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Vitrification of municipal solid waste incineration fly ash using biomass ash as additives a

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Moussa-Mallaye Alhadj-Mallah , Qunxing Huang , Xu Cai , Yong Chi & JianHua Yan a

State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, People's Republic of China Accepted author version posted online: 21 Aug 2014.Published online: 15 Sep 2014.

To cite this article: Moussa-Mallaye Alhadj-Mallah, Qunxing Huang, Xu Cai, Yong Chi & JianHua Yan (2014): Vitrification of municipal solid waste incineration fly ash using biomass ash as additives, Environmental Technology, DOI: 10.1080/09593330.2014.957245 To link to this article: http://dx.doi.org/10.1080/09593330.2014.957245

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Environmental Technology, 2014 http://dx.doi.org/10.1080/09593330.2014.957245

Vitrification of municipal solid waste incineration fly ash using biomass ash as additives Moussa-Mallaye Alhadj-Mallah, Qunxing Huang ∗ , Xu Cai, Yong Chi and JianHua Yan State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, People’s Republic of China

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(Received 3 March 2014; final version received 17 August 2014 ) Thermal melting is an energy-costing solution for stabilizing toxic fly ash discharged from the air pollution control system in the municipal solid waste incineration (MSWI) plant. In this paper, two different types of biomass ashes are used as additives to co-melt with the MSWI fly ash for reducing the melting temperature and energy cost. The effects of biomass ashes on the MSWI fly ash melting characteristics are investigated. A new mathematical model has been proposed to estimate the melting heat reduction based on the mass ratios of major ash components and measured melting temperature. Experimental and calculation results show that the melting temperatures for samples mixed with biomass ash are lower than those of the original MSWI fly ash and when the mass ratio of wood ash reaches 50%, the deformation temperature (DT), the softening, hemisphere temperature (HT) and fluid temperature (FT) are, respectively, reduced by 189°C, 207°C, 229°C, and 247°C. The melting heat of mixed ash samples ranges between 1650 and 2650 kJ/kg. When 50% wood ash is mixed, the melting heat is reduced by more than 700 kJ/kg for the samples studied in this paper. Therefore, for the vitrification treatment of the fly ash from MSW or other waste incineration plants, wood ash is a potential fluxing assistant. Keywords: vitrification; fly ash; biomass ash; fluxing assistant; melting heat

Introduction Incineration or waste-to-energy is considered to be one of the most efficient methods for municipal solid waste (MSW) treatment. Every year, a large quantity of toxic fly ash is discharged from the air pollution control system into different municipal solid waste incineration (MSWI) plants over the world.[1] The fly ashes collected from MSW power plants are usually finely sized and spherical shaped particles consisting of SiO2 , Al2 O3 , Fe2 O3 and alkali and earth alkali metal oxides (i.e. Na2 O, K2 O, CaO and MgO).[2] However, due to the presence of many toxic species, such as heavy metals and dioxins, the MSWI fly ash is classified as a hazardous material.[3] Currently, it usually ends up in two ways, landfilled or reused as secondary raw materials. Previous research has indicated that thermal melting treatment is one of the most efficient methods to convert fly ash into inert material with significantly reduced volume.[4–6] Moreover, when the fly ash is heated to its melting temperature, the contained organic species can be destroyed thoroughly and toxic heavy metals will be stabilized from leaching out.[7–10] The final inert products can be used either as an aggregate material for backfilling and road construction, or as a material for producing paving slab and roof tiles.[10–15] Many high temperature furnaces have been developed to melt incineration fly ash into non-toxic glassy slag, including surface melting furnaces, swirling-flow melting furnaces, rotary kiln melting furnaces and plasma melting furnaces.[16–20] However,

*Corresponding author. Email: [email protected] © 2014 Taylor & Francis

the high temperature melting treatment of fly ash costs a lot of extra energy. In the past few years, many researchers are working on fluxing assistants to decrease the melting temperature for saving energy cost. Park et al. [21] demonstrated that the MSWI fly ash can be vitrified at 1500°C for 30 min with the addition of around 5% of SiO2 . Dezhen et al. [22] reported that the vitrification temperature can be reduced to below 1000°C by adding B2 O3 , CaF2 and borax. Wang et al. added limestone powder to the MSWI fly ash and sewage sludge incineration ash and he found that the FT can be decreased by approximately 160°C for sewage sludge incineration ash, but there was no effect on the MSWI fly ash.[23] However, most of the previously used chemical additives are still expensive when large quantity of them is mixed with fly ash to reduce the melting temperature. In this paper, the ashes discharged from biomass combustion plants are used as a fluxing assistant to improve the melting behaviour of the MSWI fly ash and their co-melting characteristics are studied. The mineralogical, morphological and chemical properties of fly ash are analysed by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). A mathematical model is proposed to predict the melting heat of mixed ash samples. The main objective of this study is to evaluate the effects of different biomass combustion ashes on helping to decrease the melting temperatures of the MSWI fly ash for energy-saving purposes.

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Materials and methods The fly ash sample was collected at the bottom of the bag house filter from Hangzhou Jingjiang power plant, which is equipped with three 200 t/d incinerators and corresponding heat recovery units. The generated steam (3.85 MPa, 450°C) is supplied to two 3 MW turbines for generating electricity. The biomass ash samples used in this paper were, respectively, supplied by the Lanxi thermal power plant in Zhejiang province and the Shu qian Biomass power plant in Jiangsu province. These two plants are equipped with a similar circulating fluidized bed furnace for direct burning of biomass. The Lanxi thermal power plant is fuelled with mixed forest wastes which mainly consist of spruce barks. The Shu qian biomass plant burns agricultural waste and rich husks are the only fed fuels when our ash sample was collected. To evaluate the effect of biomass ash on the melting characteristics of the MSWI fly ash, the MSWI fly ash was mixed with biomass ash by mass ratios of 3:1, 2:1 and 1:1 respectively. Before co-melting, all samples were dried at 105°C for 1 h and analysed for the mass ratios of the major oxide species including SiO2 , Al2 O3 , Fe2 O3 , CaO, MgO, K2 O and Na2 O. Hereafter, the ash samples collected from biomass combustion plants are, respectively, referenced as wood ash and rice husks ash. The melting tests were carried out with an automatic ash fusion temperature analyser (Model 5E-AF4000). During the test, the DT, the softening (ST), HT and FT were identified from the change of cone shape of the samples according to the Chinese national standard method for determining the fusibility of ash (GB/T 219-2008). After melting, the morphologies of the sample were investigated by an X-ray diffractometer (XRD) (Rigaku Ultima III, Japan) using Cu Kα radiation (λ = 0.154178 nm) at 40 kV over the scanning range from 10° to 80° with a rate of 10° per minute. The micro structure of the melting products was examined by field emission scanning electron microscopy (FESEM, Model SIRION-100, FEI, USA). Results and discussion Melting behaviour of mixed fly ash samples The average approximate and ultimate analysis values of the burned waste and biomass are listed in Table 1. As

we can notice, the moisture and ash content of MSW are much higher than those of biomass. On the contrary, the received oxygen content and heating value of biomass are, respectively, three and five times higher than that of MSW. Although, the composition of the MSWI fly ash varies with the components of the burned waste and incineration system, previous studies have indicated that SiO2 , Al2 O3 , Fe2 O3 , CaO, MgO, K2 O and Na2 O are the major oxides.[24] Table 2 shows the composition of the raw ash samples. As expected, SiO2 and Al2 O3 are the most abundant compounds for the MSWI fly ash. The Fe2 O3 was originated from different types of metal scraps. The composition of biomass ash is totally different. When lignin-based wood wastes, mostly spruce barks, were burned, the ash is rich in CaO and SiO2 , along with a large quantity of alkali species. On the contrary, the rice husks mainly consist of cellulose and semi-cellulose and when they were burned in a circulating fluidized bed incinerator their ash is similar to the MSWI fly ash and is also dominated by SiO2 . The melting characteristics of different ash samples mainly depend on the acid and alkali oxides. Previous research suggests different slagging factors to predict the melting tendency according to the mass ratio of major oxides [25,26] and the three widely used factors are listed in Table 3 along with the values for the ash samples discussed in this paper. According to the recommended slagging criteria, the MSWI fly ash sample has the lightest tendency to melt associated with the highest melting temperature. On the contrary, the wood ash is expected to have the lowest melting temperature. The exception is the rice husks ash, which has inconsistent melting tendency.

Table 2. Composition of MSWI fly ash and biomass combustion ash (wt%). Components SiO2 Al2 O3 Fe2 O3 CaO MgO K2 O Na2 O

MSWI fly ash

Rice husks ash

Wood ash

52.65 28.45 8.54 3.25 6.2 0.86 0.48

76.52 5.58 2.38 3.81 7.42 4.11 0.91

27.59 6.10 2.34 37.32 6.97 17.39 2.247

Table 1. Proximate and ultimate analysis of MSW and biomass (as received, wt%).

Samples

Moisture

MSWd Wood Rice husks a b c d

VM: volatile matter. FC: fixed carbon. LHV: lower heating value, MJ/kg. Average values.

59.6 4.7 6.5

Proximate analysis Ash VMa 15.5 6.1 11.4

21.6 73.5 65.7

FCb

LHVc

C

3.4 15.7 16.3

5.1 18.4 15.8

14.6 49.6 41.1

H

Ultimate analysis O N

2.1 6.1 5

7.6 38.2 37.1

0.4 0.4 0.5

S 0.09 0.03 0.07

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Environmental Technology Table 3.

Melting tendency factor for ash samples.

Melting tendency factor SiO2 SiO2 +CaO + MgO + Fe2 O3

0.74

SiO2 Al2 O3

1.85

CaO + MgO + Fe2 O3 +Na2 O+K2 O SiO2 +Al2 O3

0.24

a

Rice husks ash 0.85

13.7

0.23

Wood ash

Criteriaa

0.37

> 0.72(L) 0.65–0.72(M) < 0.65(S)

4.52

< 1.87(L) 2.65–1.87(M) > 2.65(S)

1.97

< 0.4(L) 0.4–0.7(M) > 0.7(S)

(L) light; (M) medium, and (S) strong.

1450

1400

1350

Temperature (°C)

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MSWI fly ash

1300

1250

1200

1150

W MS

sh sh sh sh sh sh sh sh sa sa sa sa da da da da oo oo oo oo u sk u sk u sk u sk h h h h W W W W ce ce ce ce % % % Ri Ri Ri Ri 33 50 25 % % % 5 0 3 2 5 3 ash

Figure 1. Measured melting temperature for the pure and mixed ash samples.

According to the suggestion of the previous research, all influence factors should be taken into account for assessing the melting character.[27] Figure 1 shows the measured melting temperatures for the pure and mixed ash samples along with the definition of the temperature during the analysis. The temperatures, especially the flowing temperature for the pure MSWI fly ash, are dispersed and much higher than those for the other ash samples. The four temperatures for the pure rice husks ash are also dispersed and the temperature difference between DT and FT is over 120°C indicating that the compositions are dominated by one or two species whose heat capacity changes with respect to temperature. On the contrary, for the wood ash, the four temperatures are very close to each other, demonstrating the feature of forming polycrystalline. It should be noted

that, although according to the slagging tendency factors in Table 3, pure wood ash should have a relatively lower melting temperature, the measured values are higher than 1300°C. It may be that the wood ash sample was dominated by alkaline components which inhibited melting in an inert environment. When the MSWI fly ash is mixed with rice husks ash, the melting character is proportional to their linear combination due to their similar composition. Compared with the pure MSWI fly ash, the DTs have decreased by 89°C, 99°C and 158°C, respectively, when 25%, 33% and 50% of rich husks ash was mixed. The flow temperature decreased around 100°C. The difference between DT and FT is in the range of 70–148°C. When the mass ratio of rice husks ash reaches 50%, four melting temperatures dispersed very strongly suggesting a potential chemical

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(b)

(c)

1100 °C

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1300 °C

Figure 2. sample.

SEM images of molten (a) original MSWI fly ash sample, (b) 50% rice husks ash mixed sample and (c) 50% wood ash mixed

reaction. The effect of wood ash on the melting behaviour of the MSWI fly ash is totally different and the melting temperatures have decreased significantly. When 50% of wood ash was mixed, the DT, ST, HT and FT have been reduced by 189°C, 207°C, 229°C, and 247°C, respectively. It suggests that after mixing the acid component in the MSWI fly ash, it reacted with the alkaline components contained in wood ash and new polycrystalline species were formed during the heating process. The SEM pictures for typical melted ash samples are depicted in Figure 2. As the temperature was increased from 1100 to 1300°C, the morphology of all mixed ash samples was changed from porous powder to a solid glassy material. For the pure MSWI fly ash, when the sample was treated at 1300°C, a few irregular particles with various shapes in the surface can be observed. These irregular particles indicate that the temperature is not high enough for the crystals contained in the ash powder to transfer into a uniform glassy phase. Compared with the pure MSWI fly ash, the microstructure of samples mixed with rice husks ash is similar but becomes smoother when treated at 1300°C. As for the wood ash mixed sample, the morphology is totally different. It exhibited a glassy and amorphous structure even at 1100°C and when the temperature increases to 1300°C, it becomes totally uniform and no structure can be observed. XRD analysis was carried out to understand the crystalline composition and to get an insight into the transferring characteristics of major components when biomass ash was added. The result shows a significant phase transition during the melting process when the temperature was increased. The main crystalline phases for the MSWI fly ash melted at 1100°C were quartz (SiO2 ) and two polycrystalline components (Ca0.68 Na0.32 )(Al1.68 Si0.32 )Si2 O8 and mullite (Al4.984 Si1.016 O9.508 ). Although the temperature

was increased, these species remained except a small portion of quartz was melted. When the MSWI fly ash was mixed with rice husks ash, the peak of another crystal components diopside Ca(Mg,Al)(Si,Al)2 O6 appeared. However, the peaks of the main crystal silicon species all disappeared when the mixed samples were heated at 1300°C indicating that the sample was fully melted or being well covered by melted glass. Diopside ferroan (Mg0.95 Fe0.052 CaMg0.028 Si2 O6 ) was another polycrystalline component that appeared when the MSWI fly ash was mixed with wood ash and the peak disappeared with molten silicon components as shown in Figure 3.

Melting heat predicting model Energy consumption is one of the major obstacles restricting commercial application of the vitrification technologies for the fly ash treatment. Although many researches have concerned with reducing melting temperature by using different fluxing assistant additives, currently, there is no available model for predicting the melting energy cost. Because the composition of the fly ash fluctuates with the composition of wastes, incinerator types and combustion conditions, it is complicated and sometimes even impossible to rely on experimental tests to determine its melting energy cost. It is essential to establish a quantitative forecasting model for predicting the melting heat and such model can also be used for optimizing other direct melting treatment system.[28] The energy required for ash melting includes the sensible heat Qs that increases the temperature of ash and the fusion heat Qf that changes the phase of ash from solid particle to liquid. As a mixture, the sensible heat of ash can be

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(a)

Figure 3.

(b)

(c)

XRD of molten (a) original MSWI fly ash sample, (b) 50% rice husks ash mixed sample and (c) 50% wood ash mixed sample.

deduced from the heat capacity of the major components Qs =

M  i=1

xi · Qi =

M  i=1

 xi ·

T1

cpi dT

(1)

T0

M is the number of components with a mass ratio over 0.1% and in this paper, seven major acid and alkali oxide species are considered and are listed in Table 2; xi is the mass ratio and cpi is the specific heat capacity of component i. As cpi varies with the temperature, the NIST fitting equation with respect to temperature is used in this paper[29] cpi = A + B · t + C · t2 + D · t2 + E · t−2 , t=

T , T ∈ [T0 , T1 ] . 1000

(2)

The coefficients for the major components may change with the temperature and can be obtained from the chemical engineering handbook. As a multicomponent mixture, the acid and alkali oxide species may form a polycrystalline component during the heating process. The melting temperature and fusion heat of polycrystalline are very different with those of the pure components. Unfortunately, there is no efficient mathematical method to calculate the fusion heat of the polycrystalline component because its chemical structure is very complex and depends on many factors. In this paper, to provide a quantitative estimation of the energy cost for ash melting, the following empirical model is proposed to calculate the overall energy consumption for vitrifying the fly

ash samples Qf =

M 

xi · HiT1 =

i=1

M 

  T0 xi · Hi0 − QTi1 .

(3)

i=1

Here, Hi0 is the fusion heat of the pure component i at T0

melting temperature Ti0 . Term QTi1 in the bracket is used to account for the fusion heat variation due to the changed melting temperature. It has different forms under different conditions ⎧ T0 0 i ⎪ ⎪ T1 cpi dT, T1 < Ti , ⎪ ⎨ 0

T0 T − T1 T0 QTi1 = Hi0 · i 0 T1 < Ti0 and T1i cpi dT > Hi0 , ⎪ Ti ⎪ ⎪ ⎩ T1 − T0 cl, pi dT, T1 > Ti0 . i (4) Here, cl,pi is the specific heat capacity of the liquid phase. After the mass ratios of all major components are known, the melting energy consumed for heating the mixed samples from ambient temperature to its melting phase can be deduced as follows:  T1 M  Ti0 0 xi · Hi − QT1 + cpi dT . Q = Qs + Qf = i=1

T0

(5) As mentioned above, the melting characteristics of fly ash depend on the composition and heating condition. With the proposed model, we can make an empirical and quantitative estimation of the melting energy consumption at different temperatures. Figure 4 shows the specific melting heat for the MSWI fly ash when it was, respectively, mixed with rice husks ash

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Melting Heat (kJ/kg)

2600 2400

2600

0 20% 40% 60% 80% 100%

2400 Melting heat (kJ/kg)

(a) 2800

2200 2000 1800

2200

2000

1600

1800 1100

1150

1200

1250

1300

1350

1400

(b) 2600 2400 Melting Heat (kJ/kg)

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Temperature (°C)

2200

0 20% 40% 60% 80% 100%

Figure 5. samples.

2000

1800

1600 1100

sh sh sh sh sh ash ash ash ash sa sa sa sa ya ood Wood Wood Wood I fl usk usk usk usk h h h h W W e e e e MS Ric Ric Ric Ric 33% 50% 25% 25% 50% 33%

1150

1200

1250

1300

1350

1400

Temperature (°C)

Figure 4. Melting heat of MSWI fly ash mixed with (a) rice husks ash and (b) wood ash under different temperatures.

and wood ash. The averaged heat capacities for the MSWI fly ash, rice husks ash and wood ash are 2.55, 3.05, and 1.56 (kJ/kg K). The overall energy cost ranges from 1650 to 2650 kJ/kg when the melting temperature was increased from 1100 to 1400°C. These values are consistent with the data reported in the literature.[24] When mixed with rice husks ash, the required melting heat will increase at the same melting temperature and the augmentation increases with the mass ratio of rice husks ash. That is because the specific heat capacity of the rice husks ash is larger than that of the MSWI fly ash at the same temperature. On the contrary, as the heat capacity of wood ash is smaller than that of the MSWI fly ash, the melting heat required for the mixed sample at high temperature is smaller than that of the pure MSWI fly ash and there is a temperature at which their melting heat equals. The melting heat plotted in Figure 4 indicated that if the melting temperature can be decreased by using additives to form polycrystalline species, the energy cost can be reduced significantly. For the MSWI fly ash sample discussed in this paper, Figure 5 shows the calculated heat consumption with the measured melting temperature in Figure 1. Due to the decreased flowing temperature, the melting heat for all the mixed samples is lower than that of

Calculated melting heat for the original and mixed ash

the pure MSWI fly ash. But for the case of rice husks ash, the reduction is relatively small because of their similar composition and when its mass ratio is larger than onethird, the melting heat reduction is about 300 kJ/kg which stops changing with further increase in the mass ratio of rice husks ash. For the wood ash, though the pure melting heat of wood ash is higher than that of rice husks ash, its effect on the MSWI fly ash melting is dramatic and it reduced the melting heat more than 400 kJ/kg when only 25% of wood ash was mixed. Moreover, the melting heat of the mixed sample keeps reducing if the mass ratio of wood ash increases, and when the mass ratio reached 50%, the melting heat is below 1900 kJ/kg. Therefore, for the thermal vitrification treatment of the fly ash discharged from MSW or other waste incineration system, wood ash is an efficient additive for reducing the energy cost.

Conclusions The fly ash discharged from the MSWI system contains high concentration of toxic species. Current widely used cement solidification and landfilling practice may occupy large volume of the land resource and cause possible water/soil pollution due to long time leakage. Fly ash vitrification can maximally reduce the volume and produce hard slag which can be used as a construction material. To reduce the energy cost during melting, the wood and rice husks ashes were used as additives in this paper and the effect of biomass ash on the MSWI fly ash melting characteristics has been discussed. To quantitatively estimate the melting heat reduction, a new mathematical model has been proposed based on the mass ratios of major oxide species. Melting tests show that the melting temperatures of the MSWI fly ash can be reduced when rice husks or wood ash is mixed. Due to the high concentration of alkali species, the effect of wood ash is much stronger than rice husk ash and when wood ash is mixed by 50%, the melting

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flow temperature can be reduced from 1426°C to below 1179°C because the alkaline species contained in wood ash would react with the acid components to form the polycrystalline component during the heating process. By taking the sensible heat and fusion heat into consideration, the model predicts that the melting heat of the pure or mixed ash samples ranges from 1650 to 2650 kJ/kg between 1100°C and 1400°C. When wood ash is half-to-half mixed with the MSW fly ash, the melting heat of the mixture is below 1900 kJ/kg, which is about 700 kJ/kg lower than that of the pure MSWI fly ash. Therefore, for the vitrification treatment of the fly ash or other waste incineration residues, wood ash is a very promising additive. Acknowledgements Acknowledgment is gratefully extended to National Basic Research Program of China 973 Program (grant no. 2011CB201500), National Science & Technology Pillar Program (grant no. 2012BAB09B03), National High Technology Program (grant no. 2012AA063505), the Program of Introducing Talents of Discipline to University (grant no. B08026), and National Science Funding of Zhejiang Province (grant no. LY13E060003) for their financial support.

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Vitrification of municipal solid waste incineration fly ash using biomass ash as additives.

Thermal melting is an energy-costing solution for stabilizing toxic fly ash discharged from the air pollution control system in the municipal solid wa...
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