STUDY ON THE INFLUENCE OF ADDITIVES CONTANING ELEMENTS OF Na, Fe, Al, Ca TO ANTHRACITE COAL COMBUSTION EFFICIENCY

STUDY ON THE INFLUENCE OF ADDITIVES CONTANING ELEMENTS OF Na, Fe, Al, Ca TO ANTHRACITE COAL COMBUSTION EFFICIENCY

Nguyen Thi Thuc Phuong

M.S, Institute for Technology of Radioactive and Rare elements,

Vietnam, Ha Noi

Hoang Van Duc

Ph.D, Institute for Technology of Radioactive and Rare elements,

Vietnam, Ha Noi

Luu Xuan Dinh

Ph.D, Institute for Technology of Radioactive and Rare elements,

Vietnam, Ha Noi

Le Thi Mai Huong

Assoc. Prof., Ph.D, Nuclear Training Center,

Vietnam, Ha Noi

ABSTRACT

Additives enhance transferring oxygen to the carbon surface, leading to the reactions happen faster; using additives is easy to implement, flexible, takes no time to stop the furnace, ensuring continuous operation of the plant ... In this study, the effect of additives containing Na, Fe, Al, Ca to coal combustion efficiency applied on Vietnam anthracite coal sample of coal-fired thermal power plant was shown, compared, evaluated focusing on criteria of reducing the ignition temperature, based on the results of thermal analysis method. The obtained results show that the multi-element additives containing NaCl/CaCl₂/Al₂O₃/Fe₂O₃ with different ratios improves the coal combustion.

Keywords: Antraxite, additives for coal combustion.

Acknowledgements: This study was funded by project named: “Research on evaluating the possibility of using red mud as a coal-fired additive of thermal power plant in the alumin-Tan Rai Complex from the Vietnamese Ministry of Science and Technology”, Code ĐTCB. 16/19/VCNXH

I. INTRODUCTION

Currently, coal fossil fuel resources are gradually exhausted, pushing coal prices in the world market higher and higher. In order to increase the efficiency of coal combustion, solutions to improve technology have been continuously carried out, but the efficiency in saving coal consumption and reducing polluting emissions is not really high. The use of additives for coal combustion is of great interest due to its flexibility, ease of implementation, no downtime, and assurance of the continuous operation of the plant. The additives lower the activation energy of the reaction, making it happen faster or happen at lower temperatures. This results in a more exhaustive combustion reaction, reduced ash and combustion emissions.

The additives for coal combustion can be inorganic compounds such as metal oxides, salts [1-3] or organic substances [4-5]. One solution that is attracting attention today is to use industrial wastes as catalysts for coal combustion [6,7].

The chain-catalysis mechanism of coal combustion using a mixed Na - Fe - Ca catalyst from industrial wastes has been reported by Jun Cheng and colleagues [7]. In this mechanism, the oxygen atom is transported based on the order of the catalytic metal activity from Na to Fe to Ca with carbon. In the catalytic coal combustion process, catalytic oxidation is considered as one of the predominant mechanisms. These catalysts can promote coal combustion because many oxides are generated during the oxidative decomposition of metal oxides or other materials, and these oxides have high adsorption capacity of oxygen on the surface. In the coal burning process, these oxides are used as active oxygen carriers.

When using a catalyst containing a mixture of elements Na, Fe, Ca, the catalytic reaction will take place according to a chain mechanism, shown in Figure 1 [6,7]. The following reactions happen in turn:

2 Na₂O + O₂ → 2 Na₂O₂

Na₂O₂ + 2 FeO → Na₂O + Fe₂O₃

Na₂O₂ + Fe → Na₂O + FeO

2 Fe₂O₃ + CaO → CaO₂ + 4 FeO

FeO + CaO → CaO₂ + Fe

2 CaO₂ + C → 2 CaO + CO₂

Figure 1. Mechanism of chain reaction of Na-Fe-Ca composite catalyst for coal combustion

In Vietnam, the issue of improving the efficiency of coal burning is also focused at thermal power plants. Several studies on using catalytic additives to improve coal combustion efficiency have been carried out and obtained certain results [8-10]. In our previous report, the results of studying the effects of additives containing separately elements Na, Ca, Al and Fe were reported [11]. The obtained results show that the Ca additive has the highest efficiency and the Al additive has the lowest efficiency. Following the mentioned study, in this report, the influence of multi-element additives containing Na, Fe, Al, Ca on coal combustion efficiency applied on Vietnamese anthracite coal samples of coal-fired thermal power plants has been shown out, compare and evaluate on the criteria of reducing the ignition temperature and increasing the burnout level of coal.

II. EXPERIMENTALS

II.1. Objects

The object of this study is Vietnamese anthracite coal. The characteristics of the coal sample are shown in Table 1.

Table 1.

The properties of Vietnamese anthracite coal sample

II.2. Methods

II.2.1. Experimentals

• Raw coal and additives are ground and passed through a standard sieve to a particle size of about 0.015 - 0.1 mm.
• The coal powder sample is mixed with the additive powder sample according to different mixing ratios.
• The sample of the mixture of coal and additives is added water to turn into a slurry, then the slurry sample is dried and ground to a particle size of about 0.015 - 0.08 mm.
• Fine powder samples are taken for thermal analysis. The coal sample is placed in an aluminum-ceramic crucible and ignited under standard atmospheric conditions (air flow rate is about 10-50 mL/min) at a steady heating rate (approximately 10°C/min) from room temperature to 900°C.
• The weight of the sample is continuously controlled according to the change of temperature.
• Based on the results of the thermal analysis, determine the ignition temperature of the coal in the way shown below.

II.2.2. Determination of ignition temperature

The ignition temperature of coal is the temperature at which coal begins to ignite when heated at a certain rate of increase in temperature in the presence of an oxidizing agent.

The ignition temperature of coal is determined in turn through the following steps [6,7]

• First, draw a vertical line passing through the vertex A of the DTG curve. This line intersects the TG curve at point B.
• Draw a tangent to the TG curve at point B.
• Draw a horizontal line through the starting point of the loss of mass of the TG curve. This horizontal line intersects the tangent line at point C.
• From point C, draw a vertical line. This line intersects the horizontal axis at the point Ti. Ti temperature is the ignition temperature of coal.

II.2.3. Determination of coal burnout

Burnout, Bc, is used to represent the burnout property of coal. For the conventional ash-based assessment method, the ash mass is assumed to be constant before and after combustion. Therefore, the degree of burnout of the original raw coal sample is calculated as follows [6,7]

M₀.A₀ + M₁.A₁=M₂.A₂

FC₂ + A₂ = 1

M₂/(M₀+M₁) = 1 – (TG)max

B_c = 1 – (M₂.FC₂)/(M₀.FC₀) = [FC₀ + A₀ + (M₁.A₁)/M₀ – (1+ M₁/M₀)(1– (TG)_max]/FC₀

In which:

• M₀, M₁ and M₂ are the mass of the original raw coal samples, the weight of the additive and the mass of the coal slag, respectively.
• A₀, A₁, A₂ are the ash composition by mass (%) of the raw coal sample, of the additive and of the coal slag, respectively.
• FC₀ and FC₂ are the % by mass composition of C in the raw coal sample and in the coal slag, respectively.
• (TG)_max is % maximum mass loss of sample during combustion (%)

III. RESULTS AND DISCUSSION

The influence of the ratio of Na/Ca/Al/Fe components in the additive, additive content and additive size on coal combustion efficiency was studied.

• Based on the overview of additives for coal combustion, the influence of the ratio of NaCl/CaCl₂/Al₂O₃/Fe₂O_3­ components (by weight) in the multi-element additive was investigated in turn with the ratios: 0/0/0/0 (blank sample), 2/1/1/3, 1/1/5/15, 1/1/10/30. The additive/charcoal content used was 10% and the particle size of the additives was 180-200 µm. The results are shown in Table 2.

Table 2.

Effect of the ratio of elements in the additives on the coal combustion

The results shown in Table 2 show that the ratio of NaCl/CaCl₂/Al₂O₃/Fe₂O₃ in the additive is 2/1/1/3 (by weight) which is the most effective in reducing the ignition temperature of coal and improving burnout improvement. This ratio was chosen as the optimal ratio for the multi-element additive of coal.

• The influence of the additive/coal content was investigated in turn with the ratios of 0 % (blank sample), 2%, 4%, 6%, 8% and 10%. The ratio of NaCl/CaCl₂/Al₂O₃/Fe₂O₃ by weight used is 2/1/1/3 with the particle size of the additives is 180-200 µm. The results are shown in Table 3.

Table 3.

Effect of additive contents on the coal combustion

The ignition temperature of coal decreases when the additive/coal ratio increases to 8% and decreases when the ratio increases to 10%. The volatile matter content in coal also varies with the degree of burnout of coal.

• The influence of the particle size of the additive was investigated with the following sizes: <75 µm, 75-125 µm, 125-180 µm, 180-200 µm, respectively. The ratio of NaCl/CaCl₂/Al₂O₃/Fe₂O₃ by weight used is 2/1/1/3 with the additive/coal content (in %) of 10%. The results are shown in Table 4.

Table 4.

Effect of additive particle size on coal combustion

The ignition temperature of coal decreases as the size of the additive particles decreases. The volatile matter content in coal also increases similarly to the degree of burnout of coal.

IV. CONCLUSIONS

The multi-element additives containing NaCl/CaCl₂/Al₂O₃/Fe₂O₃ with different ratios improves the coal combustion. The ratio of additives/coal from 8 - 10% is a suitable condition to increase the efficiency of coal burning. The smaller the particle size of the additive, the higher the combustion efficiency of the coal, due to the better dispersion of the additive on the coal surface. However, for further improvement, there still needs to be other studies such as the formation of exhaust gases, the heat generated during the combustion of coal when using additives...

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