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Mercury Removal in Cement Industry Using Venturi Type Wet Scrubber

G.M. Ngala1, B.M. Tela2 and B.K. Highina3

1Department of Mechanical Engineering, University of Maiduguri, Nigeria 2Nigerian Nuclear Regulatory Authority, Nigeria

3Department of Chemical Engineering, University of Maiduguri, Nigeria

International Journal of Engineering & Technology Research Volume 2, Issue 6, November-December, 2014, pp. 48-54

ISSN Online: 2347-4904, Print: 2347-8292, DOA : 20122014 © IASTER 2014, www.iaster.com

ABSTRACT

A Venturi Type Wet Scrubber is air pollution control equipment that uses spray of water to remove particulate and poisonous matter from flue gas in industry. The problem of air pollution and global warming has become major policy issue especially when citing an industry. This study carried out the performance evaluation of Venturi wet scrubber for removal of mercury from Coal combustion. The methods adopted in this work includes the collection of relevant data and sample, analysis and experimentation through combustion, performance tests were carried out using the constructed scrubber and mercury contents were evaluated using standard testing procedures and equipment in National Agency for Food and Drug Administrator and Control (NAFDAC). The optimum value of the mixture was found to be 1200 g of Coal to 750 g of additive (CaCl2) at temperature of 6000C. Hence, the system optimum efficiency was determined to be 57.73%. The study demonstrated an abatement strategy for mercury produced in coal combustion in Ashaka Cement Industry using varying proportion of Coal with Calcium Chloride as an additive.

Keywords: Wet Scrubber, Mecury, Venturi, Particulate Matter.

1. INTRODUCTION

Mercury is the only metal that is liquid (pure mercury is a silvery-white liquid metal) at ordinary temperatures (Lenntech, 2009). It is a heavy metal such that a cup of mercury is nearly fourteen times heavier than a cup of water. Mercury is an element in the earth's crust and cannot be created or destroyed. It is a poor conductor of heat as compared with other metals but is a fair conductor of electricity. Mercury is a highly toxic compound commonly found in coal and municipal wastes. (University of Calgary, 2007).

Mercury is a dangerous neurotoxin, interfering with the brain and nervous system. Exposure to mercury can be particularly hazardous for pregnant women and small children. During the first several years of life, a child’s brain is still developing and rapidly absorbing nutrients. Prenatal and infant mercury exposure can cause mental retardation, cerebral palsy, deafness and blindness. Even in low doses, mercury may affect a child’s development, delaying walking and talking, shortening attention span and causing learning disabilities. There are emerging concerns over the potential adverse effects of ambient levels of environmental mercury during early childhood development (Trasande et al. 2005). There is sufficient evidence that children and other

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developing organisms are particularly susceptible to the adverse neurological effects of mercury (Raymond et al. 2008). Mercury poses a threat to adult men, as well as women and children. In adults, mercury poisoning can adversely affect fertility and blood pressure regulation and can cause memory loss, tremors, vision loss and numbness of the fingers and toes. So, there is the need to remove or reduce the mercury content in the ambient air of such environment where mercury is released.

Many of the world’s producers of cement acknowledge that: “The cement industry is at the centre of the climate change debate — but the world needs construction material for schools, hospitals and homes” (Rosenthal, 2010). Many of the world’s producers of cement acknowledge that: “The cement industry is at the centre of the climate change debate — but the world needs construction material for schools, hospitals and homes” (Rosenthal, 2010).

Mercury is a heavy metal found naturally in trace amounts in the earth’s atmosphere in differing forms—as elemental vapour, reactive gaseous compounds, or particulate matter. Studies show that background levels of environmental mercury deposition have steadily increased several folds since the pre-industrial era, with the largest source of potentially adverse exposures coming primarily from coal-fired utility plants (33%), municipal/medical waste incinerators (29%) and commercial/industrial boilers (18%) (Hilson, 2005). Other sources include hazardous waste sites, cement factories, and chlorine production plants. Mercury is now widespread in the environment (Carrasco et al. 2002).

The process for making cement often relies on fuels and raw materials that are high in mercury content. The quantity of mercury emissions from cement kilns is not widely known. To fuel this process, cement kilns burn primarily coal. Both the rock and the coal contain mercury, a highly volatile metal that evaporates at room temperature. Virtually all the mercury in the coal and limestone is vaporized in the cement production process, and the vast majority of that mercury enters the air through the kilns’ smokestacks.

The exhaust gases of combustion of coal may contain substances considered harmful to the environment, and the scrubber may remove or neutralize those substances. A wet scrubber is used to clean air, flue gas or other gases of various pollutants and dust particles. Wet scrubbing works via the contact of target compounds or particulate matter with the scrubbing solution. Solutions may simply be water (for dust) or solutions of reagents that specifically target certain compounds (Kelly Martin et al., 1999).

A Venturi type scrubber is a device that effectively uses the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of air pollution control equipment, collectively referred to as wet scrubbers (Cheremisinoff and Young, 1976). This study is aimed at the control of mercury released from coal combustion in cement industries using Venturi Type wet scrubber.

2. MATERIALS AND METHODS The model Venturi Type Wet Scrubber (Plate 1) is equipment designed and constructed for mercury trapping and removal of mercury from cement factory. To carry out this function the equipment has the following components: Combustion Chamber, Air Blower, Venturi throat assembly, water pump, and water collector (see figure 1 for details.). The combustion chamber is where the mixture of coal and calcium chloride are burned. The Chamber has a port at the top where temperature is measured. There is a pipe connecting the Chamber to Blower. The Blower supplies air to the combustion chamber at varying

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flow rate. It has a control valve where flow rate of the air is controlled by opening and closing of the air inlet to the blower. The blower also serves as temperature regulating device, since an increase in volume of air increases the rate of combustion. The Venturi throat assembly consists of nozzles, and water supply lines. It is the cavity where the flue gas mixed with water. The convergent-divergent assembly reduces the velocity of the gas where high pressure water droplets are injected through the nozzles and the oxidized Hg and dust particulates are collected via impaction. The separator chamber separates effluent from the abated gas. The clear gas (moisturized gas) from the throat which is lighter travels up through the separator and a demister which removes the moisture in the gas at the top of the separator. The water is collected at the bottom of the separator and it is recycled.

Water collector is a reservoir that store the water used for the Scrubber for the scrubbing of the flue gas produce in the coal combustion, and it allow the water to re-circulate.

3. PRINCIPLE OF OPERATION

The machine needs a 220 – 240 Volt power supply source where the pump and the blower are to be connected. Thirty (30) liters of clean water is poured to the reservoir at the bottom of the scrubber and the pump is switch on to start the circulation. The Coal is measured with Calcium Chloride and put in to the combustion chamber and ignited. As the combustion starts, the Handle of the combustion chamber is then screwed to the combustion gas inlet pipe. The system is now set for operation. The product of combustion mixes up with high pressure water droplets, which passes down to the separator and the effluent passes down to the collector while the clean air rises to pass through the demister and in to the atmosphere. This is done for each sample and the time taken is also noted.

4. EXPERIMENTATION

The experiment was conducted at the Faculty of Engineering teaching workshop university of Maiduguri and samples were collected for analysis. The analysis of the samples were done at laboratory to determine the quantity of mercury in the samples

Experimental Procedure

The experiment was conducted as follow:

First, 1200 g of Coal was dissolved in water, and the sample was taken and analysed with a mercury analysing device (Smart Specro) to determine the total quantity of mercury in the coal. Secondly 1200 g of coal was burnt in the scrubber’s combustion chamber at 600 oC and the flue gas was passed through a Venturi type wet Scrubber. The effluent was collected and analysed at laboratory. Plate 1: Model Venturi Wet Scrubber

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Similarly, at the subsequent tests, to the 1200 g of coal a 250 g, 500 g, 750 g and 1000 g of calcium chloride were separately added and burnt respectively. The effluent was collected in each case and analysed at laboratory. Three tests were carried out three times on each case and average value was taken. The results are shown in table 1 and 2 below.

Fig. 1: Detail Drawing of Venturi Wet Scrubber

Experimental Results

Table 1: Results of Set 1 Test

S/No Quantity of Coal

used (kg)

Weight of CaCl2 (kg)

Vol. of water

used (lt)

Time taken (sec)

Temperature (oC)

Quantity of mercury

removed (ppm)

Efficiency

1 1.2 0 30 4200 600 0.09 9.27% 2 1.2 1 30 5400 600 0.26 26.8% 3 1.2 0.75 30 7800 600 0.56 57.73% 4 1.2 0.50 30 5220 600 0.47 48.45% 5 1.2 0.25 30 4500 600 0.37 38.14%

Table 2: Reading Results of Hg in 1.2Kg of Coal

Readings First runing (ppm) Second runing(ppm) Third runing (ppm) 1.2kg 0.97 0.98 0.96

Average Value 0.97

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Test Results Analysis

Total Mercury in 1.2Kg of coal = 0.97 ppm When 1.2kg of Coal was burnt in combustion chamber and passed the flue gas through the Scrubber, the amount of mercury in effluent = 0.09 ppm Difference 0.97 - 0.09 = 0.88ppm When 1000g of CaCl2 was mixed with feed Coal of 1.2kg and burnt = 0.26ppm of Hg was realized Difference 0.97 - 0.26 = 0.71ppm When 750g of CaCl2 was mixed with feed Coal of 1.2kg and burnt = 0.56ppm of Hg was realized Difference 0.97 – 0.56 = 0.47ppm When 500g of CaCl2 was mixed with feed Coal of 1.2kg and burnt = 0.47ppm of Hg was realized Difference 0.97 – 0.47 = 0.50ppm When 250g 0f CaCl2 was mixed with feed Coal of 1.2kg and burnt = 0.37ppm of Hg was realized Difference 0.97 – 0.37 = 0.60ppm System Efficiency

1. η = 0.09/0.97 x 100 = 9.27% 2. η = 0.26/0.97 x 100 = 26.8% 3. η = 0.56/0.97 x 100 = 57.73% 4. η = 0.47/0.97 x 100 = 48.45% 5. η = 0.37/0.97 x 100 = 38.14%

The optimum efficiency is 57.73%

Figure 2 : Cacl

2 Content Impact on Hg Content

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Figure 3: Cacl

2 Efficiency on Mercury Removal

5. DISCUSSIONS OF RESULTS From the graph (Fig.2) it was observed that the amount of mercury captured increases as the amount of additives (CaCl2) increases from 250g to 750g and also decreases from addition of the additive from 750g to 1000g of CaCl2. This result is in agreement with the study reported by Zhunang et al (2007).

From the graph (Fig.3) it was observed that the efficiency of the system increases from 38.14% to 57.73% as the quantity of the additive increases from 250g to 750g and hence, the efficiency decreases from 57.73% to 26.8% as the additive increases from 750g to 1000g. Addition of Calcium chloride to the gas stream greatly increased the amount of HgO removal. This increase in the removal may have been due to the formation of an intermediate compound, such as nitrosyl chloride (NOCI), which could react rapidly with the HgO. The majority of the mercury was captured in the sump condensate, with most of the remainder collecting in the slurry.

The collection efficiencies for conventional scrubbers depend on the operating factors such as inlet gas loading, catalysts and energy input. Conventional scrubbers rely almost exclusively on initial impaction of gas with water. The efficiencies of scrubbers that rely on inertia impaction can be improved; however by increasing the relative velocity between the flue gas and the liquid droplets. Increasing the velocity will result in more momentum for all the gas enabling to dissolve the soluble mercury in the flue gas. In addition to HCl (g) and Cl2 (g), O2 (g) and NO2 (g) are potential mercury reactants in flue gas. Kinetic limitations and the relatively short flue gas residence time in the combustion chamber duct to the scrubber stack (4200 sec-7800 sec), preclude the occurrence of any significant homogeneous reactions involving HgO(g) and O2(g) or NO2(g). However, in the presence of inorganic and carbonaceous ash particles, these gases are apparently important in HgO (g) adsorption and oxidation processes.

The addition of CaCl2 to the solution for Hg capture is an interesting choice because it was found to be non corrosive and very cheap (N1000.00/kg), even though the pH of the effluent solution was not determined. About 90% of the CaCl2 goes to the bottom ash while only 10% is emitted into the air, which determines the ability to perform the tests successfully highlighting the high quantity of mercury removal potential from the developed Venturi type wet scrubber.

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6. CONCLUSIONS

The relationship between inlet flue gas concentration and Mercury collection efficiencies for the scrubber were found to be increasing as the additive increases to a certain level. This also suggests that scrubber removal efficiency is not constant for a given scrubber design unless it is referred to a specific inlet gas loading.

The study also concludes that flue gas from Coal combustion contains toxic waste such as mercury from Cement factories. This has been reduced to a minimum by using Venturi type wet scrubber with an addition of additives in the combustion chamber.

The aim of developing Venturi type wet scrubber for removal of mercury in coal combustion has been achieved. The efficiency of the machine was found to be 57.73% and the maximum amount of Hg removed was 0.56 ppm when 750g of CaCl2 additive was used.

REFERENCES [1] Carrasco, F., Bredin, N. and Heitz, M. (2002) Atmospheric Pollutants and Trace Gases: Gaseous

Contaminant Emissions as Affected by Burning Scrap Tires in Cement Manufacturing, Published in Journal of Environmental Quality. Vol 31, pp: 1484–1490.

[2] Cheremisinoff and Young (1976) Pollution Engineering Handbook; Second printing, pp. VI, 125 – 184.

[3] Hilson, G. (2005) Abatement of Mercury Pollution in the Small-Scale Gold Mining Industry: Restructuring, The Policy and Research Agendas, Science of the Total Environment, doi:10.1016/j.scitotenv.2005.09.065, Elsevier.

[4] Kelly, M. (1999) Elemental Mercury Removal Using a Wet Scrubber. http://www.osti.gov/bridge/purl.cover.jsp?purl=/11821-8y5zOt/webviewable/11821.pdf .retrieved on 21st December 2011.

[5] Lenntech (2009) Water Treatment Solutions http://www.lenntech.com/periodic/elements/hg.htm retrieved on 15th January 2011

[6] Raymond, F. P., Stephen, B. and Robert, W. (2008) Proximity to Point Sources of Environmental Mercury Release as a Predictor of Autism Prevalence; Health & Place (2008.02.001), Elsevier.

[7] Rosenthal, E. (2010) Cement a Global Pollutant; October 26, http//www.cemnet.com/world-cement, accessed on 5th November, 2010.

[8] Trasande, L. Philip, J. L. and Clyde, S. (2005) Public Health and Economic Consequences of Methyl Mercury Toxicity to the Developing Brain, Environmental Health Perspectives. Vol. 113, No. 5, May. May 2005

[9] University of Calgary (2007) Mercury Abatment Program, Alberta Occupational Health & Safety Act, Regulations & Code http://www.hre.gov.ab.ca/cps/rde/xchg/hre/hs.xsl/2444.html?requested-page=/hre/whs/law/ohs.asp* . Retrieved on 13th September 2011

[10] Zhunang, Y.E. (2007) Impact of Calcium Chloride addition on Mercury Transformations and Control in Coal Flue Gas. Elsevier, Vol.1 , No. 86, PP2351-2359