Biochemical and Thermochemical Conversion of Biomass.pptx

Unit3 - THERMO CHEMICAL CONVERSION BY COMBUSTION

1. KONGUNADU COLLEGE OF ENGINEERING AND TECHNOLOGY (Autonomous) Department of Agricultural Engineering Course: Biochemical and Thermo chemical Conversion of Biomass Topic : Thermo chemical Conversion by Combustion By Mr. M.Prabhu , Assistant Professor, Department of Agricultural Engineering, Kongunadu College of Engineering and Technology

2. Biomass wastes can be easily converted into other forms of energy at high temperatures, they break down to form smaller and less complex molecules both liquid and gaseous including some solid products. Combustion represents a complete oxidation to carbon dioxide (CO2) and water (H2O). By controlling the process using a combination of temperature, pressures and various catalysts, and through limiting the oxygen supply, partial breakdown can be achieved to yield a variety of useful fuels. The advantages of thermo-chemical conversion processes Rapid completion of reactions Large volume reduction of biomass Range of liquid, solid and gaseous products are produced Some processes do not require additional heat to complete the process Thermochemical Degradation Thermochemical conversion is defined as the degradation of organic matters due to heat exposition of biomass and chemical reactions. The process is mainly categorized in some processes named combustion, torrefaction, gasification, pyrolysis, and hydrothermal. Biomass having high moisture - beyond 70 % and with little lignin (say, < 10 %) qualify for bacterial conversion process dry - moisture < 30 % will ideally qualify for thermo-chemical conversion. 95 % of the waste biomass use is via thermo-chemical conversion route.

3. Most biomass at the time of harvest will be at 50 to 60 % moisture. Many agricultural residues are light with thin walls so that spreading the material on ground in the open will reduce the moisture to 20 % or less in a few days. Wood stock with sufficient girth takes longer to dry in the open air. Routes to bio-diesel Two sources for bio-diesel are: waste oils and fats used in domestic or food industry environments as well seeds from oil-seed bearing trees. The seeds are dried and subject to screw extraction to obtain oils. Residual oil extraction is performed by a solvent process. These are called Straight Vegetable oils (SVO) and can be used with minimal filtering to remove extraneous solids directly in stationary reciprocating engines. These oils as well as the waste oils can be mixed if needed and then subject to trans-esterification process which consists of reaction of these oils with ethyl or methyl alcohol and separation of the ethyl or methyl ester from glycerin These esters form the fuel for use in reciprocating engines replacing the diesel or heavy fuel oil.

4. Routes for energy conversion

5. Stoichiometric Ratio And Elemental Composition The first reaction can be stated in words as: one mole of carbon reacts with one mole of oxygen molecule to give one mole of carbon dioxide. This will be a volumetric statement. Stoichiometric ratio How much of oxidizer is needed to completely consume the fuel? It is taken that the final products in a combustion process are those in which the fuel elements are oxidized to stable products at ambient condition. 1 kg methane requires 17.2 kg air and 1 kg kerosene requires 14.6 kg air. Principles Of Combustion Combustion is a phenomenon that releases the energy in the bonds of the molecules - one called fuel and another called oxidizer into sensible heat. The combustion process occurs largely in the gas phase and in a few cases on the interface between a solid and a gas, like in the case of the oxidation of char.

6. Densities of gases and liquids The density of air is dependent on the ambient temperature and pressure governed by the equation of state: p = R/MT where p = pressure in Pascals , gas is the density of the gas in kg/m3, R is the universal gas constant (8.314 J/g−mole K),M is the average molecular weight of the gas and T is the temperature (K). Heat of combustion and heat of formation The energy released in combustion is called the heat of combustion. For biomass it is a function of ash and moisture fractions. It is measured in a laboratory using a bomb calorimeter. There are two ways of expressing the heat of combustion. These are the higher heating value (HHV) and the lower heating value (LHV) or more conventionally lower calorific value (LCV). The LCV of cellulose, hemicelluloses, lignin and extractives/oils (moisture-free conditions) are 16.4, 16, 20 - 24 and 34 - 36 MJ/kg of individual components. Gas Composition at high temperature The reactions noted above will occur more vigorously at higher temperatures because the energy contained in the species as reflected in the random velocities of the molecules and the atoms will become larger. This causes the stable molecules to decompose (or dissociate as it is termed) into smaller fragments.

7. When we burn any fuel with air, one can do so with air less than, equal to or more than the requisite amount of air. This is described by a quantity called equivalence ratio, 𝜙. It is the ratio of fuel-to-air to fuel-to-air at stoichiometry , a condition at which air is just sufficient to produce complete products of combustion. It is written as 𝜙 = (fuel/air) / (fuel/air) stoichiometry , 𝜙 = 1 means stoichiometric condition With 𝜙 < 1 , it is lean operating condition and 𝜙 > 1 is rich condition. For 𝜙 > 1, the “rich combustion” there will be incomplete combustion. The products will not be limited to the ones seen in the above equation. Combustion of solid bio-fuel and emissions The combustion process of solid fuels involves several steps since the flame that will be the final result is only in the gaseous phase. After ignition of the fuels, something that involves a process of heat up of the fuels to about > 600°C, gaseous fuel components emerge from the solid fuel. The gaseous fuel components consist of CO, 𝐻2, 𝐶𝑂2, 𝐻2O, and other complex oxygenated hydrocarbons that when condensed to ambient temperatures will become liquids called tar. This process is essentially pyrolysis . These gases mix with air around and react exothermically in the gaseous phase to release heat. This heat is transferred back to the surface of the solid fuel to release more volatiles are released. Char is composed largely of carbon (some residual hydrogen is present). Typically this char will be red hot – temperature of about 700 to 800°C and will oxygen of the surrounding air medium heterogenenously (surface combustion) with perhaps a blue gaseous flame (between carbon monoxide and oxygen of air). Burn rate of solid fuels Solid fuels are burnt in a variety of shapes and sizes as many of these are considered wastes. But in some European countries, wood stock is dried, pulverized and pelleted to make the fuel shape and quality uniform.

8. LARGE COMBUSTION SYSTEMS The development of large combustion systems is particularly relevant to countries where large amounts of agricultural residues are available for utilization. This calls for owning of large tracts of land by individuals or institutions. travelling grate-stoker type, (b) inclined grate (c) vibrating grate (d) bubbling fluid bed and (e) circulating fluid bed systems, Air can be sent through the grate uniformly or in sections in which larger air flow comes through the grate in the frontal area and much reduced flow in the downstream section to prevent low density char/ash blow off. Thus, the basic processes involve managing a near uniform bed over the grate, air supply from the bottom to facilitate gasification and subsequent char gasification-combustion and substantial gaseous phase combustion with over-fire air. By managing the air flow over various segments along the grate, it should be possible to limit the gaseous and particulate emissions. The intense movement of sand particles enhances the heat transfer to the biomass and maintains the bed at the temperature. In a few cases, the volatiles released here burn up above the bed (in a zone called free board). When the velocities that carry the sand and fuel are large – typically 5 to 10 m/s, the system operation transitions to circulating fluid bed reactor

9. The most severe of the problems are high temperature bonded deposits in the super-heater, (b) bridging and blockage of convection passes, (c) erosion of super-heater tubes, and (d)corrosion in the super-heater, air heater, and economizer. Fuel management techniques were to eliminate the worst acting fuel components where possible, more usually, dilute the “dirty” fuels with clean fuels. Boiler control was also exercised to limit temperature excursions. Cleaning and additives include the addition of limestone based on boiler condition.

10. Gaseous Emissions From Solid Fuel Combustion Devices Apart from CO2O and H2O, CO, oxides of nitrogen (NO2 ) and oxides of sulphur (SO2 ) are also emitted to varrying small fractions. The emissions of CO and NO2 have contradictory trends. CO is emitted due to lack of oxygen in the combustion zone having CO. It is essentially a product of fuelrich condition. NO2 represents a mix of NO and NO2 , though largely composed of NO (~ 90 %). Its production is high at high temperatures and when oxygen is present. It is a serious emission problem with fossil fuels that are designed to burn optimally to achieve high temperatures in combustion devices. Its emissions in the fuel rich conditions is very small.

11. Industrial plants and devices have specifications on the emissions of CO, NO𝑥, SO2 and have specific devices to deal with them. If CO is in excess, a catalytic converter is used to oxidise it. For limiting NO𝑥, emissions one of the several techniques needs to be adopted. Particulate emissions also have to be limited. Cyclones and bag filters are used to limit their emissions. Chemistry Of Combustion Classification of Fuels Primary Fuels: It occurs in nature as such. ex. coal, petroleum, natural gas. Secondary Fuels: It is derived from primary fuels ex.: coke, gasoline, coal gas. Fuel + Oxygen ⎯⎯→ Products + Heat Calorific Value Calorific Value of a Fuel is “the total quantity of heat liberated, when a unit mass (or volume) of the fuel is burnt completely”. Higher or Gross Calorific Value (GCV) It is the total amount of heat produced, when unit mass/volume of the fuel has been burnt completely and the products of combustion have been cooled to room temperature (15°C or 60°F). Lower or Net Calorific Value (NCV) It is the net heat produced, when unit mass/volume of the fuel is burnt completely, and the products are permitted to escape.

12. Emissions Combustion-derived air toxics

13. Inorganic emissions of concern include acids, such as sulfuric and hydrochloric acid, sulfur and nitrogen oxides ( NOx ), and minerals. Polycyclic aromatic hydrocarbon emissions PAHs can be formed during combustion when carbonaceous (organic) fuels are used. Aromatics can grow to PAHs by addition of non-aromatic molecules to an already existing aromatic structure, or by reacting directly with other aromatic radicals. C2H2 + C4H5 → C6H6 (benzene) Coke and char formation Coal and fuel oil combustion can lead to the formation of char and coke. These are the carbonaceous residue particles that remain if the original solid or liquid fuel does not have time to fully combust. Ash formation Coal and, to a lesser extent, heavy fuel oils contain non-combustible materials such as minerals, including silicon, nickel, aluminium , and calcium, and trace quantities of other metals like selenium, cadmium, and so on as inclusions in the fuel. Cofiring of Biomass Biomass co-firing is a promising technology to decrease the use of fossil fuels for energy generation and hence mitigate greenhouse gas emissions.

14. Biomass co-firing stands for adding biomass as a partial substitute fuel in high-efficiency coal boilers. Coal and biomass are co-combusted in boilers that have been designed to burn coal. Biomass co-firing processes Direct co-firing The biomass and the coal are burned in the same furnace This concept is most commonly used because it is the easiest to implement and most cost-effective. The biomass energy fraction is typically limited to below 10-20 %. For a high blend rate and the best performance, pre-treated fuels (e.g., white or black pellets) are used. Indirect co-firing The solid biomass is converted to a clean fuel gas, using a biomass gasifier . The gas can be burnt in the same furnace as the coal. This principle is less used compared to direct co-firing. Parallel co-firing It is also possible to install a completely separate biomass boiler in addition to the conventional boiler. In this case, the biomass boiler provides energy to the feed water or generates steam at low temperatures, while the conventional boiler tops up the superheat.

15. Incinerators Incineration, like carbon adsorption, is one of the best known methods of industrial gas waste disposal. Incineration is an ultimate disposal method in that the objectionable combustible compounds in the waste gas are converted rather than collected. Thermal Incinerators The heart of the thermal incinerator is a nozzle-stabilized flame maintained by a combination of auxiliary fuel, waste gas compounds, and supplemental air added when necessary. Up on passing through the flame, the waste gas is heated from its inlet temperature (e.g., 100°F) to its ignition temperature. The mixture continues to reacts as it flows through the combustion chamber.

16. Recuperative Incinerators Recuperative incinerators have improved energy efficiency as a result of placing heat exchangers in the hot outlet gas streams. recuperative incinerator is comprised of the combustion chamber, the waster gas preheater , and , if appropriate, the secondary, energy recovery heat exchanger. Primary Energy Recovery (Preheating Inlet Streams) – Considerable fuel savings can be realized by using the exit (product) gas to preheat the incoming feed stream, combustion air, or both via a heat exchanger. plate and shell-and-tube. Place-to-plate exchangers offer high efficiency energy recovery at lower cost than shell-and-tube designs shell-and-tube exchangers usually have lower purchase costs than plate-to-plate designs Most heat exchangers are not designed to withstand high temperatures, so that most of the energy needed to reach ignition is supplied by the combustion of fuel in the combustion chamber and only moderate preheat temperatures are sought in practice (<1200℉). Regenerative Incinerators A distinction in thermal incinerators can now be made on the basis of this limitation in the preheat temperature. The operation of the regenerative systems is the inlet gas first passes through a hot ceramic bed thereby heating the stream (and cooling the bed) to its ignition temperature. If the desired temperature is not attainable, a small amount of auxiliary fuel is added in the combustion chamber. The hot gases then react (releasing energy) in the combustion chamber and while passing through another ceramic bed, thereby heating it to the combustion chamber outlet temperature. The process flows are then switched, now feeding the inlet stream to the hot bed. This cyclic process affords very high energy recovery ( upto 95%).

17. Catalytic Incinerators The catalyst has the effect of increasing the reaction rate, enabling conversation at lower reaction temperatures than in thermal incinerator units. Nevertheless, the waste stream must be preheated to a temperature sufficient high (usually from 300 to 900℉) to initiate the oxidation reactions. The waste stream is preheated either directly in a preheater combustion chamber or indirectly by heat exchanger with the incinerator’s effluent or other process heat or both The preheated gas stream is then passed over the catalyst bed. The chemical reaction (combustion) between the oxygen in the gas stream and the gaseous pollutants takes place at the catalyst surface. Catalytic incineration can, in principle, be used to destroy essentially any oxidizable compound in an air stream.

18. Combustion of wastes and MSW Incinerator systems can be classified by the types of wastes incinerated: municipal solid waste incineration; medical and pathological waste incineration; hazardous waste incineration; sewage sludge incineration; tire incineration; and biogas flaring. Description of Municipal Solid Waste Incineration Technologies Furnace Types Mass Burn Modular Incinerator Refuse-Derived Fuel Air Pollution Control Devices Electrostatic precipitator (ESP) The ESP is generally used to collect and control particulate matter that evolves during MSW combustion, by introducing a strong electrical field in the flue gas stream Fabric filter (FF), FFs are also particulate matter control devices, which remove dioxins associated with particles and any vapors that adsorb to the particles. Six- to 8 inch diameter bags, made from woven fiberglass material, are usually arranged in series. Dry scrubber (DS) DSs, also called spray dryer adsorption, involve both the removal of acid gas and particulate matter from the post-combustion gases. By themselves, these units probably have little effect on dioxin emissions.

19. Wet Scrubber (WS): WS devices are designed for acid gas removal GASIFIER STOVES Stoves are made of metal, mud, refractories of various qualities with and without chimney. Chimney based stoves are considered superior to chimney-less stoves because any emission is taken out of the kitchen thus preserving the indoor air quality. Most of these stoves had utilization efficiencies assessed by water boiling tests between 10 to 20%. while one slightly complex design shows an efficiency of 40% at small power levels. It has been known that both kerosene and LPG(liquefied petroleum gas) stoves have efficiencies as high as 65 and 70%. The inner wall is made of ceramic composition that removes the limitations of the material limited life issues in the combustion chamber. The bottom grate is made of cast iron that ensure longlife . The primary air comes through the grate and the secondary air issues out of holes seen at the top. The cup-like flames are those formed around the air jets issuing from the wall. Except for the initial lighting process during which the flames are yet to acquire the character of the combustion of a gasified fuel. Even an open fire is often 90% efficient at the work of turning wood into energy. But only a small proportion, from 10% to 40%, of the released energy makes it into the pot. Improving combustion efficiency does not appreciably help the stove to use less fuel.

20. Ten Design Principles Whenever possible insulate around the fire using lightweight, heat- resisitant materials. Place an insulated short chimney right above the fire. Heat and burn the tips of the sticks as they enter the fire. High and low heat are created by how many sticks are pushed into the fire. Maintain a good fast draft through the burning fuel. Too little draft being pulled into fire will result in smoke and excess charcoal. The opening into the fire, the size of the spaces within the stoves through which hot air flows, and the chimney should all be about the same size. Use a grate under the fire. Insulate the heat flow path. Maximize Heat Transfer to the pot with properly sized gaps.