Larry Gray Homework #7 SHWPCE - Spring 2014 27-Mar-14 7.1 Difference between pryrolysis and gasification Pyrolysis decomposing of material to gas, liquid and solid by adding heat in the absence of oxygen gasification limited quantity of oxygen (less than stoichiometric combustion) plus heat to decompose material to gas and solid 7.2 What is similar equation for pyrolysis C6H10O5 + heat CH4 + H2 + CO2 + C2H4 + C +H2O End products will be gas (methane, hydrogen, carbon dioxide and water vapor) a liquid (ethylene) and a solid (carbon) 7.3 a. What specific questions would you ask your own power plant engineer about the possibility of accepting Podunk's MSW? b. What specific additional data would you need from the city before you discuss further? Could MSW be used as a supplemental fuel? What would your concerns be? Due to the heterogenous nature of MSW, can we get an estimate of the heating value? Is the heating value sufficient to provide enough energy for our operations? What type of additional air pollution equipment would we need? What type of MSW processing equipment would we need? What is the volume of MSW that we could expect to receive? 7.4 A furnace dedicated to paper (assume pure cellulose, C6H10O5) operates at 15 tons/hr, at 100% excess air. How much air is required? C6H10O5 + 6 O2 MW cellulose = mass of stoichiometric O2 100% excess O2 = 6 CO2 + 5 H2O 162 g/mol 192 grams 384 grams Amount of stoichiometric O2 needed = '= 192 / 162 * 15 tons / hr = 17.8 tons/hr of O2 (Stoichiometric) 100% excess O2 = 35.6 tons/hr of O2 (100% excess) Percent O2 in air 21% 1 mol air has 0.78 mol N2 and 0.21 mol O2 amount of O2 per g air = 0.235 g 7.5 Draw flow diagrams of unit operatrions for producing from raw MSW the following products: a) RDF-1 b) RDF-2 c) RDF-3 c) RDF-4 Raw MSW (RDF-1) Shredding (RDF-2) sepaarate organics Organic fraction of shredded MSW (RDF-3) Fine shredded organic MSW (RDF-4) 7.6 Describe Waste-to-Energy operations Plant receives MSW from trucks and dumped in storage pits Waste, depending on plant, might be processed, such as shredding or sorting to produce RDF waste or RDF then burned in a furnace which heats water to form steam The steam drives a steam turbine which in turn drives a generator to produce electricity The flue gas then needs to be treated to remove air pollutants before emitting to atmosphere 7.7 auto tires burning of tires produce a lot of soot and PM and other air pollutants the release of benzene, heavy metals and dioxins 7.8 If the WtE plant has the latest technology air pollution controls, then bring in the specs of the equipment and show collected data that shows the quality of the flue gasr being emitted from the stack 7.9 burning of plastic containers while true that burning of plastic would emit air pollutants Plastics can be recycled or collected to be burned in WtE plants The WtE plant would have air pollution controls to filter out air pollutants from flue gas 7.10 a). Stoichiometric equation for combustion of celluose C6H10O5 + 6 O2 6 CO2 + 5 H2O 6 mols of O2 b) calculate stoichiometric air air composed of 21% O2 the answer would still be 6 mols of air, but the MW would look like 1.185 5.037 1 mol air has 0.78 mol N2 and 0.21 mol O2 amount of O2 per g air = 0.235 g g O2 per g cellulose g air per g cellulose for stoichiometric combustion 7.11 draw the organic chemical formula for a polychlorinated dibenzodioxin and describe how dioxins are controlled in MSW combustion O O EPA has developed Maximum Achievable Control Technology standards which apply to MSW incinerators. MSW WtE plants with incineration must install latest available air pollution control technologies 7.12 sample of refuse 20% moisture content 1.2 gram sample is placed in calorimeter 6200 Btu / lb, gross calorific value 0.3 gram of ash remains in bomb calorimeter 25% ash content Calculate the HHV in terms of (a) moisture-free and (b) moisture- and ash-free 7.13 HHV, dry = 7750 Btu / lb HHV, daf = 10,333 Btu / lb to electrical contact Thermometer Water Bomb high pressure O2 Bomb Calorimeter RDF Sample RDF sample is weighed and placed in bomb and then bomb screwed back together High pressure O2 is injected into bomb Bomb placed in container as shown with water an electrical spark is sent to the sample within the bomb and the spark initiates combustion within the bomb the heat from combustion of the material in the bomb heats the water the temperature rise is recorded as a function of time the heat energy is calculated as the temperature increase of the water times the mass of the water plus the bomb one calorie is defined as the amount of energy necessary to raise the temperature 1 deg C knowing the weight of the RDF sample, the heat value can be calculated 7.14 difference between HHV and LHV. Why is the HHV always higher than LHV? LHV (Lower Heating Value) is the heat energy released from combustion of a material where the products (CO2 and H2O) are formed and the H2O is as water vapor HHV is the LHV plus the incremental heat energy released when the water vapor condenses HHV includes the contribution due to the latent heat of vaporization 7.15 Underfiered air in SW combustion underfire air assists in combustion and cools the grates. underfire air is used to control the operating temperature in a combustion chamber most SW combustors operate in the range of 980 to 1090 degrees C Temperature, F 4000 3000 2000 1000 0 -50% control underfire air to maintain temp at ~2000F 0% 50% 100% 150% Excess Air, % above stoichiometric 7.16 Mass burn is taking raw MSW and loading in feed chute to combustor directly because of the large variation in fuel value between MSW items, the excess air needs to be about 100% of stoichiometric to ensure complete combustion RDF (refuse-derived-fuel) is MSW that has been processed by shredding and sorting such that it has a higher heating value it is more uniform, therefore the excess air only needs to be 50% of stoichiometric A benefit is the cost of emission control is reduce with RDF due to lower amount of excess air required. Less air that needs to be treated for air pollution 7.17 Hartford operates a WtE plant and it uses RDF the capacity is 2850 tons per day of refuse derived fuel
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