Exploring the Dynamics of Natural Sodium Bicarbonate (Nahcolite), Sodium Carbonate (Soda Ash), and Black Ash Waste in Spray Dry SO₂ Capture ^†

Abstract

The efficacy of spray dry systems compared to wet flue gas desulphurisation (FGD) units depends on applying a highly reactive scrubbing reagent. This study assessed sodium-based compounds derived from natural sources and waste by-products as potential agents for treating sulphur dioxide (SO₂). Sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃) were acquired from mineral deposits, whereas the black ash waste (Na₂CO₃·NaHCO₃) was obtained from the pulp and paper sector. The sorbents introduced in slurry form were subject to SO₂ absorption conditions in a lab-scale spray dryer, including an inlet gas phase temperature of 120–180 °C, flue gas flow rate of 21–34 m³/h, and sodium to sulphur normalised stoichiometric ratio (Na:S) of 0.25–1. The comparative performance was evaluated using the metric of %SO₂ ($\%{\eta }_{DeS{O}_X}$) removal efficiency. The results showed that NaHCO₃ had the highest overall result, with a removal efficiency of 62% at saturation. Black ash was the second best-performing reagent, with a 56% removal efficiency, while Na₂CO₃ had the lowest efficiency (53%). The maximum degree of SO₂ reduction achieved using NaHCO₃ under specific operating parameters was at an NSR of 0.875 (69%), a reaction temperature of 120 °C (73%), and a gas inlet flow rate of 34 m³/h. In conclusion, the sodium reagents produced significant SO₂ neutralisation, exceeding 50% in their unprocessed state, which is within acceptable limits in small- to medium-sized coal-fired power plants considering retrofitting pollution control systems.

1. Introduction

Spray drying devices are a subset of SO₂ mitigation methods proven to be as competitive as wet FGD units. Under optimal circumstances, SO₂ collection can surpass 90% while producing dry by-products, eliminating the need for post-treatment procedures [1]. Hydrated lime (Ca(OH)₂) is primarily employed in spray dry absorption operations, given its excellent performance compared with typical limestone or lime [2]. However, it is limited by high prices and low reagent conversion rates. Considering their heightened activity, sodium reagents can be utilised as alternatives to address this drawback. Nahcolite (sodium bicarbonate–NaHCO₃), trona (Na₃H(CO₃)₂·2H₂O), and soda ash (sodium carbonate–Na₂CO₃) constitute sodium salts applied in SO₂ removal processes. Their widespread availability enables their incorporation despite being relatively more costly than calcium-based sorbents [3].

Since sodium-based sorbents are deployed in slurry form, high-quality water is essential to prevent scale and fouling in spray delivery systems. The order of preference for such sorbents is governed by the ability of the reacting particle to facilitate SO₂ diffusion after product layer formation. The bicarbonate (HCO₃⁻) species sodium bicarbonate and trona initially break down at temperatures above 100 °C to sodium carbonate (Na₂CO₃) with an evolved pore architecture [4]. This phenomenon advocates for the selection of NaHCO₃ as the preferred reagent over other sodium compounds due to its elevated concentration of HCO₃⁻, contributing to the formation of a higher number of voids. Furthermore, operating conditions such as the amount of reagent feed, residence time, slurry atomisation pattern, and competing reactions contribute to the overall efficiency of the spray dry process. Hence, it is necessary to conduct parametric investigations to moderate the rise in operational expenses and the duration of system interruptions, particularly in extensive applications. Additional sodium substances that can be exploited include sodium chlorite, sodium hydroxide, or sodium bisulphite [3]. Black ash is a residue emanating from incinerating sludge in the pulp and paper industries. It is usually disposed of in landfills, which adds to harmful industrial practices. The fundamental chemistry of black ash consists of sodium oxide (NaO), which is analogous to a mixture of NaHCO₃ and Na₂CO₃. Due to its reactive nature, black ash is a suitable candidate for controlling SO₂ in desulphurisation units.

This study evaluated and compared the performance attributes of sodium-based sorbents (nahcolite, soda ash, and black ash) obtained from various sources using a laboratory spray dry setup. The objective was to harness existing natural materials and waste by-products for sulphur dioxide management. The selected sorbents were identified and subjected to experiments under different conditions, such as the flue gas flow rate, desulphurisation temperature, and stoichiometric molar ratio.

2. Materials and Methods

The sulphur dioxide gas (99.9%) was supplied by Afrox, South Africa. Nahcolite and soda ash were obtained from Sua Pan mines, Botswana, while the black ash was sourced from paper and pulp manufacturing. Major mineralogical composition, as determined by X-ray diffraction, revealed a chemical composition of 93 wt.% NaHCO₃ in nahcolite, 87.1 wt.% Na₂CO₃ in soda ash, and 62.9 wt.%:36.5 wt.% of Na₂CO₃:NaHCO₃ in black ash waste.

The absorption experiments were conducted in a Buchi mini spray dyer B-290 system. The reagent solutions were formulated by agitating a blend of a predetermined quantity of sorbent and distilled water in a slurry preparation vessel, yielding a known slurry concentration. The normalised stoichiometric ratio (NSR), where ideally 2 moles of Na⁺ react with 1 mole of SO_x⁻², was calculated by varying the slurry solid content when injected at a flow rate of 0.8 kg/h, constituting an NSR range of 0.25–1. This slurry was injected into the spray chamber from the top using a two-fluid spray nozzle (tip diameter of 0.7 mm and a cap diameter of 1.4 mm) via a peristaltic pump. A simulated flue gas of 1000 ppm was prepared by mixing 99% pure SO₂ with ambient air at controlled flow rates (21–34 m³/h). This was subsequently heated to temperatures between 120 and 180 °C using an electric heater and directed into the reactor in a co-current flow, resulting in an average residence period of 1.0 to 1.5 s. Following an instantaneous drying and evaporation process during sulphation, a dry salt entrained in the treated gas stream was collected in the product holder chamber after separation using a cyclone. The inlet and outlet gas concentrations were recorded using a Testo-340 combustion analyser fitted with an inbuilt cold trap to mitigate the effects of moisture during measurement. The removal efficiency was calculated as illustrated in Equation (1) below.

$$\% {\mathrm{SO}}_2 removal=\left(1-C_{out}/C_{in} \right)\times 100$$

(1)

where $C_{out}$ and $C_{in}$ are the effluent and feed SO₂ concentrations (ppm), respectively.

3. Results and Discussion

3.1. Effect of the Stoichiometric Ratio

The outcome presented in Figure 1 shows the influence of the ratio of the sorbent feed to SO₂ for efficient neutralisation. It is specific to the alkaline species taking part in the reaction. For sodium materials, two moles of sodium ions (Na⁺), Equation (2), is required for the complete conversion of SO₂ to a sulphite (SO₃²⁻) or sulphate (SO₄²⁻) salt [5], formulating a normalised stoichiometric ratio (NSR). Generally, SO₂ removal was enhanced as the NSR approached 1, with NaHCO₃ having the highest result (69% at NSR = 1). Black ash had its maximum removal (64%) at NSR = 0.75, and no further change was experienced beyond this point. Clogging issues were encountered when an NSR of 1 was implemented with the black ash due to insoluble and coagulated particles. The Na₂CO₃ was the least efficient, with the highest SO₂ capture (58%) at an NSR = 1. Only a 1% increase was observed from NSR = 0.5 to 1 with the Na₂CO₃ reagent. The absolute solubility of NaHCO₃ enables the formation of ultra-small particles upon the evaporation of the water droplets. Additionally, at temperatures exceeding 100 °C, NaHCO₃ decomposes into Na₂CO₃ with a denser pore structure. These traits, when combined, enhance the overall performance [6]. This aligns with congruent study findings on spray dry setups [7,8].

$$2{\mathrm{Na}}_{\left(aq\right)}^{+}+2{\mathrm{OH}}_{\left(aq\right)}^{-}+{\mathrm{SO}}_{3\left(aq\right)}^{2-}+{\mathrm{H}}_2{\mathrm{O}}_{\left(l\right)}\to {\mathrm{Na}}_2{\mathrm{SO}}_{3\left(aq\right)}+2{\mathrm{H}}_2{\mathrm{O}}_{\left(l\right)}$$

(2)

Theoretically, an NSR = 0.5 is projected to neutralise 50% of SO₂. All sorbents surpassed this threshold (Figure 1), suggesting that secondary neutralisation occurs. During the reaction, the acidic gas at constant concentration is contacted by freshly added sorbent slurry. Hence, some of the SO₂ molecules can directly react with the excess OH⁻ species from water and sorbent dissociation (Equations (3) and (4)), adding to the SO₃²⁻ that reacts in Equation (2) formulated from SO₂ dissolution (Equations (6)–(8)) [9].

$${\mathrm{H}}_2{\mathrm{O}}_{\left(l\right)}\leftrightarrow {\mathrm{H}}_{\left(aq\right)}^{+}+2{\mathrm{OH}}_{\left(aq\right)}^{-}$$

(3)

$${\mathrm{NaOH}}_{\left(aq\right)}\leftrightarrow {\mathrm{Na}}_{\left(aq\right)}^{+}+{\mathrm{OH}}_{\left(aq\right)}^{-}$$

(4)

$${\mathrm{SO}}_{2\left(g\right)}+{\mathrm{OH}}_{\left(aq\right)}^{-}\leftrightarrow {\mathrm{HSO}}_{3\left(aq\right)}^{-}$$

(5)

3.2. Effect of Flue Gas Flow Rate

Flue gas flow rate governs the reagent residence time, material dilution, and the evaporation of water droplets on the particle. According to Figure 2, the removal efficiency of NaHCO₃ increased by 25% and black ash increased by 17% when the gas flow rate increased from 21 m³/h to 33 m³/h. This phenomenon can be attributed to turbulent conditions during the mixing of gas and sorbent induced by turbulence kinetic energy. Such conditions fostered mass transfer, specifically of the SO₂, from the gas phase to the liquid interface, where it could then dissolve and undergo neutralisation [10]. The effectiveness of Na₂CO₃ reached its highest point at a flow rate of 31 m³/h, achieving a maximum removal rate of 62%. However, beyond this point, the functionality dropped. Higher flow rates introduce more heat, leading to accelerated droplet evaporation and decreased gas solubility due to minimised humidification [11]. The nahcolite and black ash maintain their enhanced performance because of the NaHCO₃ composition, which can break down to Na₂CO₃ with enhanced pore surfaces and reaction area (popcorn effect) if total sorbent dissolution is not achieved. Soda ash, consisting solely of Na₂CO₃, exhibits less reactivity when in contact with the gas at these conditions, as opposed to the Na₂CO₃ slurry solution.

3.3. Effect of Temperature

The desulphurisation temperature in spray dry devices regulates the rate at which the liquid film evaporates. SO₂ dissolution on the liquid layer yields ionised sulphur species (Equations (6)–(8)) accessible for the absorption reaction (Equation (2)) with the Na⁺. When the temperature was raised from 120 to 180 °C, the removal efficiency of NaHCO₃, Na₂CO₃, and Na₂CO₃ dropped by 22%, 20%, and 22%, respectively (Figure 3). At 120 °C, the reaction environment facilitated the condensation of SO₂ on the surface of the reagent particles, enhancing the intimate gas–solid interaction and the neutralisation reaction [6]. Higher temperatures lead to an increased rate of evaporation, which in turn decreases the amount of liquid layer available for the breakdown of SO₂ (lowering SO₂ solubility) and the availability of water for humidification. Wu et al. [12] also observed an identical correlation during their numerical simulation of spray drying in a powder particle spout bed. The authors noted an increase in water loss at elevated bed temperatures.

$${\mathrm{SO}}_{2\left(aq\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(l\right)}\to {\mathrm{H}}_2{\mathrm{SO}}_{3\left(aq\right)}$$

(6)

$${\mathrm{H}}_2{\mathrm{SO}}_{3\left(aq\right)}\leftrightarrow {\mathrm{H}}_{\left(aq\right)}^{+}+{\mathrm{HSO}}_{3\left(aq\right)}^{-}$$

(7)

$${\mathrm{HSO}}_{3\left(aq\right)}^{-}\leftrightarrow {\mathrm{H}}_{\left(aq\right)}^{+}+{\mathrm{SO}}_{3\left(aq\right)}^{2-}$$

(8)

Further experimental runs were undertaken for 8 min to determine performance until no additional change was observed. The tests were conducted in triplicate and operated under the following conditions: a slurry flow rate of 0.8 kg/h, DeSOx temperature of 120 °C, flue gas flow rate of 31 m³/h, NSR of 0.5, 10% solid weight fraction, and an inlet SO₂ concentration of 1000 ppm. Figure 4 shows the average outcome and reveals that nahcolite (NaHCO₃) had the highest efficiency at 62%. This performance outperformed black ash (Na₂CO₃·NaHCO₃), which had a 56% efficiency, closely followed by soda ash (Na₂CO₃) at 53%. The apparent efficiency of NaHCO₃ is due to its increased solubility, which allows for greater availability of dissolved sodium ions. The data gathered emphasise the practicality of the nahcolite mineral, soda ash, and the utilisation of black ash waste from the pulp and paper sector as suitable candidates for spray dry SO₂ mitigation.

4. Conclusions

The presence of sulphur in energy sources such as coal causes extensive environmental hazards. Hence, desulphurisation is essential to remove SO₂ from energy sources. The present investigation on desulphurisation covered a spray dry configuration that sought to estimate the efficiency of black ash waste, soda ash, and nahcolite as SO₂ neutralisation agents. The investigation findings revealed that lower temperatures promote the overall reaction, which is linked to slower evaporation of slurry water droplets. This condition encourages gas dissociation and dissolution when in contact with slurry droplets. The peak sorbent activity was reported at 120 °C, with nahcolite attaining a removal efficiency of 73%, 69% with black ash, and 62% with soda ash. Elevated NSR improved the neutralisation reactions for all sorbents. However, the sorbent derived from black ash was restricted to a maximum normalised stoichiometric ratio of 1 due to solubility impediment over this point. Augmenting the flue gas flow rate resulted in greater material blending, which improved the desulphurisation for all sorbents. Conversely, the performance of soda ash declined above a flow rate of 31 m³/h due to diminished gas contact. Overall, nahcolite showed a higher performance across all experimental conditions.