A brief overview of soot formation theory and soot reduction methods in steam assisted flares

Transcription

1 International Journal of Environmental Science and Toxicology Research (ISSN: ) Vol. 3(3) pp , March, 2015 Available online Copyright 2015 International Invention Journals Review A brief overview of soot formation theory and soot reduction methods in steam assisted flares Rekhapalli Srinivasarao 1 * and KVSG Murali Krishna 2 1 Research Scholar at JNTUK University, Kakinada, A.P., India 2 Professor of Civil Engineering, JNTUK University, Kakinada, India Abstract Soot is a black powdery substance that consists mainly of carbon and is formed through the incomplete combustion of wood, coal, diesel oil or other materials. Soot particles absorbs energy from sunlight rather than reflecting it, Soot is believed to be a cause of global warming, especially when it settles on snow and ice, reducing their reflectivity. Soot formation is quite common in industrial flares those which are not controlled properly. Unwanted waste gases are burn in flare stacks. Flaring is a technique used extensively in the oil and gas industry to burn unwanted flammable gases. Oxidation of the gas can preclude emissions of methane, however flaring creates other pollutant emissions such as particulate matter in the form of soot or black carbon. Smoke formation may vary from different gases which are flaring and have different carbon components. Smoke formation depends on distribution of combust air mixed in the flare tip nozzles. Most of the flare stacks are assisted with steam to get complete combustion to avoid black smoke and unburnt hydrocarbons. The amount of steam is insufficient to the flare stack, leads to generate more smoke. It was estimated that, soot particles life time in the atmosphere is 1-2 weeks to disperse. This paper explains the steam reforming reactions in the flame zone of a flare stack and methodology of control of soot from flares by steam to carbon ratio control. The economical benefit of smokeless operation of flare stacks are energy saving in terms of minimization of excess steam and indirectly reduce the amount of CO2 to the atmosphere. Keywords: Black Carbon, Green house gas, Poly Aromatic Hydrocarbons, Soot particle. INTRODUCTION In most of the oil and gas industries, the flare is manually observed by the utility operator for any abnormality. Manual observation of the flare in CCTV on a 24 7 basis is a difficult job, and is not a reliable way to detect abnormalities. In case of plant shut down/emergency or pressure relief valves pops up, sudden flaring cause smoky flare. It may take some time for the operator to respond to inject steam as process operator s manual action. During this time, often flares are smoky. New scientific evidence has led to recognition of the significant role of black particles (black carbon BC) as one of the short-lived climate forcers. Measures focused on BC and methane is expected to achieve a significant short-term reduction in global warming (WHO, 2012). *Corresponding Author Rekhapalli_sri@yahoo.co.in According to US Environmental Protection Agency, Rule 401 prohibits visible emissions in excess of Ringelmann 1 or 20 percent opacity for periods exceeding more than three aggregate minutes within any hour. 40CFR and (USEPA 1984; 1985; 1986) requires flares to have no visible emissions except for periods of time up to five minutes during two consecutive hours. If waste gases venting with black smoke and its opacity is above Ringlemann no#2 and smoky flare is more than 5 minutes, it should be reported as flare incident. It is difficult to adjust steam to flare manually during this 5 minutes crucial period of process plant upset. Automatic control of steam flow to flare by adopting S/C ratio control with feed forward action reduces the soot formation from flare stacks. Soot particles in the air are a contributing factor in respiratory diseases. The fine particles (<3µ) are the worst causing of lung damage due to their ability to penetrate into the deep air passages. Larger particles (>3µ) are trapped in

2 50 Int. J. Environ. Sci. Toxic. Res. the nose and throat from which they are easily eliminated, but finer particles can stay intact for years in the innermost regions of the lungs, which have no effective mechanism for particle removal. the tail pipe by coagulation to form aggregated chains, and further by condensation of the simultaneously released semi-volatile organic substances on their surfaces in the atmosphere. Figure 2. MATERIALS AND METHODS Emissions from flaring include carbon particles (soot), unburned hydrocarbons, CO, and other partially burned and altered hydrocarbons. Also emitted are NOx and, if sulphur-containing material such as hydrogen sulphide or mercaptans is flared, sulphur dioxide (SO2) is emitted. The quantities of hydrocarbon emissions generated relate to the degree of combustion. The degree of combustion depends largely on the rate and extent of fuel-air mixing and on the flame temperatures achieved and maintained. Properly operated flares achieve at least 98 percent combustion efficiency in the flare plume, meaning that for smokeless combustion, sufficient primary air must be supplied, this varying from about 20 percent of stoichiometric air for paraffin to about 30 percent for an olefin. If the amount of primary air is insufficient, the gases entering the base of the flame are preheated by the combustion zone, and larger hydrocarbon molecules crack to form hydrogen, unsaturated hydrocarbons, and carbon. The carbon particles may escape further combustion and cool down to form soot or smoke. Olefins and other unsaturated hydrocarbons may polymerize to form larger molecules which crack, in turn forming more carbon. The fuel characteristics influencing soot formation include the carbon-to-hydrogen (C-to-H) ratio and the molecular structure of the gases to be burned. All hydrocarbons above methane, i. e., those with a C-to-H ratio of greater than 0.33, tend to soot. Branched chain paraffin s smoke more readily than corresponding normal isomers. The more highly branched the paraffin, the greater the tendency to smoke. Unsaturated hydrocarbons tend more toward soot formation than do saturated ones. Soot is eliminated by adding steam or air; hence, most industrial flares are steam-assisted and some are air-assisted. Emission factors For a very rough order of magnitude estimate, considering gas flared volumes of 139 billion m 3 /year as estimated from satellite data ( )) and estimating a single valued soot emission factor of 0.51 kg soot/103 m 3, flaring might produce 70.9 Gg of soot annually. This amounts to 1.6% of global black carbon emissions from energy related combustion, based on estimates of 4400 Gg for the year Soot in concentration values ( ppt; USEPA, 1983): Non smoking flares, 0μg/l; Lightly smoking flares, 40μg/l; Average smoking flares, 177 μg/l; and Heavily smoking flares, 274μg/l. Steam to hydrocarbon reforming reaction The reforming process is based on the reaction between methane and higher hydrocarbons present in the natural gas with steam thereby generating CO, CO2 and Hydrogen. After reforming reaction, combustion reaction takes place. Reforming Reactions The reforming of any hydrocarbon is as follows: Combustion The direct combustion of hydrocarbons: After reforming: Soot formation pathway Smoke burns up in high temp region (smokes if soot exits this region before being consumed).smoke forms when C-C bonds in hydrocarbon crack and aromatic structures grow in to multi ring molecules (>3 ring=primary soot particle). Other Poly aromatic hydrocarbons (PAH) form a long reaction route to soot. The optimal combustion of fuel at high temperature, such as the current low-sulfur fossil diesel fuel in modern diesel engines, results in the emission of large numbers of very small soot particles (aerodynamic diameter 1 5 nm) that rapidly grow in size ( nm) in (figure 1) Soot Reduction Mechanism A diffusion flame receives its combustion oxygen by diffusion of air into the flame from the surrounding atmosphere. The high volume of fuel flow in a flare may require more combustion air at a faster rate than simple gas diffusion can supply. High velocity steam injection nozzles, positioned around the outer perimeter of the flare tip, increase gas turbulence in the flame boundary zones, drawing in more combustion air and improving combustion efficiency. For the larger flares, steam can also be injected concentrically into the flare tip. The

3 Srinivasarao and Krishna 51 Figure1. Soot particles. Figure 2. Soot formation pathway injection of steam into a flare flame can produce other results in addition to air entrainment and turbulence. Three mechanisms (Guide for Pressure-Relieving and Depressurizing Systems (1982).) in which steam reduces smoke formation have been presented. Briefly, one theory suggests that steam separates the hydrocarbon molecule, thereby minimizing polymerization, and forms oxygen compounds that burn at a reduced rate and temperature not conducive to cracking and polymerization. Another theory claims that water vapour reacts with the carbon particles to form CO, CO2, and H2, thereby removing the carbon before it cools and forms smoke. An additional effect of the steam is to reduce the temperature in the core of the flame and suppress thermal cracking (Letter from David Shore (Flare gas Corp., Spring Valley, NY) to William M. Vatavuk (U.S. Environmental Protection Agency, Research Triangle Park, NC), October 3, (1990)). The physical limitation on the quantity of steam that can be delivered and injected into the flare flame determines the smokeless capacity of the flare. Smokeless capacity refers to the volume of gas that can be combusted in a flare without smoke generation. The smokeless capacity is usually less than the stable flame capacity of the burner tip. Significant disadvantages of steam usage are the increased noise and cost. Steam aggravates the flare noise problem by producing high-frequency jet noise. The jet noise can be

4 52 Int. J. Environ. Sci. Toxic. Res. Figure 3. Feed forward control signal on ratio controller reduced by the use of small multiple steam jets and, if necessary, by acoustical shrouding. Steam injection is usually controlled manually with the operator observing the flare (either directly or on a 7-17 television monitor) and adding steam as required to maintain smokeless operation. To optimize steam usage infrared sensors are available that sense flare flame characteristics and adjust the steam flow rate automatically to maintain smokeless operation. Automatic control, based on flare gas flow and flame radiation, gives a faster response to the need for steam and a better adjustment of the quantity required. If a manual system is used, steam metering should be installed to significantly increase operator awareness and reduce steam consumption. As infrared cameras function is erratic in case of wind direction changes i.e., smoky flame direction changes according to wind and it will not be in the focal point of IR camera lenses. Ultrasonic flow meters are best suit in these scenarios. UFM are working with high turn down ratio 2000:1. Ultrasonic flow meter measures the gas flow to the flare. Different molecular weight gases having different carbon numbers. Gaschrometography measures the carbon numbers of different gases. Sum of all different carbon numbers gives total number of hydro carbon to flare. Carbon flow in kg/hr can be achieved as follows, Carbon flow (kg/hr) = hydrocarbon flow (N m 3 /hr)* total carbon no./ Gas Chromatography takes 15-20minutes to calculate the complete gas composition. Feed forward signal compensates this time lag. In a process unit any vent to flare header control valve output >0% and predetermined carbon number will be taken from the loop is used as a feed forward signal to ratio controller (Srinivasarao and Krishna, 2014). If multiple vents to flares are operate at the same time, higher carbon number output will be taken by ratio controller. Then once FF signal taken from a loop, steam flow set point will be calculated and steam control valve will open. After getting gas Chromatograph results (approx 15-20min) actual carbon number will be calculated and it will optimize the steam to flame zone. Figure 3. Steam flow required to achieve smokeless flame will be calculated as follows Required Steam flow (kg/hr) = S/C * Carbon flow (kg/hr) Health Effects of Soot Soot particles in the air are a contributing factor in respiratory diseases. The fine particles of size less than 3micron can penetrate easily into air passage. Larger size than 3 micron are entrapped in the nose. But very fine particles stay for longer duration in the lungs (Srinivasarao and Krishna, 2012).). These are causing cancer and bronchiole problems. Larger particles which are trapped in the nose may create breathing problems and creates irritation. Emissions of fine particulate matter i.e., black smoke from flare stacks is almost same chemical characteristics of diesel engine exhaust which have been growing in community, industry and government concern. Combination of small size and chemical composition increases the likelihood that particles will carry irritants and toxic compounds into the deepest and most sensitive areas of the lungs. Pneumonia, bronchitis, and asthma can causes from these fine particles. Occupational health studies depicts lung cancer is causing from black carbon particles which are from diesel engine exhausts. Traffic studies suggest

5 Srinivasarao and Krishna 53 increased rates of respiratory and cardiovascular disease and risk of premature death near busy urban streets or highways and thus must be addressed by industry and government. CONCLUSIONS Based on simulation study, steam injection achieved less than 5minutes by feed forward control action and S/C ratio control. This control action keeps the correct stoichiometric amount of steam to hydrocarbon and makes zero soot formation from the flare stack in any emergency situation and regardless of any weather conditions. So control action takes place within 5 minutes, meet or exceed government legislation, and eliminate risk of non-compliance, as flare opacity is always less than Ringlemann index#2. Automatic control of soot by ratio controller keeps always zero soot formation from the flare stack in any emergency situation. US Environmental Protection Agency proposal would strengthen the annual health standard for harmful fine particle pollution (PM2.5) to a level within a range of 13μg/ m3 to 12μg/m 3. The current annual standard is 15μg/ m 3. By proposing a range, the agency will collect input from the public as well as a number of stakeholders, including industry and public health groups, to help determine the most appropriate final standard to protect public health. ACKNOWLEDGMENT REFERENCES Srinivasarao R, Krishna KVSGM (2012). Particulate Matter from Refinery Flares and Health Effects of Soot, International Journal of Information Technology and Business Management (ISSN ), Vol. 6, No.1. pp 78-83, 29 October. Srinivasarao R, Krishna KVSGM (2014). Simulation study to minimize soot and unburnt hydro carbons from steam assisted flares and Health effects of Soot, International Journal of Science and Research (IJSR), Volume 3, Issue 10, October. USEPA (1983). Flare Efficiency Study, EPA-600/ , U. S. Environmental Protection Agency, Cincinnati, OH, July. USEPA (1984). Evaluation of the efficiency of industrial flares: Test results, United States Environmental Protection Agency, Office of Air quality planning and Standards, EPA-600/ , May. USEPA (1985). Evaluation of the efficiency of industrial flares: Flare Head design and gas composition, US EPA Office of Air quality planning and Standards, EPA-600/ , September. USEPA (1986). Evaluation of the efficiency of industrial flares: H2S gas mixtures and pilot assisted flares, US EPA, Office of Air quality planning and Standards, EPA-600/ , September. WHO (2012)Report: Joint World Health Organization/Convention task force on health aspects of air pollution, Health Effects of Black carbon, page-1 Estimated Flared Volumes from Satellite Data ( ). Guide for Pressure-Relieving and Depressurizing Systems (1982). Refining Department, API Recommended Practice 521, Second Edition, September Letter from David Shore (Flare gas Corp., Spring Valley, NY) to William M. Vatavuk (U.S. Environmental Protection Agency, Research Triangle Park, NC), October 3, (1990). How to cite this article: Srinivasarao R, Krishna KVSGM (2015). A brief overview of soot formation theory and soot reduction methods in steam assisted flares. Int. J. Environ. Sci. Toxic. Res. Vol. 3(3):49-53 Profound gratitude and sincere thanks to all who contributed to the success of this work.