Fuels, Combustion, and Flue Gas Analysis (3rd Class Edition 3 - Part A2)

Fuels, Combustion, and Flue Gas Analysis • Chapter 3 ^ OBJECTIVE 9 Explain the use and combustion characteristics of alternatives to traditional fossil fuels, including biomass fuels, coke, and oil emulsions. The Power Engineer should be aware of Canadian and international trends and policies rel...

Fuels, Combustion, and Flue Gas Analysis • Chapter 3 ^ OBJECTIVE 9 Explain the use and combustion characteristics of alternatives to traditional fossil fuels, including biomass fuels, coke, and oil emulsions. The Power Engineer should be aware of Canadian and international trends and policies relating to energy and fuels. These issues have a major effect on power and heating plant operation. Although traditional fossil fuels (coal, oil, and gas) have long been the dominant energy source in the industrial sector, there is a push to develop alternative sources of energy. Biomass fuels are one of the main technologies developed in the last several decades. Biomass refers to substances derived from living organisms. Since it is derived from living things, such as trees and crops, biomass is a renewable form of energy. Biomass substances have always been used as a source of fuel, but due to the demand for alternative energy, there is now an increased interest in biomass research, production, and utilization. Increased costs of fossil fuels, shortages of landfills, advances in technology, and the use of cogeneration systems make biomass fuels viable as alternative sources of heat and electricity. Biomass fuels may be fired alone or in combination with gas, oil, or coal. The heat and power produced through the combustion of biomass materials is utilized within the same facility that produces the biomass. However, facilities that do not require all the energy released by the combustion ofbiomass may have the option to sell the excess electricity or heat to external clients. The following questions are often asked when considering biomass as a fuel: a) WTiat is the ability to renew the resource? In many areas, seasonal availability and renewal rates may impact the economics ofbiomass use. b) How much does it cost? Since it is a waste product, the cost of the biomass is often quite low. However, there are hidden costs and unrealized values if the burned biomass would normally be used to produce other products. It is also generally uneconomical to transport biomass fuels over long distances. c) What are the emissions and effluents? As a natural source, biomass is often promoted to have low emissions. There is no increase in atmospheric carbon dioxide (€02) since, if still alive, it would naturally produce this compound anyway. However, processing activities and conditions may produce non-natural effluents, such as sulfur dioxide (,SO^) and nitrogen oxides (NOx), which can have detrimental effects on emission quality. d) What are the production and maintenance challenges? Biomass fuels have complex and variable compositions. Without constant care, there is a high tendency for fouling or the formation of slag, also known as slagging, during combustion. 3rd Class Edition 3 • Part A2 149 ?& Chapter 3 • Fuels, Combustion, and Flue Gas Analysis TYPES OF BlOMASS The main types ofbiomass are: 1. Wood products (hog fuel, wood chips, pellets, pulping liquids, and waste wood) 2. Agricultural biomass, such as: • Crops such as grains, corn, sugar cane, sugar beets, switchgrass, and sorghum • Crop residue such as bagasse, straw, leaves, and husks • Animal manure 3. Municipal waste (garbage containing food, paper, and wood waste) 4. Marine biomass (seaweed and algae) Wood and Agricultural Biomass The most common biomass fuels are wood pellets and wood chips. The wood and agricultural product industries produce large amounts of bark, chips, stems, and other refuse. Wood products with moisture content as high as 65% may produce stable combustion in water-cooled furnaces. Preheated combustion air reduces the time required to dry the fuel prior to ignition. Air entering above the grate or burner area is utilized to ensure that the volatile combustion gases produced are completely burned. The biomass is burned directly in boiler furnaces. The combustion technologies in these power plants include rotary kilns, water-cooled rotary combustors, spreader and stoker-fired furnaces, suspension-fired boilers, fluidized bed boilers, and cyclone furnaces. The following are typical heating values for a variety of wood and agricultural waste products. Since the burning quality and content of biomass fuels tend to vary depending on their source, these are approximate values only: • Dry wood bark 20000kJ/kg • Corn leaves and talks (stover) 18 000 kj/kg • Sugarcane leaves and stalk (bagasse) 18 000 kj/kg • Bamboo 19000kJ/kg • Sweet sorghum 15 000 kj/kg Municipal Waste Municipal wastes contain large amounts of biomass material that may be used as fuel. Due to environmental issues and changes in the way goods are packaged, the heating value of municipal waste is increasing, and the moisture content is decreasing, making it even more attractive as a fuel. The heating value varies from about 6000 kj/kg to 15 000 kj/kg, depending on the moisture content (normally 20% to 35%) and the combustible components of the waste (15% to 35%). There are two general methods used to burn municipal wastes (refuse derived fuel). One method involves the removal of large non-combustibles, such as metal and appliances, with the remaining waste products pushed onto stoker grates. The ash and other non-combustibles are reclaimed in an ash pit for reclamation or disposal. The other method involves more preparation of the fuel prior to it entering the furnace, with recyclable products first removed and then the combustibles sorted before going to the furnace. The latter method achieves a higher heating value per tonne of waste. 150 3rd Class Edition 3 • Part A2 Fue/s, Combustion, and Flue Gas Analysis • Chapter 3 BlOFUELS Fuels derived from biomass maybe in liquid or gaseous forms. Biofuels, such as biodiesel, ethanol, and pyrolysis oils, are liquid fuels made from biomass. Biodiesel Biodiesel is similar to diesel fuel, except it has a wider range of characteristics depending on its source. This fuel is produced by combining vegetable oils, animal fats, or recycled greases with methanol (or ethanol) in the presence of a catalyst, such as sodium hydroxide. Biodiesel can be used as a fuel for vehicles in its pure form, but is usually used as a diesel additive to reduce levels ofparticulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles. Ethanol Ethanol (ethyl alcohol) is a renewable biofuel made from a variety of biomass materials. The feedstocks include grains, corn, sorghum, barley, sugar cane, and sugar beets. Ethanol can also be made from agricultural and forestry residues, such as corn cobs and stocks, rice straw, sawdust, and wood chips. Fermentation is the most common method to produce fuel ethanol. Due to its abundance and low price, corn is the main feedstock for fuel ethanol in North America. The starch in corn kernels is converted to sugar, which is then fermented into alcohol. In other continents, sugar cane and sugar beets are more widely used to make ethanol. Ethanol can also be produced by breaking down cellulose in plant fibres. This ceUulosic ethanol is considered an advanced biofuel and involves a more complicated production process than fermentation. Pyrolysis Oils Pyrolysis oil, also called bio-oil or bio-crude, is a synthetic fuel obtained from biomass. Fast pyrolysis is the process in which biomass is rapidly heated to about 500°C in a reactor in the absence of oxygen. As the biomass thermally decomposes, it converts mostly into vapours and some charcoal. Subsequent cooling and condensation form a liquid fuel with a heating value of about half that of conventional fuel oil. Pyrolysis oils are often economical as liquid fuel sources where organic feedstocks are plentiful. GASEOUS FUELS FROM BlOMASS It is advantageous to convert biomass into more versatile and economic forms, such as liquid and gaseous fuels. The main gaseous fuels produced from biomass are biogas (methane-C02) and synthesis gas (syngas). Biogas Biogas is a methane-C02 mixture produced when biomass is broken down by bacteria in a process called anaerobic digestion. Agricultural biomass includes animal manure; cellulosic crop residues, such as corn stalks, wheat straw, wood waste, fruit and vegetable culls; and food-processing wastewater. Agricultural biomass is processed in an anaerobic digestion plant to create biogas fuel and a by-product called digestate. Digestate refers to the solids and liquids remaining after anaerobic digestion. Acidogenic digestate is fibrous matter consisting of lignin and cellulose. JVtethanogenic digestate is a liquor or sludge containing nutrients such as phosphates and ammonium compounds. Digestate can be processed for nutrient recovery. This reduces the nutrient intake requirements for an agricultural business. 3rd Class Edition 3 • Part A2 151 ^ Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Biogas varies significantly in its composition with typical compositions of: • 55% to 65% methane • 30% to 35% carbon dioxide • Minor amounts of hydrogen, hydrogen sulfide, halogens (chlorides, fluorides), aromatic compounds, and air The heating value ofbiogas is around 600 BTU/ft3 (22 350 kj/m3). There are two typical configurations ofbiogas plants: 1. Industrial biogas plants, often at landfill sites, which receive either separated or mixed materials from several sources. The gas produced is usually sold to combined cycle power plants in the nearby vicinity. 2. Farm biogas plants, which utilize the waste (manure) from a single farm. Synthesis Gas (Syngas) Synthesis gas (syngas) is a gaseous fuel derived from coal, biomass, and waste products through a process called gasification. Synthesis gas is a versatile product that can be used on its own or converted to liquid fuels. Syngas derived from biomass is mainly made of hydrogen (N2), carbon monoxide (CO), carbon dioxide (C02), and methane (€N4) with lesser amounts of nitrogen (N2), hydrogen sulfide (l-^S), ammonia (NH3), water (H20), carbon particles, tar, and ash. A typical syngas has a composition by volume of CO 35%, H^ 30%, C02 23%, CI-LI 7%, and other 5%. The heating value depends on the method with which the heat is supplied to the gasifier. When air is used as the oxidant, the heating value is typically around 5 MJ/m3-6 MJ/m3. When oxygen is used, the heating value is typically 13 MJ/m3-15 MJ/m3. For indirectly-heated gasifiers, the heating value of the syngas may be as high as 18 MJ/m3-20 MJ/m3. The major biomass source ofsyngas is wood products. Cellulose, hemicellulose, and lignin are the principal components of wood product biomass. Gasification breaks these complex molecules down into simpler ones which causes the thermochemical conversion ofbiomass into syngas. Syngas can be used as a fuel for power generation and heating. It can also be converted to liquid fuels used in automobiles and other vehicles. The main technology for conversion is the Fischer-Tropsch process, which uses catalysts to convert carbon monoxide and hydrogen into liquid fuels. Syngas can also be converted to hydrogen which can be burned or used in fuel cells. Syngas from Black Liquor Gasification Plant The gasification of black liquor is a recently emerging technology for pulp mills. In this design, a gasification plant is used in place of the black liquor recovery boiler. As a black liquor boiler nears the end of its 30 to 40 year life, replacing the boiler with a gasification plant has the potential for greater energy efficiency and environmental benefits. As shown in Figure 9, the process to create syngas involves the gasification of black liquor at temperatures above the melting point of the inorganic chemicals. The evaporated black liquor is gasified in a pressurized reactor under reducing conditions. The gas is separated from the inorganic smelt and ash. The smelt falls into the quench bath where it dissolves to form a green liquor; this is similar to the dissolving tank of a recovery boiler. The raw fuel gas leaves the quench bath and is further cooled in a counter-current condenser. Hydrogen sulfide is removed from the fuel gas in an absorption stage. The resulting gas is a nearly sulfur-free synthesis gas (syngas) made up of carbon monoxide, hydrogen, and carbon dioxide. A typical application for black liquor syngas is as the fuel in a combined cycle plant. This concept is known as black liquor gasification combined cycle. The syngas first enters the combustor of a gas turbine generator. The hot flue gas from the gas turbine is then used to generate steam in a heat recovery steam generator (HRSG) and the high-pressure steam is used in a steam turbine plant for additional power generation. 152 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 -^ Black liquor gasification is an emerging technology. Compared to a conventional recovery boiler, a black liquor gasifier can increase the total energy efficiency of a chemical pulp mill, reduce waste heat at the mill, and produce a synthesis gas that can be used as a fuel for power generation, as described above, or other applications such as the production of automotive fuel. Figure 9 - Black Liquor Gasification Plant Gasification plant Black liquor Contactors Oxygen Atomizing media Green liquor Steam turbine Gas turbine To stack Combined cycle plant 3rd Class Edition 3 • Part A2 153 ?& Chapter 3 • Fuels, Combustion, and Flue Gas Analysis NON-TRADITIONAL FOSSIL FUELS Besides biomass, there are other alternatives to traditional fossil fuels (coal, conventional oil, and natural gas). The term non-traditional fossil fuels includes oil shales, gas hydrates, oil sands, coal bed methane, in situ coal gasification, methane clathrate (hydrate), coke, coal gasification, and oil emulsions. Coke Coke is a carbonaceous fuel derived from coal or crude oil. Depending on its origin, coke is generally classed as either metallurgical coke or petroleum coke. JVtetallurgical coke is made from bituminous coal low in ash and sulfur and with special properties. Coking involves heating the coal in coke ovens, in the absence of oxygen, to 540°C to drive off most of the volatile matter. This process is called thermal distillation or pyrolysis. The final product is a nearly pure carbon source ranging in size from foundry coke to a coke breeze (fine powder). JVtetallurgical coke is primarily used to smelt iron ore in blast furnaces, acting both as a source of heat and as a chemical reducing agent to produce pig iron. Petroleum coke (pet coke) is derived from petroleum refining or bitumen and heavy oil upgrading in the process of coking. Unprocessed petroleum coke is called green coke (or fuel grade) and may be produced by delayed coking or fluid coking. Delayed coke contains 8%-15% volatiles and 2%-8% sulfur. Fluid coke typically contains about 5% volatiles and is smaller in size. Calcined coke (anode grade) is thermally treated petroleum coke used to produce carbon anodes and graphite electrodes. Petroleum coke is a highly flexible and useful blending fuel that can be used in conventional pulverized coal, cyclone, and fluidized bed boilers, as a fuel for cement kilns, or as a feedstock for metallurgical coke in the steel industry. Pet coke can also be gasified to syngas. Typical composition for fuel-grade petroleum coke is 90% C, 4% Hz, 2% 02,1 % N, 2% S, and 0.25% ash. The low volatile content, in comparison to coal and other fossil fuels, makes petroleum coke more difficult to ignite and sustain combustion. The ash content in petroleum coke is relatively low compared to coal, but much of it is in the form of heavy metals such as nickel and vanadium. Some notable examples of coke fired plants in Canada are: • Nova Scotia Powers Point Aconi Generating Station • Suncor Base Plant located north of Fort McMurray (this plant used coke-fired Foster Wheeler boilers for many years but is being replaced by a natural gas-fired cogeneration plant.) Conventional coal-fired boilers can blend petroleum coke with coal, and some newer boiler designs have replaced coal with petroleum coke entirely. Typical boilers used to fire petroleum coke are as follows: • Conventional pulverized coal boilers • Cyclone furnace (coke is mainly co-fired with coal) • Circulating fluidized bed (CFB) (coke may be co-fired with coal or heavy oil, or burned alone) • Atmospheric fluidized bed (AFB) 154 3rd Class Edition 3 - Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 ^ As shown in Figure 10, the coke-fired plant with a CFB boiler can be integrated with oil refinery operations for the greatest economic benefit. There are no transportation costs because the fuel is used on site. Vacuum residue from the refinery is sent to the delayed coking unit. The coke is extracted, and the remaining hydrocarbons are returned to the refinery. The coke is fired in a circulating fluidized bed boiler as part of a cogeneration plant. Surplus electrical power is sold to the local utility. Figure 10 - Integrated Coke-Fired CFB Plant Crude oil < ^r ^ ^ Vacuum residue C1-C2gas LPG Naphtha Oil refinery Gas oil Petroleum coke Steam Power T ^. ^ Refined products Electrical power to grid Coal Gasification Coal gasification is an important industrial process which converts solid coal into a fuel called synthesis gas or syngas. This technology emerged around the nineteenth century when gas companies produced coal gas (also called town gas) to use in heating and gas lighting. At present, synthesis gas is produced by gasification plants. The syngas derived from gasification has a heating value of 10 MJ/m3 to 20 MJ/m3 for oxygen-blown syngas and somewhat lower for air-blown syngas due to nitrogen dilution. The gas can be further processed through methanation to produce a synthetic natural gas with a heating value close to actual natural gas. Although there are various types of gasifiers (gasification reactors), most industrial gasifiers fall into the three main classifications affixed (moving) bed, entrained flow, and fluidized bed gasifiers. 3rd Class Edition 3 • Part A2 155 ^ Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Fixed (Moving) Bed Gasifiers Figure 11 illustrates a fixed bed gasifier with an updraft or counterflow pattern. Large coal particles and fluxes are loaded into the top of the refractory lined gasifier vessel and they move slowly downwards through the bed. Oxygen and steam enter at the bottom of the gasifier and flow upwards through the coal bed. Reactions within the gasifier occur in four different zones: • Drying: At the top of the gasifier, the entering coal is heated and dried to remove moisture while cooling the product gas before it leaves the reactor. • Pyrolysis: The hot syngas from the reduction zone reacts with the downwards flowing solids. The pyrolysis (or carbonization) reaction causes thermal decomposition of the coal into volatile gases, tar, and charcoal. • Reduction: In this high temperature (above 700°C) zone, there are several reduction reactions which occur in the absence of oxygen. These reactions produce carbon monoxide and hydrogen, the main components of syngas. • Combustion (also called the oxidation zone): This is the zone of highest temperature where combustion of the coal takes place. Entrained Flow Gasifiers In entrained flow gasifiers, fine coal, an oxidant (air or oxygen), and steam are fed into the top of the gasifier. This system allows the oxidant and steam to surround or entrain the coal particles as they flow through the gasifier. Entrained flow gasifiers operate at a high temperature and pressure with extremely turbulent flow. Because of the high operating temperatures, gasifiers of this type melt the coal ash into vitreous inert slag. The fine coal is fed into the gasifier in either a dry or slurry form. The slurry feed results in a product syngas with a higher hydrogen to carbon monoxide ratio, but with a lower gasifier thermal efficiency. Fluidized Bed Gasifiers In a fluidized bed gasifier, the coal particles are suspended in an oxygen-rich gas. Although the bed of coal is made of solid particles, it has the properties of a fluid. Coal enters at the side of the reactor, while steam and an oxidant enter near the bottom with enough velocity to fully suspend or fluidize the reactor bed. Fluidized bed gasifiers are best suited to relatively reactive coals, low rank coals, and other fuels such as biomass. Figure 11 - Coal Gasification Synthetic gas Steam Oz or Air COMBUSTION Ash 156 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 ^ Oil Emulsions Heavy fuel oil (HFO) is a low-grade fuel primarily used in industrial boilers and other direct-source heating applications. Terminology HFO is also sometimes called #6 fuel oil, bunker C, or residual oil. 0 HFO is typically generated by blending diesel or gas oil with the residue from the crude oil refining process. Those residues, which are solid at atmospheric temperatires, are often caUed asphalt, tar, residual oil, or tower bottoms. The name of the residue depends on the process that the residue originates from. Blending these residues with diesel or gas oil (cutting oil) produces a molasses-like fluid at atmospheric temperature. Given its high boiling point and tar-like consistency, HFO typically requires heating before it can be moved through pipes or sent to a burner. Combustion temperatares are typically very high and produce noxious emissions, particularly nitrogen oxides (N0^;) and particulates. HFO does not burn cleanly; it leaves significant quantities of carbon residue, which fouls combustion chambers and reduces furnace and boiler efficiencies. Using chemicals, very small droplets ofHFO can be suspended in water to prevent the oil from coming together. These oil emulsions can be used as a fuel with similar properties and handling characteristics to standard grades of fuel oil. As a replacement for HFO, oil emulsions offer the following advantages: • Improved efficiency for large industrial boilers • Simultaneous reduction of N0^: and particulate emissions • Better combustion and improved carbon efficiencies • Reduced maintenance costs and downtime Side Track Concentrated Solar Power: In one hour/ the output of the sun over Earth's surface is enough to supply human energy demands for an entire year. One way of harvesting this free energy is concentrated solar power (CSP). CSP uses mirrors to reflect sunlight to a central collector tower, where it is used to heat a fluid—either water or molten salt. Water systems operate at temperatures up to 440°C and feed steam directly to a power turbine. Molten salt systems operate up to 570°C and supply steam by indirect exchange with water. The mirrors are in a wide (B array surrounding a tower and are focused to all four sides of the solar receiver at the top of the tower. Operating time is limited by daylight, but can be extended by the thermal storage capacity of molten salt. 3rd Class Edition 3 • Part A2 157