Energy Engineering - Power Production from Renewable Energy

Ex 3 - USE OF BIOMASS - Text

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USE OF BIOMASS Power Production from Renewable Energy – A.Y. 20 20 /21 Prof. Paolo Silva – Ing. Elena Crespi Part 1 - Biomass -fired ORC plant Consider a biomass -fired power plant made of a covered storage area, a wood chipper, a belt conveyor , a biomass boiler and an Organic Rankine Cycle generating both electricity and heat (CHP). On the basis of the following data: - the feedstock (green oak at the harvest condit ions as cha racterized in the table) arrive s to the plant covered storage area as timbers (with an average diameter of 40 cm, average length of 2.5 m) and is arranged into log -piles whose height is 5 meters and void fraction is 15%. - during the storage period , wood timbers are naturally dried to a moisture content (wet basis) equal to 35% ( the required residenc e time is equal to half of a year) - after leaving the storage area , wood timbers are chipped into 5 mm particles by means of an electrically driven chippe r, absorbing 170 kJ el/kg wood - wood particles are transported from the chipper to the boiler by a belt conveyor absorbing 45 kJel/kg wood - the boiler is designed fo r a stack temperature of 130 °C, it works with 40% of excess air and the thermal losses are eq ual to 0 .8% of the thermal input (referred to the oak LHV) - about 3% of the inlet carbon is not oxidized (unconverted carbon in the ash ), the carbon LHV is assumed equal to 32.8 MJ/kg - flue gases , air and ash specific heat capacity at constant pressure are respectively: cp,flue gases = 1. 11 kJ/kg -K, c p,air = 1 .02 kJ/kg -K, cp,ash = 7 .5 kJ/kg -K - the evaporation heat of the water is considered equal to 2450 kJ/kg - the ash boiler outlet temperature is 450 °C - the ORC net electric efficiency is 25% - Ambient temperature is equal to reference temperature: 25°C Oak at harvest conditions (‘as received biomass’) Weight percentage on dry basis [%] Moisture content [%] dry basis LHV [kJ/kg ] Density [kg/ m3] C H O N S ash 48 .80 6.09 45.00 0.00 0.00 0.11 50 17769 850 1) Evaluate the mass flow rate of ‘as received biomass ’ required by the plant to generate 1 MW of net electric power, and assess the land footprint of the storage area. 2) Repeat the same evaluation assuming that ‘as received biomass ’ is directly chipped and burned (no covered storage area). 3) Assume that the biomass, instead of being naturally dried in the storage area, is dried by providing heat burning some of the biomass itself . Determine the amount of ‘as received biomass’ required , assuming an overall thermal efficiency of the heater equal to 0.8.