Scholarly article on topic 'Development of a Process for Aqueous Mineral Carbonation on Municipal Solid Waste Incinerator Bottom Ash with Recovery of Useful Chemicals'

Development of a Process for Aqueous Mineral Carbonation on Municipal Solid Waste Incinerator Bottom Ash with Recovery of Useful Chemicals Academic research paper on "Materials engineering"

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{CCS / "Mineral carbonation" / MSWI / "Bottom ash" / "Sodium bicarbonate" / "Potassium chloride" / "Cement raw material"}

Abstract of research paper on Materials engineering, author of scientific article — Shuji Hamano, Satoshi Okumura, Tsunehira Yamamoto, Mamoru Kondo

Abstract We propose a cost-effective aqueous mineral carbonation process for municipal solid waste incinerator (MSWI) with using CO2 captured from MSWI flue gas. In this process, sodium bicarbonate (NaHCO3) and potassium chloride (KCl) are recovered. The former can be used as a neutralizing agent for acid gas treatment in the MSWI, which can contribute to reducing the operating cost of MSWI and also reducing the cost of the CO2 fixation to the bottom ash. First, the basis of a new mineral carbonation process was designed, and the system continuity was investigated by equilibrium simulations with data from salt crystallization tests. Then, the carbonation/extraction tests on bottom ash were performed. In the result, the amount of CO2 fixed to ash is 0.12 tonnes CO2 per tonne of the ash, the dissolved rates of Na, K and Cl were 48, 27 and 95%, respectively. Though this process can sequester relatively small amount of CO2, which is 2,880 tonnes CO2 per year for the MSWI of the size of 230 thousand tonnes of waste per year, the cost of the process was calculated to be US$ 58 (JPY 4,600) per tonne CO2 avoided, while the produced useful chemical, NaHCO3, might compensate the cost of the MSWI operations.

Academic research paper on topic "Development of a Process for Aqueous Mineral Carbonation on Municipal Solid Waste Incinerator Bottom Ash with Recovery of Useful Chemicals"

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Energy Procedia 37 (2013) 6696 - 6703

GHGT-11

Development of a process for aqueous mineral carbonation on municipal solid waste incinerator bottom ash with recovery of useful chemicals

Shuji Hamanoa*, Satoshi Okumuraa, Tsunehira Yamamotoa, Mamoru Kondob

aHitachi Zosen Corporation, Business & Product Development Center, Osaka 559-8559, Japan bHitachi Zosen Corporation, Environmental EPC Business Unit, Osaka 559-8559, Japan

Abstract

We propose a cost-effective aqueous mineral carbonation process for municipal solid waste incinerator (MSWI) with using CO2 captured from MSWI flue gas. In this process, sodium bicarbonate (NaHCO3) and potassium chloride (KCl) are recovered. The former can be used as a neutralizing agent for acid gas treatment in the MSWI, which can contribute to reducing the operating cost of MSWI and also reducing the cost of the CO2 fixation to the bottom ash. First, the basis of a new mineral carbonation process was designed, and the system continuity was investigated by equilibrium simulations with data from salt crystallization tests. Then, the carbonation/extraction tests on bottom ash were performed. In the result, the amount of CO2 fixed to ash is 0.12 tonnes CO2 per tonne of the ash, the dissolved rates of Na, K and Cl were 48, 27 and 95 %, respectively. Though this process can sequester relatively small amount of CO2, which is 2,880 tonnes CO2 per year for the MSWI of the size of 230 thousand tonnes of waste per year, the cost of the process was calculated to be US$ 58 (JPY 4,600) per tonne CO2 avoided, while the produced useful chemical, NaHCO3, might compensate the cost of the MSWI operations.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: CCS; mineral carbonation; MSWI; bottom ash; sodium bicarbonate; potassium chloride; cement raw material

1. Introduction

The development of carbon capture and storage (CCS) technology has advanced worldwide as a way to sequester a large amount of anthropogenic carbon dioxide (CO2). In 2005 Intergovernmental Panel on

* Corresponding author. Tel.: +81-6-6569-0196; fax: +81-6-6569-0197. E-mail address: hamano_s@hitachizosen.co.jp.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi: 10. 1016/j .egypro .2013.06.602

Climate Change (IPCC) released "Special Report on Carbon dioxide Capture and Storage (SRCCS) [1]", and in 2006 Ministry of Economy, Trade and Industry, Japan (METI) also released "CCS2020 [2]", the first Japanese political document about CCS, which showed domestic underground storage potential and the target cost in the future.

Table 1 shows the amount of CO2 emissions from burning facilities in Japan. The total CO2 emission amount of MSWI is 41 million tonnes of CO2 per year (generally showed as "14 million tonnes of CO2 per year" because the rest of the CO2 derived from biogenic sources is not counted as an incremental GHG emission), however, applying CCS technologies commercially for smaller plants such as MSWI has not been proceeded yet. One of the reason is from their higher unit cost which typically depends on the scale of the plant.

Table 1. CO2 emissions from different types of burning facilities in Japan [3][4]

Facility type Total amount (thousand tonnes of CO2 per year, 1999) From an average scale facility (thousand tonnes of CO2 per year) From a large scale facility (thousand tonnes of CO2 per year)

Coal-fired power plants 140,500 2,300 10,300

Oil-fired power plants 62,200 360 9,200

Gas-fired power plants 102,000 880 4,800

Integrated steelworks 131,500 3,100 3,400

Cement plants 32,500 1,700 3,500

Municipal solid waste incinerators (MSWI) 41,000 a 44 b 450 b

a. Data in 2004. 14,335 thousand tonnes derived from fossil fuel and 26,700 from biogenic sources [4].

b. The author calculated as 144 tonnes of waste per day for an average scale facility and 1,500 tonnes of waste per day for a large scale one, and 300 days operations per year for each.

As a first step in this development, an aqueous mineral carbonation technology for MSWI was discussed. Mineral carbonation, one of the CCS technologies, is a process in which magnesium-rich or calcium-rich minerals, such as serpentine or olivine rocks, react with CO2 at temperature 100-200 °C and pressure below 20 MPa to form geologically stable mineral carbonates which might be disposed of or used for applications such as roadway materials [1]. There are two big advantages for its cost and energy in using MSWI bottom ash. One is that it can be easily react with CO2 in milder condition, the other is that there is no need to mine and transport the materials.

However, while in aqueous condition, soluble salts are released much more from MSWI ash than from other rock materials, so it must be needed a low cost way to process salty solution. Evaporative drying method might be unrealistic, because of not only high energy consumption, but the difficulties of selling off as mixed salts with reasonable cost.

In this paper, we propose a cost-effective aqueous mineral carbonation process in which useful chemicals are recovered with using CO2 captured from MSWI flue gas. In this process, sodium and potassium in reaction solution are separately recovered as sodium bicarbonate (NaHCO3) and potassium chloride (KCl). NaHCO3 can be used for acid gas neutralization at dry or semi-dry flue gas treatment (FGT) processes in MSWI [5], so there might be no need to purchase extraneous alkaline agents such as NaHCO3 or calcium hydroxide, and this leads to reducing the operating costs of the MSWI.

2. Process design

This process is separated into two main parts: the carbonation/extraction reactor and the CO2 absorbing tower (Fig. 1). The salt solution with buffering ability containing 1-2 mol carbonate/bicarbonate ions is circulated among these parts. Other main ions in the solution are sodium, potassium and chloride. While bottom ash contacts with the solution, salts in bottom ash are extracted and simultaneously carbonate/bicarbonate ions react with calcium oxide (CaO) and magnesium oxide (MgO) in the bottom ash, then each carbonates are generated with increasing pH of the solution.

The increased level of pH depends on bicarbonate ion concentration in the solution and reactive calcium and magnesium concentration in the ash. The solution increased pH up to 11-11.5, then reacts with CO2 in the absorbing tower with pH decreasing to 10, and is recirculated to carbonation/extraction reactor. The salts extracted from ash are recovered as KCl and NaHCO3 by cooling crystallization of the solution. Since NaHCO3 consists of sodium ion and bicarbonate ion, with or without the crystallization depends a great deal on pH of the solution (bicarbonate ion concentration is as a function of pH). We can prevent NaHCO3 crystallization by increasing pH of the mother solution, consequently, KCl and NaHCO3 can be crystallized out separately by pH swing of the salt solution. In addition, KCl crystallization is basically independent on pH of the solution, so KCl must be crystallized at the lowest temperature in this system. The control parameter of this system is as follows: i) Temperature of cooling crystallizer for KCl, ii) Temperature of cooling crystallizer for NaHCO3, iii) pH of the solution after CO2 adsorption tower, iv) Balance of K/Cl molar.

Bottom ash

_1 60-80 °C

Carbonation/ Extraction

(crystallized at 30 °C) pH 11.5 f

Flue gas (CO2: 10 vol.%)

55-75 °C

Ash Residue

(CO2 fixed, low Cl conc.)

Solution circulated I CO2 absorption

pH 10.0

NaHCO3 (crystallized at 35 °C)

Low CO2 gas (CO2 < 2 vol.%)

Fig. 1. Schematic drawing for the proposed mineral carbonation process in MSWI

The equilibrium simulations were performed with data from some salt crystallization tests using commercially available reagents. The results are shown in Fig. 2. We can see that the appropriate operating condition area in which only KCl can be crystallized is present enough (economically inefficient in the other condition areas).

The process in more detail is illustrated in Fig. 3. MSWI Bottom ash contacts with salt solution in extraction/carbonation reactor in 15-30 minutes at 60-80 °C. This temperature is determined as the highest temperature in this system to avoid generating blockages since most of calcium compounds have lower solubility at higher temperature. Ash residues after carbonation/extraction reaction and crystallized compounds are rinsed, and the rinse solution is returned back to the process. Heavy metals are unlikely to present as high concentration in this solution in which the pH value is 10-11, and consequently most of them are immobilized and discharged with ash residues. Depending on the conditions, the crystallizer for KCl might be designed smaller while more KCl can be crystallized by lower temperature and excess solution would be bypassed. It should be noted that the proposed process might generate no wastewater on the grounds that main salts are recovered and trace metals are discharged along with ash residues.

Control pH of absorber outlet 9.5

Control pH of absorber outlet 10.0

a § 10 H

KCl precipitation 1

atNaHCO3 crystallizer -

- / KCl only (appropriate condition) / o . §

- + KHCO3 - o o M o

+ Na2CO3 J

......................- + KHCO3+Na2CO3 / e H

1 /+ KHCO3 + NajCO3 / +NaHCO3 / 3

30 35 40 45 50 55 Temperature ofNaHCO crystallizer (°C)

30 35 40 45 50 55 Tempeaature ofNaHC O crystallizer (°C)

Fig. 2. The simulation results of crystallized compounds at steady state in the KCl crystallizer

Washed residues to landfills or cement raw material

Fig. 3. Block diagram of this system which is controlled temperature from 25 to 80°C at normal pressure

3. Experimental

3.1. Materials

The amounts of CO2 fixed and salt extracted were investigated by the laboratory scale test using bottom ash collected from stoker type MSWI plant in Japan. Specifically, it was collected after cooling equipment using a small amount of sprayed water (no drainage), a sieving machine for removing large blocks and a magnetic separator. Table 2 shows the elemental composition of the bottom ash. For the tests, we used smaller than the 0.106 mm sieve size, containing a somewhat higher concentration of salts than upper particle size.

Table 2. Elemental composition of bottom ash on a dry weight basis.

Particle size < 0.106 mm < 5 mm

Particle size fraction (wt.% dry) 15.0 81.7

Na (wt.% dry) 2.3 2.1

K (wt.% dry) 2.4 1.3

Ca (wt.% dry) 15.0 16.5

Mg (wt.% dry) 1.6 1.5

Cl (wt.% dry) 3.7 1.3

Total-C (wt.% dry) 2.1 1.3

3.2. Methodology

The examination process is shown schematically in Fig. 4. Batch carbonation tests were performed using the sieved ash and salt solution (Na 1.4, K 5.5, Cl 2.0, HCO3" 1.7, CO32" 1.7 mol/kg H2O). The solution had been verified in advance that NaHCO3 and KCl could be crystallized out separately using control of pH and temperature. The tests imitated the carbonation/extraction reactor were performed at 5.0 L/S ratio, 0.5 h of time and 60 °C with magnetic stirring. After the reaction, the filtered residue was rinsed by 25 °C and 200 ml water at five times. Finally, concentrations of Na, K, Cl and total carbons (Total C) in the raw ash and the washed ones were examined.

5 times water washing

water washing)

Fig. 4. Experimental scheme for aqueous mineral carbonation on MSWI bottom ash. Salt solution composition: Na 1.5, K 5.5, Cl 2.3, HCO3- 1.1 and CO32- 1.1 mol/kg H2O.

4. Results and discussion

4.1. Amount of CO2 fixed

Total C concentrations of bottom ash and treated one are shown in fig. 5(a). The 70% of total C in raw bottom ash was specified as CaCO3 or MgCO3 by the CO2 decreasing in 590-670 °C at TG-MS analysis. By the salt solution treatment, the total C concentration increased from 2.1 to 5.3 wt.%, or the amount of

fixed CO2 is 0.12 tonnes of CO2 per tonnes of the raw bottom ash. It can be estimated that a sequestration capacity is approximately 1.2% of the CO2 gas emissions of the MSWI, which is some equal level of another method (weathering) for bottom ash carbonation [7].

4.2. Amount of salts extracted

The extraction ratio of Na, K and Cl are shown in fig. 5(b). They are 48, 27 and 95 % of the initial amount of bottom ash by using the salt solution. This extractable Na might be only 33% of the requisite amount as neutralizing agent (sodium bicarbonate), we performed cost evaluation of the process using not only bottom ash but with fly ash for covering the shortage of Na recovery (the cost evaluations are described later). In addition, K extraction ratio is much lower than that of Cl, the molar K/Cl ratio of these extract amounts is 0.25 although the molar amounts of K and Cl in the bottom ash is nearly equal. The reason for the low extraction ratio of K may be that the forms of K compounds in ash are glass or ceramic like. This process was designed that the emission form of Cl is only the compound "KCl", so it is found that the process needs to make up K source such as Potassium hydroxide (KOH) as much as the molar deficiency of K. On the contrary, the Cl extraction ratio by salt solution is so high and the residual Cl concentration can be become less than 0.2 wt.%, so the treated ash might have an advantage for the utilization as cement raw materials.

Bottom ash

After treatment (Water, 60°c)

After treatment (Salt solution,

(a) Carbon concentrations of bottom ash and treated ones

(b) Extraction ratios of Na, K and Cl from bottom ashes by water and salt solution washing

Fig. 5. The results of laboratory tests on bottom ash treatment

4.3. Cost evaluation

The sequestration costs of the process applied to MSWI plant were roughly calculated. The numerical parameters of an 800 tonnes per day MSWI are shown in Table 3. They and the elemental composition data (shown in Table 2) of the fraction less than 5 mm were used in the calculations. NaHCO3 generated from the process would be used as neutralizing agent instead of the common use agent, high reactive Ca(OH)2 (US$ 0.56 per kg, JPY 80 to US$). The saving cost of unused high reactive Ca(OH)2 was considered as a profit.

The estimated two cases are shown in figure 6. One is that only bottom ash is used in the process, the other is that bottom ash and 55% of the fly ash is used. In the latter, all the requisite amount of NaHCO3 as neutralizing agent in the MSWI can be generated in the proposed process. Compared to using only bottom ash, the latter process has 19% more throughput and but only 5% more amount of the fixed CO2 because fly ash has less concentration of reactive Ca than bottom ash. The disposal costs are considered

as landfill in Japan. The amounts of sequestered CO2 were 2,740 and 2,880 tonnes CO2 per year (2,040 and 2,110 tonnes CO2 avoided per year), respectively.

Table 3. Numerical parameters of an MSWI for cost evaluations

Parameters MSWI

Municipal solid waste throughput 230,000 t/y

(800 t/d)

Generating amount of bottom ash 5.3 t/h

Amount of bottom ash used for the proposed process (< 5mm) 3.3 t/h Generating amount of fly ash

- In the case of high reactive Ca(OH)2 (common use in Japan) used as 1.18 t/h neutralizing agent, whose stoichiometry for acid gas neutralization is 2.0

- In the case of NaHCO3 used as neutralizing agent, whose stoichiometry for acid 0.88 t/h gas neutralization is 1.0

Price of high-reactive Ca(OH)2 US$ 0.56 /kg

MSWI (in existence) MSWI (in existence)

(a) Bottom ash (b) Bottom ash + 55% of fly ash

Fig. 6. Two schemes for cost calculations which are (a) Bottom ash only and (b) Bottom ash and 55% of the fly ash. In the (a) scheme, the flue gas treatment in MSWI needs additional NaHCO3 to achieve stoichiometrically enough acid gas neutralization because of the Na shortage. In the (b) scheme, all the necessary amount of NaHCO3 can be produced by using 55% of fly ash while the Na extraction rate from fly ash is 80%.

Table 4 gives the result of cost calculations of the proposed process. These are the sequestration costs consisted of depreciation costs and operating costs. The depreciation costs were determined by linear depreciation of the fixed capital investments over a period of 15 years. The costs of the process are basically more expensive than using other materials such as serpentine or steel slag [6] with respect to the depreciation, electricity, chemicals and the other items, respectively, because of the significantly small plant scale and the small amount of sequestered CO2 per unit material volume. However, adding a saving cost of unused Ca(OH)2 by using NaHCO3 which is generated in the process, the total sequestration costs of the process can be decreased to US$ 58 per tones CO2 avoided, which is the same level of the large scale CCS plants. The cost of chemicals, relatively high in this process, depends mainly on the amount of the use of potassium hydroxide (KOH). The cost might be improved if waste alkali or biomass-derived materials (ashed, charred or torrefied biomass) would be used instead of KOH as sources of potassium.

Table 4. CO2 sequestration costs of this process with decrease in operating cost of MSWI using NaHCO3 reproduced in this process

Costs (US$ /tonnes CO2) (a) Bottom ash (b) Bottom ash + 55% of fly ash

Sequestered Avoided Sequestered Avoided

Plant depreciation 120 161 130 179

Electricity 120 161 131 179

Chemicals 59 80 251 344

Staff 91 122 87 119

Maintenance, other 42 56 45 62

Decrease in chemical and disposal -269 -361 -602 -825

costs of flue gas treatment at MSWI

Sum 164 219 42 58

5. Conclusions

In this paper, a new process using aqueous mineral carbonation technology with recovering useful salt is proposed as a CCS technology for MSWI. This process can sequester relatively small amount of CO2 of 2,880 tonnes CO2 per year for 230 thousand tonnes of waste per year of MSWI, however, a calculated cost of CO2 sequestration of the process is US$ 58 (JPY 4,600) per tonnes CO2 avoided which is nearly the same cost level of larger scale CCS technologies. The produced useful chemical, NaHCO3, which can be used for flue gas treatment, might compensate the costs of MSWI operations.

There still has some room for reducing cost, hence, this process might be economically viable and applied as an industrial CO2 utilization technology with business continuity.

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