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Chapter 1 Evaluation of Combined Heat and Power Technologies for Wastewater Treatment Facilities 1.2 Technical Background As the name suggests, the purpose of power plants is to produce power, most often in the form of either electricity or mechanical power. A common consequence of power generation is the production of heat. A common means of dealing with unused heat is releasing it to water bodies or the atmosphere. However, many processes, including anaerobic digestion at POTWs, require heat input or process heat. One of the distinguishing features of CHP systems is the capture of unused heat and its subsequent conversion to usable process heat. Only systems that couple electric or mechanical power generation with thermal energy for process heating can be defined as CHP, or “cogeneration.” The following paragraphs describe the conversion of digester gas, a by-product of the decomposition of volatile organic matter in wastewater, into cogeneration fuel. Chapter 2 of this document provides a detailed discussion of common CHP technologies, or prime movers, in use at POTWs. Prime mover is a term that is commonly used to describe the central element of a CHP system that is responsible for the conversion of fuel to heat and power. In the simplest of terms, wastewater treatment is the removal of solid and dissolved materials from raw sewage. The residuals removed from the wastewater downstream of the POTW’s headworks are commonly referred to as sewage sludge. Proper disposal of sewage sludge requires additional treatment or stabilization. One of the most common sewage sludge stabilization methods is anaerobic digestion, which results in pathogen kill and the reduction of the mass of sewage sludge solids through the breakdown of volatile solids. The anaerobically stabilized solids are often referred to as biosolids and are typically land-applied as a soil amendment, incinerated or disposed of at municipal landfills. Table 1-1 presents some of the common characteristics of anaerobic digester gas. Table 1-1. Characteristics of Anaerobic Digester Gas Item or Parameter Methane, CH4, percent (dry basis) Carbon dioxide, CO2, percent (dry basis) Nitrogen, N2, percent (dry basis) Hydrogen, H2, percent (dry basis) Water vapor, H2O, percent Hydrogen sulfide, H2S, ppmv (dry basis) Siloxanes, ppbv Specific gravity (based on air = 1.0) Ignition velocity, maximum, feet per second Higher heating value, HHV, Btu per cubic foot Lower heating value, LHV, Btu per cubic foot Range 60 – 70 30 – 45 0.2 – 2.5 0 – 0.5 5.9 – 15.3 200 – 3,500 200 – 10,000 0.8 – 1.0 0.75 – 0.90 600 – 650 520 – 580 Digester Gas Common Value 65 39 0.5 0.2 6 500 800 0.91 0.82 620 560 Notes: All percentages are by volume. The abbreviation ppmv is parts per million, by volume. The abbreviation ppbv is parts per billion, by volume. As produced, mesophilic digester gas at 98 degrees F is water saturated and contains about 6 percent water vapor. As produced, thermophilic digester gas at 131 degrees F is water saturated and contains about 15 percent water vapor. A fuel’s higher heating value (HHV) includes the heat of vaporization of water. A fuel’s lower heating value (LHV) does not include the heat of vaporization of water. 1-3PDF Image | Combined Heat and Power Technologies for Wastewater Facilities
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