COMBUSTION LAB OVERVIEW Level: Grades 9-16 Estimated Time to Completion: 90 Minutes Prior Knowledge: Background Provided In this lab investigation, students will employ the methods of calorimetry to approximate the amount of energy contained in a selection of biomass types and other food items. The heat given off from the reaction will be absorbed by water that is suspended above the burning sample. This method indirectly measures the amount of heat given off by combustion through observing the changing temperature of the water. Upon completion, students will be able to: • Understand and employ the techniques of calorimetry. • Calculate how many calories of energy/g for several biomass/biofuel items and compare these values to established literature values. • Determine ash content and its impact on energy. • Understand the impact of moisture on biomass energy content. • Discuss how this experiment can be improved to provide more accurate results. MATERIALS REQUIRED Lab Balance Test Tube Test Tube Holder Ring Stand Thermometer and Thermometer Clamp Source of Distilled Water Evaporating Dish Wire Screen Lighter or Matches Heat Resistant Gloves or Tongs NOTES TO INSTRUCTOR • It is recommended that safety protocols are discussed before the lab, including the importance of safety equipment to avoid burns or other fire hazards. Additionally, experiments should be conducted in a well-ventilated area such as a hood. • Provide a variety of biomass types for the students to choose from. Have the % moisture recorded from the biomass along with the literature value for the biomass. • A food item with a “calories” content makes for an interesting biomass sample. Note calories are really kilocalories and food calories are related to, but not the same as, calories measured through bomb calorimetry. • Have the students burn both a relatively dry (<10% moisture) and wet (20 – 30% moisture) sample to determine the double impact of moisture (change in mass content and decreased net energy content). • If you have done the biodiesel and ethanol labs, the fuel obtained may be burned instead of a solid biomass. In this case, it works best to replace the wire mesh with a kerosene lamp wick. A spirit burner is a convenient method to burn liquid biofuels such as ethanol or biodiesel and can be weighed preand post-experiment to determine the mass of the biofuel combusted. • Dense biomass such as wood chips or biomass pellets can be used but are difficult to fully combust.
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COMBUSTION INTRODUCTION In a science lab, you may have heated a test tube over the flame of a Bunsen burner fueled by natural gas. At home, perhaps you have charred vegetables, fish, or meat on a propane grill. You’ve seen candles burning on a dinner table or perhaps oak logs burning in a wood stove. On a daily basis, cars and motorbikes drive around on streets, most running on gasoline. . Have you ever wondered how these fuels compare in terms of their energy content? Is one fuel “better” or more efficient than the others? On what basis might you compare different fuels? In this investigation, you will answer questions such as these. Fuels burn to give off energy. They do so by combining with oxygen to form compounds of lower energy, typically carbon dioxide and water. For example, below is a chemical equation that represents the complete combustion of methane, CH4, to produce CO2 and H2O: CH4 + 2O2 -> CO2 + 2H2O Methane burns cleanly. We call this complete combustion because the two products, CO2 and H2O, are not flammable. That is, they cannot burn any further. Other fuels, however, may burn incompletely unless supplied with plenty of oxygen. In addition to carbon dioxide, the combustion products also include carbon monoxide and/or soot, including black carbon. For example, some hydrocarbons when burned, including candles, burn with a sooty flame. Below is the chemical reaction in which hexane (a hydrocarbon, C6H14) burns to produce carbon monoxide: C6H14 + 2O2 -> 6CO + 7H2O When either soot or carbon monoxide or both are formed as products, this is called incomplete combustion. The energy when fuels are burned is given off primarily in the form of heat, with some light as well. Although sometimes quite bright, the light emitted by a flame or fire is hard to quantify. In contrast, it is straightforward to measure the heat released when a fuel burns. For these two reasons, you will evaluate the energy content of fuels in this experiment by the heat they give off rather than by the light. So how do you measure the heat? Rather than doing this directly, we will use the heat released to increase the temperature of a known quantity of water. From this, we can estimate the heat released by burning a particular fuel. With these measurements, you can calculate the heat (q) absorbed by the water: q = mass of water (g) × 4.184 joules/g-ºC × temp change (ºC) q = m × 4.184 joules/g-ºC × ΔT Finally, a reality check. In theory, the amount of heat liberated by the burning fuel is all absorbed by the water. But this doesn’t happen. As part of this experiment, think about what happens to the heat that is lost.
PROCEDURE
NOTES • Safety glasses must be worn at all times. • Only perform experiments in a well ventilated area (best done in a hood). • A fire extinguisher should be present, but a box of baking soda or sand to pour on any excessive flames works best. • Combustion apparatus will be hot after combustion. Heat resistant gloves or tongs should be used to handle samples.
1. Weigh your biomass and then suspend it above an evaporating dish using a wire mesh. Try to get at least 1g of biomass. Record in data table. 2. Measure the mass of the biomass with evaporating dish/ wire mesh setup prior to burning it. Record in data table. 3. Using a graduated cylinder, measure ~15mL of distilled water and pour it into a test tube. Record the mass of H2O in the data table (1mL of H2O = 1g). Alternatively, you may weigh the H2O directly, less the mass of the test tube. 4. Place the biomass setup at the base of the ring stand. Adjust the test tube height so that it will be directly above the chip. Insert a thermometer into the water and secure. 5. Measure and record the initial temperature of the water. 6. Momentarily move the test tube. Use a lighter to ignite the bottom of the chip and then put the test tube back over the burning biomass. The idea is to get as much of the heat from the burning biomass to rise up and heat the water in the test tube. 7. With a stirring rod, stir the water in the test tube while the chip burns. Measure the highest temperature of the water and record it in the data table. Re-light biomass if it goes out. 8. Measure the final mass of the ash in the evaporating dish after burning. Record data. 9. Repeat steps 1-8 using a different biomass item each time.
COMBUSTION
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DATA AND OBSERVATIONS PART 1: BULK DENSITY Sample: Empty Container (g): Volume of Container (cm3) Container + Sample (g): Bulk Density (g/cm3): Bulk Density (lb/ft3):
BIOMASS DENSIFICATION AND QUALITY OF DENSIFIED BIOMASS
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DATA AND OBSERVATIONS PART 2: MOISTURE CONTENT Sample: Empty Container (g): Container + Moist Sample (g): Container + Dry Sample (g): % Moisture: PART 3: PERCENT FINES Sample: Sample Weight (g): Empty Pan Weight (g): Pan + Fines (g): % Fines:
BIOMASS DENSIFICATION AND QUALITY OF DENSIFIED BIOMASS
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DATA AND OBSERVATIONS PART 4: LENGTH AND DIAMETER OF PELLETS Sample: Sample Weight: Longest Sorted Pellet (in): Weight of Pellets >1.5in (g): % Exceeding 1.5in: Pellet 1 Diameter: Pellet 2 Diameter: Pellet 3 Diameter: Pellet 4 Diameter: Pellet 5 Diameter: Average Pellet Diameter (in):
BIOMASS DENSIFICATION AND QUALITY OF DENSIFIED BIOMASS
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DATA AND OBSERVATIONS PART 5: DURABILITY Sample: Sample Weight: Empty Sieve Weight (g): Sieve + Durable Pellets (g): % Durable: DATA SUMMARY Sample: Bulk Density: % Moisture: % Fines: % Exceeding 1.5 in: Average Pellet Diameter (in): % Durable: BIOMASS DENSIFICATION AND QUALITY OF DENSIFIED BIOMASS
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DISCUSSION 1. Calculate the energy from combustion using q=mCpâ&#x2C6;&#x2020;T.
2. Calculate the amount of ash from your sample.
3. Calculate the effect of moisture on usable energy content.
4. Did your measurements come close to literature values?
5. How could your measurements be made more accurate?
6. Describe methods to create improved fuels for combustion.
COMBUSTION
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