The catalytic effect of the Na and Ca-rich industrial wastes on the thermal igni

State Key Lab of Clean Energy Utilization,Zhejiang University,Hangzhou 310027,China

Keywords:Industrial waste Catalysis Coal combustion Thermal ignition

ABSTRACT The catalytic effects of four industrial wastes,namely,the soap residue(SR),brine sludge(BS),calcium carbide residue(CCR),and white lime mud(WLM),on coal thermal ignition were investigated.The acidity of palmitate anion associated with Na+in SR was lower than that of chloride anion combined with Na+in BS,which resulted in an improved the combustion of SR.The acidity of OH-anion combined with Ca2+in CCR was lower than that ofanion combined with Ca2+in WLM,resulting in CCR exhibiting a better catalytic effect on coal ignition.The alkaline metal Na had lower initial ionisation energy than the alkaline earth metal Ca.Therefore,the Na-rich SR exhibited higher catalytic activity on coal ignition than Ca-rich CCR.The ignition temperature of coal with 0.5%SR decreased from 544 to 503°C.

1.Introduction

It is reported that the in 2013,coal accounted for 65.7% of the primary energy consumption in China[1].The total coal consumption of China is approximately 3.6 billion tons per year,and approximately 50%of this coal is used for power generation[2].Thus,improving the coal combustion efficiency while reducing energy consumption and polluting emissions need continuous research to be achieved.Using the combustion-supporting agents is an effective strategy to enhance the coal combustion efficiency.It was reported that the metal oxides,metal salts,alkaline earth metal,alkali metal,and transition metal oxides,as well as their compounds can effectively promote coal combustion.Such catalysts include MnO4,K2CO3NClO4,MnO2,CaO,La1-xCaxMnO3,CeO2,KNO3,CeO2,Fe2O3,KClO3,NaNO3,CuO,ZnO,and BaCO3[3,4,5-10,11,12].There are several studies conducted on these pure chemical reagents as coal combustion catalysts,whereas only few studies have been reported on the industrial wastes rich in the compounds,which can be used as efficient coal combustion catalysts.For instance,Wu et al.investigated the influence of an industrial waste resulted from the steel production on the burnout rate and NOxrelease during the catalytic combustion of pulverised coal[13].The combustion experiments were performed in a lab-scale drop tube furnace.The experimental results showed that the use of an appropriate ratio between the reagents,such as NaClO4,MnO4,and industrial waste generated in steel industry effectively improved the coal burnout rate[14].It is worth mentioning that in China,in addition to steel slags,large amounts of other industrial wastes rich in alkaline earth metals,alkali metals,and transition metals are produced every year,and they can be recycled for the coal combustion.

Soap residue(SR)is a sodium-rich industrial waste produced during soap making.The main components of SR are NaCl and other salts.Brine sludge(BS)is generated during the production of hydrogen,chlorine,and caustic soda when NaCl is used as a raw material[15].Calcium carbide residue(CCR)is a calcium-based industrial waste produced during acetylene fabrication.White lime mud(WLM)is an industrial waste produced during caustic soda rebirth process,whereas reeds and straw are used as raw materials for paper in paper mills.Two tons of dry WLM are produced for each ton of caustic soda.WLM,which usually is less used,is normally piled around the paper mills.As underlined above,the chemical compounds,such as alkaline metals and alkaline earth metal oxides and their salts,have catalytic effects on coal combustion.The aforementioned industrial wastes rich in Na and Ca salts can be effectively reused as coal-combustion-supporting agents through a strict control of storing,transporting and management of chemically hazardous wastes.The low cost of these wastes makes them suitable for large-scale industrial applications to decrease the energy consumption during the coal combustion.The catalytic effects of Na-,Fe-and Carich individual and composite industrial wastes on Jincheng anthracite coal combustion were comparatively analysed using the thermogravimetric method[16,17].In China,the fly ash,SOx,and NOxemissions in flue gas derived from coal-fired boilers are controlled at less than 5,35,and 50 mg·m-3,respectively,after cleaning processes withmodern environmental protection equipment.In the pulverised coal furnaces in power plants,when pulverised coal flows into the furnace chamber,the rate of temperature increase can be as high as 104°C·s-1,because the coal is heated with high-temperature air in an extremely high speed.This study reports on the catalytic combustion conducted in a tube furnace at a constant temperature.To simulate the real conditions of the ignition and combustion in a pulverised coal furnace,the experiments were performed with high heating rates to heat the coal sample.The effects of four industrial wastes,namely,SR,BS,CCR,and WLM,on coal combustion were investigated.

2.Experimental

Generally,many power plants use blending coal to ensure the relative stability of coal quality and improve the coal combustion efficiency.The coal used in this study is a blending coal.The results of chemical analysis and the caloric value of this coal are listed in Table 1.The industrial wastes used in this study,namely,SR,BS,CCR,and WLM,were obtained from the factories in Hangzhou,China.SR is a waste produced during soap making in a soap factory in Hangzhou.BS is obtained during the production of caustic soda electrolysis in a salt alkaline industry in Hangzhou,and its main component is NaCl.CCR is obtained from a chloralkali industry in Hangzhou during the reaction of calcium carbide with water for acetylene production;its main component is Ca(OH)2.WLM is the waste produced in the paper-making industry.The chemical components of the four industrial wastes are shown in Table 2.BS and SR are rich in sodium,whereas WLM and CCR are rich in calcium.

The catalytic combustion was performed using a single-particle coal sample and a vertical tube furnace with a diameter of 40 mm and a height of 330 mm,as represented in Fig.1.The furnace can move up and down with high speed,ensuring the impulsive heating of the coal particles for rapid coal ignition.This step simulates the actual thermal environment in a pulverised coal furnace.The furnace temperature can be adjusted within the range of 200-1000°C,and the natural convection of the air provides the required oxygen for combustion.The industrial wastes are grinded into small pieces with diameters of 180 μm and mixed with pulverised coal to reach a mass ratio of 0.2 and 0.5%.Thereafter,water was added to the obtained pieces to transform them into a coal slurry with a concentration of 70%.The coal slurry drops and then sticks at the top of a thermocouple wire,which has a diameter of 0.1 mm.The mass of each small coal ball was controlled at 100 mg by adjusting the diameter of the coal ball.After being air-dried,the sample was placed into the furnace and then the combustion experiment was started.The thermocouple was attached to an electronic balance connected to a computer to record in real time the temperature and mass of coal.

Fig.1.Schema of experimental system used for coal combustion.

To study the thermal ignition of coal based on the heat released during coal oxidation at the minimum furnace temperature,but not on forced ignition of coal at very high furnace temperatures,a limited temperature range(the temperature of the furnace wall increased from room temperature to 560°C)was used as external temperature to ignite coal samples.Then,the furnace is suddenly raised,which makes the coal ball to quickly enter the furnace and heated for rapid ignition.The ignition temperature of coal can be recorded during the gradual decrease in the temperature of furnace wall.This value is obtained based on the temperature-time curve as follows.After recording the temperature-time curve,the corresponding first derivative is analysed.The thermal ignition temperature is given by the minimum of this first derivative,which correspond to the moment when the rate of temperature increase changes from acceleration to deceleration.These two curves,i.e.,T-t curve and its first derivative,used to determine the ignition temperature are shown in Fig.2.Hence,on the basis of the experimental results,the lowest temperature required by the blending coal without additives is 380°C while the thermal ignition temperature of blending coal is 544°C.

Fig.2.The temperature-time curve and its fist derivative used to determine the ignition temperature of coal.

3.Results and Discussion

3.1.Chemical compositions of industrial wastes

The X-ray diffraction(XRD)patterns of the four industrial wastes are illustrated in Fig.3.According to the X-ray patterns displaying the diffraction peaks of crystalline components of the industrial wastes,SR and BS are rich in sodium compounds.Hence,the sodium palmitate(C16H31NaO2)and NaCl in SR account for approximately 60%,and 40%,respectively.The main component of BS is NaCl,whose content is above 80%,followed by CaCO3with a content of 10%.The main component in CCR is Ca(OH)2(＞80%),followed by CaCO3with less than 10%.In addition,for this sample,about 3% impurities,including α-FeCO3,Fe3O4,and α-SiO2,were found.WLM consists of CaCO3with a proportion of＞95% and less than 0.5%impurities,which are not highlighted in the XRD pattern.Summarising,it can be affirmed that the principal components with high catalytic potential in combustion are NaCl,C16H31NaO2,Ca(OH)2,and CaCO3in BS,SR,CCR and WLM,respectively.

Fig.3.XRD patterns of four industrial wastes:(a)SR,(b)BS,(c)CCR,and(d)WLM.Data are from Ref.[16,17].

3.2.The effect of the industrial wastes on the temperature curves of coal combustion

The impact of the industrial wastes on the temperature curves of coal combustion is shown in Fig.4(the experimental temperature of the furnace wall is 380°C).Accordingly,the temperature of the raw coal increases gradually at the initial heating stage.At 60 s,the central temperature of the coal particle is only 110°C,which allows the water vaporisation.Between 60 and 120 s,the central temperature of the coal increases to 490°C with a constant rate of 6.36°C·s-1.In this stage,the volatile compounds in coal are removed and the coal ignites.In the next stage,i.e.,between 120 and 305 s,the temperature still increases,but with of much lower heating rate of 0.84°C·s-1.In this stage,the maximum reached temperature is 640°C.This period is known as the ignition stage of the coal coke.The stage between 305 and 500 s corresponds to the intensive coal coke combustion.Although the central temperature of the coal reduces gradually from 642 to 560°C,the coal still has a high temperature.During the interval between 500 and 600 s,the coke burns out.With the loss of the heat source,the temperature of the coal sample sharply decreased to 340°C.The temperature of the inert ash,which is composed of mineral ash,becomes constant at 340°C.

Fig.4.The effect of the four industrial wastes and their loading degree(i.e.,0.2 and 0.5%)on the ignition temperature of coal:(a)SR and BS,(b)CCR and WLM.

The addition of different industrial wastes has no effect on the temperature before 120 s(i.e.,moisture precipitation,volatile precipitation and ignition stages).After 120 s(i.e.,coke ignition,combustion,and burnout stages),the effect of the industrial wastes on the temperature is obvious.Therefore,an increase in temperature is noticed.In addition,the interval in which the temperature increases is enlarged in comparison with that observed in the absence of any of the investigated wastes.The activation energy of the volatilised precipitation and the ignition are relatively low,and thus,these reactions can efficiently proceed in the low-temperature stage,i.e.,between 300 and 400 °C.On the other hand,the activation energy of coke ignition and the reaction temperature are relatively high.Those issues can be addressed by using metal ions from the industrial wastes,such Na+and Ca2+,which can exhibit a catalytic effect on the reactions taking place at the coal surface during combustion,as will be discussed below in the description of the reaction mechanism.

The ignition temperature is critical when the heat resulted from the chemical reaction involved in fuel combustion is emitted to the surrounding environment, and this heat is balanced under a particular thermal condition. In addition, the ignition temperature is usually used to verify whether the coal can easily ignite.When the amount of the industrial wastes added to the coal is 0.2%,SR and BS reduce the thermal ignition temperature of the blending coal by 31°C from 544 to 513°C.CCR and WLM reduce the temperature by 26°C and 21°C,respectively.When the amount of the industrial wastes added to the coal is 0.5%,a different combustion trend was noticed.Thus,SR can reduce the ignition temperature of the blending coal by 41°C,whereas the addition of BS has no effect.CCR can reduce the ignition temperature of the blending coal by 40°C,whereas the WLM has no effect on the temperature.SR and BS are rich in Na+.However,Cl-combines with Na+in BS,andcombines with Na+in SR. The acidity ofis weaker than that of Cl-.Therefore,the effect of the SR on the combustion temperature is better than that of BS. CCR and WLM are rich in Ca2+, however, the OH-ion that combines with Ca2+in CCR has a weaker acidity than thethat combines with Ca2+in WLM.Therefore,CCR has a better effect than WLM on the combustion temperature of coal.On the basis of these results,SR and CCR are considered as the most effective wastes for the combustion of coal among the four investigated herein.

The blended coal sample used in the experiments was a Chinese bituminous coal with a volatile matter of 17.61%and ash content of 30.6%.The thermal ignition temperature of this coal sample enriched with 0.5%SR decreased from 544°C to 503 °C.Currently,most researchers use pure chemical reagents as coal combustion promoters.Cui et al.lowered the coal combustion temperature by 41.5°C when 1%La0.6Ca0.4MnO3was used as catalyst[3].The ignition temperature of anthracite modified with CeO2and Fe2O3decreased by 50 and 53 °C,respectively(Gong et al.).For the various coals with different volatile matters and ash contents existing in other countries,it was shown that Na+in SR had a better promotion effect on coal combustion than Na+in BS,whereas Ca2+in CCR exhibited a better catalytic effect on coal ignition than Ca2+in WLM.However,the thermal ignition temperature of anthracite(such as Vietnamese coal with a very low volatile matter)enriched with 0.5%SR decreased by more than 41°C,and thermal ignition temperature of lignite(such as Indonesian coal with a very high volatile matter)containing 0.5%SR decreased by less than 41°C.These results highlight the importance of the chemical composition and origin of the coal used in the process,as well as the necessity to perform detailed experiments to compare the promotion effects of industrial wastes rich in Na and Ca compounds on the combustion of various coals from different countries.

3.3.The effect of the industrial wastes on the mass loss curves of coal during combustion

Fig.5 shows the effects of industrial wastes on the mass loss curves of coal combustion(the experimental furnace wall has a temperature of 380°C).Accordingly,the industrial wastes can positively influence the combustion reaction.The relative mass loss rate of the coal sample with industrial wastes added is higher than that of the raw material,and the final mass loss rate can also be higher than that of the raw material.When the amount of industrial waste added is 0.2%,the final mass loss increases from 60% to 87% and 75% for SR and BS,respectively.When the amount of industrial wastes added is increased to 0.5%,the SR can additional increase the final mass loss rate of the coal to 98%,whereas the BS improves the mass loss to only 79%.The promotion effects of CCR on the coal combustion are better than those of WLM.When the amount of industrial wastes added is 0.2%,the final mass loss of the blending coal can be raised to 91% and 87% for CCR and WLM,respectively.Hence,it can be concluded that the catalytic effect of SR on the ignition temperature of coal is better than that of BS,and that of CCR is better than that of WLM.

Fig.5.The effect of the four industrial wastes and their loading degree(i.e.,0.2 and 0.5%)on coal mass loss during combustion:(a)SR and BS,(b)CCR and WLM.

3.4.Catalytic mechanisms of individual wastes on coal combustion

The general process of coal combustion involves two main steps,i.e.,the coal devolatilization(homogeneous combustion)and char combustion(heterogeneous combustion)[18,19].In the reaction of coal with oxygen,the gas-phase oxygen transporting to the surface of the pulverised coal is considered the control step.During the coal combustion process,these oxides can be used as active oxygen carriers,which will carry the oxygen from the surface of carbon in high temperature.Moreover,in this step,the full contact between oxygen and coal is achieved,and a high amount of oxygen is adsorbed on the surface.A sufficient amount of oxygen ensures a high combustion rate of the coal,thereby lowering the coal ignition temperature.

The mechanisms involved in the catalytic phenomena associated with SR and BS during coal combustion are shown in Fig.6(a).When SR is used as a coal combustion catalyst,the NaCl and C16H31NaO2in SR are initially oxidised to Na2O2during the combustion process.Na2O2then transports oxygen to carbon while it is reduced to Na2O by carbon.The regeneration of Na2O2will occur by an oxidation process and able to start a new catalytic cycle.In other words,Na in BS(chemically pure NaCl)is always in the cycle of oxidation and reduction processes involved in the coal combustion.This step enables a continuous transport of oxygen to the surface of carbon,which promotes and maintains the carbon combustion.The equations explaining this mechanism are shown as follows:2C16H31NaO2+44O2→Na2O+31H2O+32CO2,2NaCl+O2→Na2O2+2Cl,2Na2O2+C→2Na2O+CO2,2Na2O+O2→2Na2O2.The catalytic reactions of CCR and WLM involved in coal combustion are shown in Fig.6(b).The CaO produced from CCR will be oxidised to CaO2.CaO2carries oxygen to the surface of carbon that will reduce it again to CaO and a new catalytic cycle will start.When WLM is used as a coal combustion catalyst,the CaCO3from its composition is decomposed to CaO.Hence,the Ca in CCR and WLM will change between CaO and Ca,thereby ensuring a continuous transport of oxygen to the carbon surface.

Fig.6.Catalytic mechanisms of four industrial wastes on coal combustion process:(a)SR and BS,(b)CCR and WLM.

4.Conclusions

Four industrial wastes,i.e.,SR(rich in C16H31NaO2),BS(rich in NaCl),CCR(rich in Ca(OH)2),and WLM(rich in CaCO3)were investigated in this work to understand their catalytic effect on the coal combustion.Although the main component that promoted coal ignition in SR and BS was the alkaline metal Na.The acidity of palm acid radical anion that associated with Na+in SR was weaker than that of the chloride anion that associated with Na+in BS,which led to a better combustion promotion effect of SR.The acidity of OH-anion that associated with Ca2+in CCR was weaker than that ofanion associated with Ca2+in WLM,resulting in CCR exhibiting a better catalytic effect on coal ignition than WLM.The alkaline metal Na had lower initial ionisation energy than the alkaline earth metal Ca.Therefore,the Na-rich SR exhibited a higher catalytic activity on coal ignition than Ca-rich CCR.The thermal ignition temperature of coal with 0.5% SR decreased from 544 °C to 503°C.