.328 m/s 4.193 m/s 0.0000038 m2/s 0.000012 m2/s

for complete conversion of unreacted particles to a product. The HCl and SO2 sorption occurs inside the Venturi reactor. The inlet flue gas properties and the flow rates of the acidic gases are summarized in Table 2, including the kinetic data used in the model. The diffusivities and the mass-transfer coefficients were obtained from previous work by De Nitto.20 These data were used for a large-scale EfW combustion plant currently in operation in Italy that also was designed by Germana` & Partners Consulting Engineers. The operating conditions, which include the temperature and pressure of the Venturi reactor, used in that EfW plant are identical to the operating conditions in the plant under investigation in this paper. The average particle diameter of both sorbents can be in the range of 15-300 µm. Because this part of the study only examines the modeling of HCl and SO2 sorption, a particle diameter of 120 µm is assumed for both sorbents (similar to that which was also used by De Nitto20). Other reported work in the literature21,22 that was conducted on laboratory-scale reactors have reported diffusivities in the range of 10-14-10-9 m2/s for gas-solid systems with reaction rates

Figure 2. Mass and energy balance of the combustion plant.

parameter

value

Proximate Analysis moisture combustibles inerts lower heating value, LHV

31.94 wt % 56.59 wt % 11.47 wt % 12.56 MJ/kg

Ultimate Analysis C H O N S Cl

56.69 wt % 7.92 wt % 32.43 wt % 1.38 wt % 0.35 wt % 1.24 wt %

of >10-3 m/s. Note that the values of the diffusivities used in the present work are 2 orders of magnitude bigger, because they refer to an industrial-scale plant. 3. Results and Discussion In this section, the mass and energy balances are performed using the proximate and ultimate analyses of the waste reported in Table 3. Results from applying the model using the conversion-time equations summarized in Table 1 are analyzed, and the results obtained from the economic analysis of applying Ca(OH)2 and NaHCO3 are reported. 3.1. Mass and Energy Balance. The proximate analysis shows the moisture content, combustibles, ash content, and lower heating value (LHV) of the waste. The ultimate analysis gives the elemental compositions of the waste (in dry wt %), in terms of carbon, hydrogen, and oxygen, as well as nitrogen, sulfur, and chlorine. Mass and energy balances were performed based on the compositions shown in Table 3, and the results are shown in Figure 2. Thirty four megawatts of electricity are generated from a thermal capacity of 120.2 MWth, with an energy efficiency of 28.3%. This is high for a medium- to large-scale MSW combustion plant, because the MSW is slightly pretreated. Approximately 183 000 Nm3/h of air, operating with 53% excess air, is fed into the combustion process. The amount of solid residues and exhaust gases generated from both process lines are 6.3 t/h and 216 000 Nm3/h, respectively. The ash and residues from the boiler and flue gas cleaning system comprise the fly ash stream, which is treated with binder materials and additives, hence rendering the contaminants immobile and nonleachable.

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Figure 3. Predicted effects of controlling steps on the conversion rate of (a) HCl using Ca(OH)2 and (b) SO2 using Ca(OH)2.

3.2. Model Predictions. The results obtained from the application of the model presented in the previous section to predict the conversion rates of HCl and SO2 using Ca(OH)2 and NaHCO3 are discussed in this section. Figures 3a and 3b describe the predicted effects of the controlling steps on the conversion rate when using Ca(OH)2 as the sorbent for HCl and SO2, respectively. Alternatively, the predicted effects of the controlling steps on the reaction rate when using NaHCO3 as the sorbent are shown in Figures 4a and 4b for HCl and SO2, respectively. Figures 3a and 3b show that, for both reactions of HCl and SO2 using Ca(OH)2, diffusion through the gas film controls the early stages of the conversion process, while this becomes

subsequently controlled by diffusion through the ash layer. The latter process step is accelerated by the formation of new product layers as the reaction progresses, hence preventing the reactant gas from reaching the unreacted core of hydrated lime. This is in agreement with experimental work performed by other researchers such as Weinell et al.,21 who also showed that the reactions of HCl with hydrated lime were also controlled by diffusion through the ash layer. Figures 4a and 4b show that the reactions between NaHCO3 and the acidic gases are entirely controlled by the chemical reaction step. The neutralization reaction slows after the sodium carbonate on the surface has reacted with HCl and SO2, due to pore blockage; nevertheless, the particle decomposes further and

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Figure 4. Predicted effects of controlling steps on the conversion rate of (a) HCl using NaHCO3 and (b) SO2 using NaHCO3.

H2O and CO2 gases evolve into the surrounding atmosphere. This creates a network of void spaces throughout the particle, which exposes fresh reactive sites and allows the acidic gases to diffuse through them again. Although the model assumes that only one process step at a time controls the overall reaction rate, in practice, all of the process steps may play their roles simultaneously and the relative importance of each process step may vary with the extent of the reaction. Diffusion through a gas film in a fixed-size particle case, for example, may control the early stages of the process, but it becomes less significant as the thickness of the product layer increases with time. In this regard, Duo et al.22 suggested that the rate-determining step for the reaction of a calciumbased sorbent with HCl is altered with extent of the reaction.

They found that the chemical reaction controls the early stages of the reaction, which is followed by the combined control of chemical reaction and diffusion through product layer, and then by a final stage where the reaction becomes entirely controlled by diffusion through the product layer. In Figure 5, the individual process steps are summarized to account for their simultaneous effects and, hence, obtain the overall time required for the conversion of HCl and SO2 using Ca(OH)2 and NaHCO3. Note here that, for graphical presentation only, conversion times up to 500 s are shown, while the full conversion times are reported in Table 4. The results show that the conversion times for HCl and SO2 using Ca(OH)2 are one order of magnitude greater than the conversion times using NaHCO3. This clearly demonstrates the superior efficiency of

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Figure 5. Predicted overall conversion rate of HCl and SO2 using NaHCO3 and Ca(OH)2. Table 4. Conversion Times and Number of Recycle Stages Conversion Time (s)

Number of Recycle Stages

sorbent

HCl

SO2

HCl

SO2

Ca(OH)2 NaHCO3

515 12

1960 95

78 2

296 14

NaHCO3 in removing the acidic gases from the flue gas stream, compared to the conventional Ca(OH)2. Other work reported in the literature has experimentally observed the incomplete maximum conversion of hydrated lime.10,11,22 In this regard, Duo et al.22 presented a “crystallization and fracture” model to explain this occurrence and reported that the reaction stops when the critical size of a nucleus becomes so large that the reaction kinetics no longer allow sufficient product molecules to gather and form the nucleus. Therefore, the departure of the experimental results from the linear formulation of the unreacted-core model is thus attributed to the model not taking into account the physical structure of the particles and pore blockages. The model will only formulate and project the full conversion of the reactants with time. Maximum sorption of HCl and SO2 does not occur in the Venturi reactor in just one pass. The dry Ca(OH)2 sorbent is injected into the reactor in excess and some of the unreacted material is recycled back to the reactor for greater utilization of the sorbent, as shown in Figure 1. In this study, the theoretical number of recycle stages required for the sorption or neutralization of 95% of the acidic gases and the time taken have been calculated for both Ca(OH)2 and NaHCO3. The results are shown in Table 4. The number of recycle stages is a function of the residence time, which, in turn, is dependent on the diameter of the Venturi reactor and the flue gas flow rate. The larger the reactor diameter, the longer the residence time and the fewer the number of recycle stages. The results reported in Table 4 have been calculated based on a reactor diameter of 5 m and a flue gas flow rate of 108 000 Nm3/h, with a residence time of ∼7 s. The results show that, for both types of reactions using Ca(OH)2 and NaHCO3, the sorption or removal of the acidic gases is far more efficient using NaHCO3 and requires fewer recycle stages than Ca(OH)2. Furthermore, it can also be observed that the conversion of SO2 requires considerably longer

times and many more recycle stages than HCl, because it has a greater tendency to cause pore blockage. A comparison of the results obtained from the industrial case study reported in this paper and the results reported in the literature for laboratory-scale experiments, such as those by Chin et al.,18 reveals a substantial difference in the predicted conversion times obtained. The conversion times obtained for the HCl sorption by Chin et al.18 was determined to be on the order of ∼8 h, which is contrary to conversion times of the order of magnitude of a few minutes (∼5 min), as shown in Figure 3a for the industrial case. The combustion plant investigated in this work has just been commissioned in Italy; thus, experimental data from the plant are not available for a direct comparison against the model predictions presented here. Thus, to validate the capability of the model used in this work to predict the conversion times at different conditions and scales, a test case was conducted where the conditions used in the experiments by Chin et al.18 were simulated and compared with the model predictions. Although the full operating conditions used to perform the experiment could not be exactly reproduced and taken into account in the model, the predicted conversion times were of the same order of magnitude as those determined experimentally, as shown in Figure 6. These results validate the ability of the model to predict the conversion times for different conditions and scales and they also highlight the importance of taking into account the shift of the ratedetermining step with the extent of reaction when scaling-up from laboratory scale to full, commercial scale. 3.3. Treatment Costs. The results obtained from the model and the proven higher efficiency of NaHCO3 over hydrated lime led us to examine the economic feasibility of using NaHCO3 in the combustion plant that has been investigated. This evaluation was conducted using standard chemical engineering calculations,23 in which mass and energy balances of the plant were used. The amount of sorbent required for the neutralization of the acidic pollutants, which are reported in Table 2, was calculated with 10% excess to account for any material losses. The plant was assumed to be in operation for 312 working days per year. The results reported in Table 5 show the amount of sorbents required per tonne of MSW and their cost. It is observed that 7.14 kg of NaHCO3 is required for the treatment

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Figure 6. Comparison between predicted and experimental conversion times for lime-HCl system, using data from Chin et al.18 Conditions: flow rate, 150 mL/min; HCl content, 540 ppm; O2 concentration, 9%. Table 5. Amount of Sorbents Required and Their Treatment Cost sorbent

amount (kg/tonne of waste)

cost (euros/tonne of waste)

Ca(OH)2 NaHCO3

63.51 7.14

5.27 1.43

In conclusion, although NaHCO3 is a more expensive sorbent, it is more efficient and economically a more attractive option than hydrated lime for removing acidic pollutants. Acknowledgment

of 1 tonne of waste and costs 1.43 euros, whereas 63.51 kg of Ca(OH)2 is needed for the same treatment and costs 5.27 euros. This demonstrates the attractive economic feasibility of using NaHCO3 instead of hydrated lime to remove acidic pollutants.

The authors wish to acknowledge the financial support from the Engineering and Physical Science Research Council (EPSRC), through Grant No. GR/S79626/01.

4. Conclusions

a ) stoichiometric coefficient of gaseous acids b ) stoichiometric coefficient of sorbent CAg ) concentration of gaseous acids in the gaseous bulk (kmol/ m3) CAs ) concentration of gaseous acids in the solid-gas surface (kmol/m3) dp ) particle diameter (m) De ) effective diffusivity (m2/s) kg ) mass-transfer coefficient (m/s) ks ) first-order rate constant (m/s) MB ) molar mass of sorbent (kg/kmol) R ) particle radius (m) t ) time (s) xB ) fractional conversion of sorbent

The use of municipal solid waste (MSW) to produce energy or fuel has an important role in an integrated and sustainable waste treatment system. It reduces our reliance on landfills and also is an alternative source of energy, which, by displacing fossil fuels, can help reduce greenhouse emissions and increase the share of renewables in power generation. Within this framework, the process design and efficiency of an energy-fromwaste (EfW) combustion plant scaled to treat 250 000 tonnes of MSW per year and generate 34 MWe of electricity were studied. The flue gas cleaning system of the plant was examined to investigate the effect of replacing the conventional calcium hydroxide (Ca(OH)2) sorbent with sodium bicarbonate (NaHCO3) in the removal of acidic pollutants from the flue gas. A simplified mathematical model was applied to evaluate both the efficiency and cost implications of using these sorbents. The results show that diffusion through the gas film controlled the early stages of the reactions of HCl and SO2 with Ca(OH)2. The process then becomes controlled by diffusion through the ash layer. On the other hand, reactions between NaHCO3 and the acidic gases are entirely controlled by the chemical reaction step. When the individual process steps were summed to account for their simultaneous effects, results showed that HCl and SO2 conversion times using Ca(OH) were one order of magnitude greater than the conversion times obtained when using NaHCO3. The treatment cost of using NaHCO3 in the plant was also calculated and was found to be more cost-effective than hydrated lime. Thus, selecting the most effective gas cleanup technology has a significant impact on the economics of the EfW plant.

Nomenclature

Greek Letters θ ) time required for complete conversion of unreacted particle into a product (s) F ) density (kg/m3) Note Added after ASAP Publication. The version of this paper that was published on the Web 3/20/2007 had an incorrect value in section 3.1. The corrected version was published on the Web 3/22/2007. Literature Cited (1) Eurostat. Waste Generated and Treated in Europe, 2003 Edition; European Communities: Luxembourg, Belgium, 2003. (2) Yassin, L.; Lettieri, P.; Simons, S. J. R.; Germana`, A. Energy recovery from thermal processing of waste: a review. In Engineering

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ReceiVed for reView July 17, 2006 ReVised manuscript receiVed October 27, 2006 Accepted February 16, 2007 IE060929D