Session 01 Combustion Technologies for Bioenergy - PDF

Session 01 Characterization of fuel BST 31212-Combustion Technologies for Bioenergy Bachelor of Biosystems Technology Honors (BBST Hons) Degree Programme Faculty of Technology Sabaragamuwa university of Srilanka...

Session 01 Characterization of fuel BST 31212-Combustion Technologies for Bioenergy Bachelor of Biosystems Technology Honors (BBST Hons) Degree Programme Faculty of Technology Sabaragamuwa university of Srilanka Prepared by: Eng.Prasad Amarasinghe Module Outline Content Lecture Time 1. Characterization of fuel 06 Hours 2. Combustion fundamentals 14 Hours 3. Boiler technology for 10 Hours bioenergy Assessment : Continuous Assessment……..40% Final Assessment………60% Practical: Performance evaluation through an energy audit for a biomass boiler plant (24 Hours) Laboratory Sessions: (i) Bomb Calorimeter - Measure energy content of biofuel (03 Hours) (ii) Proximate Analyzer - Determine the moisture, ash, volatile matter and fixed carbon in coal and coke (03 Hours) References/Reading Materials Rosendahl, L. ed., 2013. Biomass combustion science, technology and engineering. Woodhead Publishing. Quaak, P., Knoef, H. and Stassen, H.E., 1999. Energy from biomass: a review of combustion and gasification technologies. Murgai, M.P. and Chandra, R., 1990. Boiler operations (Vol. 1). New Age International Learning Outcomes At the completion of this course student will be able to Identify biofuel types and their combustion properties Understand biomass combustion technologies and their applications Apply combustion principles for bio-based thermal energy generation Operate low pressure steam boilers, steam distribution, and their maintenance activities Evaluate the performance of biomass combustion technologies Combustion Combustion is the process of burning of a substance in the presence of air or oxygen with the liberation of light and heat. Calorific Value Amount of heat release from the complete combustion of a unit mass(or volume) of the fuel is known as calorific value. Gross or Higher calorific value (G.C.V or H.C.V) Total quantity of heat release when one unit of fuel is burnt completely and product of combustion has been cooled to room temperature Net or Lower calorific value (N.C.V or L.C.V) Total quantity of heat release when one unit of fuel is burnt completely and products of has been permitted to escape N.C.V = G.C.V – Latent heat of water vapor formed Estimation of Calorific value of fuel The bomb calorimeter is the instrument that is used for determining the calorific value of a fuel The principle of all the calorimeters is the transference of heat of combustion of the given weight of fuel to water and the vessel. From the observed rise of temperature of the water and the container the calorific value of the fuel can be determined by equating the heat given out by the fuel to the heat taken by the water and the container. Estimation of Calorific value of fuel Advantage of using this king of instrument to determined calorific value of fuel Complete combustion happens inside the vessel due to high presser of oxygen supplied , excess oxygen is available All heat released from the combustion will be given to the surrounding water The additional oxygen and very small amount of water in the bomb do not effect the combustion The test procedure and the arrangement employed favor the accurate computation of temperature loss correction (cooling correction). Estimation of Calorific value of fuel Calculations for Calorific Value of Fuel Heat given away by the fuel = heat gained by water Q1 = Q2 m1 * CV = m2 * specific heat of water * (T2-T1) Heat = mc∆t Total heat released = Total heat absorbed Heat released by fuel + Heat released by fuse wire + Heat released by cotton thread = Total heat absorbed by (Water + Bomb + Calorimeter) (m𝑓×cv)fuel +(m×cv)fues wire+(m×cv)Thread = (total weight of water +water equivalent of calorimeter )×CW×(Temperature difference) (g×j/g)+(g×j/g)+(g×j/g) = g×j/(g×0c)×0c Calorific value taken from the bomb calorimeter test is the gross/high calorific value of the fuel Net/low calorific value of fuel can be taken as NCV/LCV = GCV/HCV- Latent heat of water vapor formed Estimation of Calorific value of fuel This is the standard instrument use to measure calorific value of gas Hot gases produce by combustion pass up the copper chimney. It is surrounded by double coil of metal tube. Known weight of Cooling water is circulate through this tube. After passing upwards the hot gas are deflected downwards through the space containing the inner coil. By the time the gases reach the top of this passage, practically the whole of their heat has been absorbed by the circulating water. The gases are then passed away into the atmosphere. The cooling water enters the outer coil. After circulating through both coils it leaves at the water collecting tank Its temperature is measured at inlet and outlet by thermometers Any water formed by combustion is drained through the pipe Estimation of Calorific value of fuel Heat release from fuel (gas) = Volume of gas(V) ×High calorific value(H.C.V) Heat absorb by water = Mw×C×∆T water condensed during the experiment=Mc latent heat of steam= L GCV/HCV = Mw×C×∆T /𝑉 NCV/LCV= GCV- Mc×L Estimation of Calcific value of fuel Calorific values of some fuels Estimation of Calorific value of fuel Different formulae are available HHV (MJ/kg) based on contents of C, H, O, and N (wt%) HHV (MJ/kg) based on fixed carbon (wt%) HHV (MJ/kg) based on fixed carbon (wt%) and volatile matter (wt%) Estimation of Calorific value of fuel Exercise ….. Calculate the calorific value of below mentions compounds 1. Tea waste 2. Beech wood 3. Soft wood Characteristics of ideal fuel High calorific value If it is good fuel, it will give us more heat per unit mass. that’s means it should have a high calorific value Burn without giving harmful gases An ideal fuel does not pollute air on burning by giving out smoke or poisonous gases A good fuel can be burned easily so that it should have a proper ignition temperature The ignition temperature of ideal fuel should neither be too low nor too high. If ignition temperature of the fuel is very low, then the fuel will catch fire too easily and hence it will be very unsafe to use it. If the ignition temperature too high, then it will be very difficult to light the fuel Characteristics of ideal fuel Good fuel should be easily available and cheap Good fuel is not more expensive and it is available in everywhere It should be easy to handle, safe to transport and convenient to store Ideal fuel will not create safety risk during handling ,transporting one location to another or during its storage. Good fuel should not leave much ash behind after burning An ideal fuel should have low percentage of non volatile materials which do not burn. It may burn completely without leaving much ash It should burn smoothly Ideal fuel should have moderate rate of combustion, and burn at steady rate. The fuel should not burn either too fast or too slow Proximate and ultimate analysis of fuel Proximate analysis From proximate analysis it will give idea about the physical properties of the fuel and it consist of the moisture content, ash content, volatile matter as well the fixed carbon. Moisture content Determination of moisture is carried out by placing a sample in an uncovered crucible and it is placed in the oven kept at 108±2°C. Then the sample is cooled to room temperature and weighed again. The loss in weight represents moisture. (𝑔−𝑥) %MC= ×100 𝑔 g is the Weight of sample, x is the Weight of dry matter Ash Content After removing moisture remaining sample is ignited until getting a constant weight that is the Ash content of the sample (𝑥) %𝐴𝑠ℎ = ×100 𝑔 g is the weight of sample and x is the weight of ash Proximate and ultimate analysis of fuel Volatile Matter Get sample of material is weighed, place in a crucible and heated in a furnace at 925 ± 25°C. The sample is cooled and weighed. Loss of weight represents moisture and volatile matter (𝑥−𝑦) %𝑉. 𝑀 = ×100 𝑔 g is the weight of sample, X is the Weight of dry matter and y is the Weight of residue, Fixed Carbon Above mentioned test ,remaining residue is fixed carbon %FC= 100 − (𝑉𝑀 + 𝐴𝑠ℎ + 𝑀𝐶) Proximate and ultimate analysis of fuel Proximate analysis of coal Initially take air drayed coal sample = w1 It was taken in silica crucible, Then heated in an electric oven (105-110) 0c. Residue weight =w2 The residue was ignited at (700-750) 0c until getting a constant weight =W3 Get another sample from this same coal sample = w4 it was heated in a silica crucible covered with a vented lid at a temperature (925-950) oc After cooling weight of residue was taken =w5 Proximate and ultimate analysis of fuel (𝑤1−𝑤2) % moisture in coal = ×100 𝑤1(𝑏𝑒𝑓𝑜𝑟𝑒 ℎ𝑒𝑎𝑡𝑖𝑛𝑔) 𝑊3 % of Ash in coal = ×100 𝑤1(𝑏𝑒𝑓𝑜𝑟𝑒 ℎ𝑒𝑎𝑡𝑖𝑛𝑔) (w4-w5)= Weight of moisture + weight of volatile matters (𝑤4−𝑤5) % of VM in coal = ×100- %moisture 𝑤4 % of fixed carbon =100-(%Moisture +%volatile matter +%Ash) Proximate and ultimate analysis of fuel Exercise Initially take air drayed coal sample its weight is 24kg and It was taken in silica crucible, Then heated in an electric oven (105-110) 0c. remaining residue weight was 16kg.Then residue was ignited at (700-750) 0c until getting a constant weight 2kg Get another sample from this same coal sample 15kg and it was heated in a silica crucible covered with a vented lid at a temperature (925-950) 0c. After cooling weight of residue was taken 6kg (01.) Calculate percentage of %moisture ,%Ash, % VM and % Fixed carbon in this sample. Impact of Moisture, Volatile material ,Ash and Fixed carbon for fuel Moisture Content: Moisture contain in coal must be transported, handled and stored and it will replace combustible materials. It will decrease heat content per kg of coals. Moisture will increase heat loss due to evaporation and superheating of vapor It will reduce binding property of fuel Help to heat transfer by radiation Ash Content: Ash is an impurity that will not burn Increase handling cost Reduce handling and burning capacity Affects combustion efficiency and boiler efficiency Impact of Moisture, Volatile material ,Ash and Fixed carbon for fuel Volatile Matter Volatile matters are the methane, hydrocarbons, hydrogen and carbon monoxide, and incombustible gases like carbon dioxide and nitrogen found in coal. Thus the volatile matter is an index of the gaseous fuels present It will help to easier ignition of coal and increases flame length Sets minimum limit on the furnace height and volume. Influences secondary oil support Fixed carbon: Fixed carbon is the solid fuel left in the furnace after volatile matter is distilled off. It consists mostly of carbon but also contains some hydrogen, oxygen, sulphur and nitrogen not driven off with the gases. Fixed carbon gives a rough estimate of heating value of coal. Proximate and ultimate analysis of fuel The heating value of a fuel directly decreases with moisture content. If moisture content is more than 55wt% ,it is very difficult to maintain combustion. The water content will increase residence time of a fuel in a furnace. In wood pallet moisture content has reduced to below 10wt% then heating value is higher than woodchips. The torrefaction of wood is a relatively new process, in this process volatiles and moisture are evaporated from the fuel. This upgrades the wood fuel quality compared to palletization. Fixed carbon composition of coal is higher than wood then it has high heating value Proximate and ultimate analysis of fuel The basic combustion route of a solid fuel Proximate and ultimate analysis of fuel Ultimate Analysis From ultimate analysis it will give idea about the chemical properties of the fuel and it consists of the carbon content, oxygen content, hydrogen content, nitrogen content and Sulphur content. Proximate and ultimate analysis of fuel All types of biomass feedstock, contains a large fraction of oxygen compared to coal. Wood also has very little fuel- bound nitrogen and Sulphur According to the biomass feedstock nitrogen and Sulphur contain will varies. The main elements of biomass are cellulose, hemicelluloses and lignin. Biomass also contains small amounts of lipids, proteins, simple sugars and starch as well as inorganic constituents and moisture Combustion efficiency Combustion efficiency is defined as the heat release by the fuel to the heat input by the fuel. Combustion efficiency will vary with below mentioned points Fuel type Bed temperature Gas velocity Excess air levels Combustion efficiency increase with volatile matter content of a fuel and bed temperature Ignition point of liquid biofuels Ignition point is the minimum temperature at which combustible substance in the air will continue to burn without addition external heat. For high volatile matter contain fuels generally ignition temperature is low. Ignition temperature is not a unique property of a fuel. It is depend on several other factors (oxygen, partial pressure, particle size, rate of heating) flash point of liquid biofuels Flash point is minimum temperature at which volatile combustible substance will start to burn in the presence of external heat source. This temperature is not directly depend on the temperature of the ignition source. Ignition source temperature is higher than the flash point. Flammable liquid has a vapor pressure, with an increasing temperature vapor pressure increases. Flammable liquid requires a certain concentration of vapor in the air to sustain the combustion. By knowing flash point we can minimize explosion and fire risk , we can get idea for storing and transportation. To measure flash point we can use open cup or close cup method flash point of liquid biofuels Open cup method In this test use a vessel, or container, that is exposed to the outside air. Material sample place inside the vessel then gradually raise its temperature, and pass an ignition source over it It will flashes and ignites at a certain temperature. That is the flash point of that sample The most common open cup method is the Cleveland open cup (COC). Tag and Setaflash are other methods. In this method vapors are free to escape into the atmosphere flash point of liquid biofuels Closed cup method In this test we use vessel that is sealed off from the outside atmosphere. Then heat both the vessel and sample then introducing an ignition source into a sealed container here are four main closed cup flash point testing methods: Pensky Martens, Abel, Tag and Setaflash.