Introduction to Oxidation and Mass & Energy Balances

Transcription

1 Introduction to Oxidation and Mass & Energy Balances Michael Mannuzza OBG

2 Combustion Combustion is an oxidation reaction: Fuel + O 2 ---> Products of Combustion + Heat Energy In addition to traditional burner fuels, incinerator fuel can include solid, gaseous and liquid waste. The Products of Combustion (POC) are the primary concern from an Air Pollution Control (APC) perspective.

3 Air Pollutants and Regulatory Drivers The type of APC system required for an incinerator will be decided based on the system s POCs and the regulatory limits mandated for specific pollutants. Emissions of the following pollutants are typically regulated for most incinerator applications: Dioxin/Furans Mercury (Hg) Semi-Volatile Metals (Cd, Pb) Low-Volatility Metals (As, Be, Cr) SOx NOx Particulate Matter VOCs & Total Hydrocarbons Carbon Monoxide HCl & Cl 2 Others

4 Identifying APC Requirements Three Steps: 1. Quantify anticipated POCs 2. Identify Regulatory Emission Constraints (establish abatement requirements) 3. Quantify the discharge flow rate of the system.

5 Waste Feed Properties I. Begin by identifying the properties of the waste feed and quantifying the constituency of the waste: 1. Material Take-Offs 2. Proximate, Ultimate, and Ash Analysis 3. Sample & Analyze for Specific Compounds 4. Employ Other Empirical Methods PROXIMATE ANALYSIS ULTIMATE ANALYSIS ASH ANALYSIS Category wt % Category wt % Category wt % Moisture 3.3 Carbon (C) 61.1 Silicon Dioxide (SiO 2 ) 74.1 Ash 22.1 Volatile Matter 27.3 Hydrogen (H) 3.0 Nitrogen (N) 1.35 Aluminum Oxide (Al 2 O 3 ) 20.0 Iron Oxide (Fe 2 O 3 ) 3.25 Fixed Carbon 47.3 Sulfur (S) 0.4 Calcium Oxide (CaO) 0.68 Gross Calorific Value Oxygen (O) 8.8 Magnesium Oxide (MgO) 0.48 Other Other 1.49 * Ash Fusion Point = 1104 ⁰C

6 Waste Feed Properties If possible, identifying specific compounds in the waste is the best approach. GASEOUS WASTE STREAMS COMPOUND NAME % VOLUME Methane Ethane Propane Butane Octane Pentane Hexane Carbon Monoxide Benzene Toluene Methylene Chloride Water (g) LIQUID WASTE STREAMS COMPOUND NAME Wt. % Toluene Xylene Propanol Isopropylbenzene Acetic Acid ,2 Dichloroethane Methanol Methyl Bromide Formaldehyde Sodium Fluoride Hydrogen Chloride Water (L)

7 Waste Feed Properties I. Waste feed rates and constituencies are rarely constant. It is important to establish a realistic waste feed design basis. The waste feed design basis must address the following: I. Worst case feed rate II. Highest heating value condition (kj/kg of waste) III. Lowest heating value condition (kj/kg of waste) IV. Worst case sulfur condition, chlorine condition, NOx condition, metals condition, particulate condition, etc. V. Any other critical regulatory or production related criteria associated with the waste.

8 Stoichiometry of Combustion I. As a starting point, it is necessary to make some Initial assumptions. If adequate Oxygen will be made available for complete combustion, assume the following: I. All Carbon converts toco 2 II. All halogens convert to acids (e.g., Cl HCl, Br HBr) III. All alkali metals convert to hydroxides (e.g., Na NaOH) IV. All remaining hydrogen converts to H 2 O V. All non-alkali metals convert to metal oxides in their most common oxidation state (e.g., MgO, Fe 2 O 3 ) VI. Bound Nitrogen in the waste converts to NO 2 VII. All Sulfur converts to SO 2 VIII. Non-combustible constituents pass through unchanged or thermally decompose to known compounds

9 Stoichiometry of Combustion I. In reality, it is possible that additional products or congeners can be formed. I. SO 3 II. II. Diatomic Halogens (Cl 2, Br 2 ) III. N 2, NO, N 2 O IV. CO V. Dioxin/Furans VI. Others Formation of these tertiary compounds can be a function of: I. Temperature II. Contaminant Concentrations III. O 2 and H 2 O Concentrations IV. Catalysts V. Other factors

10 Stoichiometry of Combustion I. The initial assumptions outlined usually provide a reasonable estimate of the products of combustion that are generated. I. Allows the elemental contaminants to be quantified. II. Provides a mechanism for identifying combustion air requirements and exhaust flow rates II. The resulting POCs can then be refined if necessary by applying: I. Empirical Data II. Advanced Methods

11 Stoichiometry of Combustion Example: Dichlorobenzene, C 6 H 4 Cl 2 C 6 H 4 Cl 2 +?O 2?CO 2 +?H 2 O +?HCl C 6 H 4 Cl O 2 6CO 2 + 1H 2 O + 2HCl 1. Balance Carbon atoms first 2. Balance halogens, metals, Nitrogen & Sulfur next. 3. Balance Hydrogen atoms 4. Balance Oxygen atoms last

12 Stoichiometry of Combustion 1. Balance Carbon & CO 2 first: C 6 H 4 Cl 2 +?O 2?CO 2 +?H 2 O +?HCl How many moles of CO 2? Answer: 6 C 6 H 4 Cl 2 +?O 2 6CO 2 +?H 2 O +?HCl

13 Stoichiometry of Combustion 2. Balance halogens, metals, Nitrogen & Sulfur next: C 6 H 4 Cl 2 +?O 2 6CO 2 +?H 2 O +?HCl How many moles of HCl? Answer: 2 C 6 H 4 Cl 2 +?O 2 6CO 2 +?H 2 O + 2HCl

14 Stoichiometry of Combustion 3. Balance Hydrogen atoms: C 6 H 4 Cl 2 +?O 2 6CO 2 +?H 2 O + 2HCl How many moles of H 2 O? Answer: 1 C 6 H 4 Cl 2 +?O 2 6CO 2 + 1H 2 O + 2HCl

15 Stoichiometry of Combustion 4. Balance Oxygen atoms last: C 6 H 4 Cl 2 +?O 2 6CO 2 + 1H 2 O + 2HCl How many moles of O 2? Answer: 6.5 C 6 H 4 Cl O 2 6CO 2 + 1H 2 O + 2HCl

16 Stoichiometry of Combustion Mass Balance C 6 H 4 Cl O 2 6CO 2 + 1H 2 O + 2HCl Compound: C 6 H 4 Cl 2 O 2 CO 2 H 2 O HCl Molecular Wt. (g/mol): No. of Moles: Total Wt. (g): Normalized Ratio: Total Wt. = (Molecular Wt.) * (No. of Moles) 2 Normalized Ratio = Total Wt. divided by the molecular wt of C 6 H 4 Cl 2 ( g/mol). Thus: 1kg C 6 H 4 Cl kg O kg CO kg H 2 O kg HCl Same applies for grams, pounds or any unit other unit of mass.

17 Stoichiometry Calculations This approach can be applied to each constituent identified in a waste stream: STEP 1: Balance the moles STEP 2: Balance the mass

18 Stoichiometry Calculations Each constituent can be factored by its mass ratio and then summed to generate a representative compound that reflects the properties of the overall waste stream. Similarly, a weighted calculation can be performed to determine the net heat of combustion of the waste stream.

19 Heat of Combustion Definitions: Heat of Combustion: The heat released by combustion of a unit quantity of fuel with its stoichiometrically correct amount of combustion air, measured either in calories or Btu. Gross Heating Value: The heat released by combustion of a unit quantity of fuel with both the combustion air and fuel at a known reference temperature prior to combustion (e.g. 60 ⁰F) after the products of combustion are allowed to cool to the initial temperature. Also known as Higher Heating Value (HHV). Net Heating Value: The heat release measured prior to the products of combustion being allowed to cool. Also known as Lower Heating Value (LHV).

20 Heat of Combustion For most incinerator applications, we are concerned only with the LHV of the fuel/waste. Numerous data sources are available (reference books, internet, etc.). Can be estimated or obtained through testing. Heat of formation Dulong s Approximation (Btu/lb = 14,544C+62,028(H O 2 )+4,050S) Empirical formula based on coal. Can be applied to other carbonaceous waste, but accuracy is questionable. 410 Btu (103.3 kcal) per Mole of O 2 Consumed Good approximation for hydrocarbons. Accuracy diminishes if Oxygen, Nitrogen, Halogens and other elements are present.

21 Mass & Energy Balance Mass in = Mass out Energy in = Energy out Mass of exhaust stream constituents (solid, gas) Heat content of constituents Thermal losses Control volume Mass of waste stream constituents (solid, liquid, gas) Heat content of constituents Net heat of combustion of combustibles Mass of burner fuel and combustion air Heat content of constituents Net heat of combustion of burner fuel

22 Mass & Energy Balance Conservation of Mass & Energy m stream1 + m stream2 + m stream3 =m stream4 Q stream1 +Q stream2 +Q stream3 =Q stream4 + Thermal Losses

23 Mass & Energy Balance Key Points I. Mass in must equal mass out (m in m out = 0). II. Energy in must equal energy out (Q in Q out = 0). III. Combustion air volume will generally be a direct function of the fuel input, however, additional air may be needed to maintain O 2 levels or control temperature. IV. Adjust the mass of the fuel input until the system energy is balanced. I. Cannot solve directly - must be an iteration. II. The Goalseek function in Excel is useful for this approach (modulate fuel input until Q in Q out = 0 III. Determine energy of waste gas streams by applying specific heats.

24 Mass & Energy Balance Conservation of Mass & Energy m stream1 + m stream2 + m stream3 =m stream4 Q stream1 +Q stream2 +Q stream3 =Q stream4 + Thermal Losses

25 Applying Specific Heats Q=m* C p *DT Q = Heat flow (Btu/hr, MJ/hr, Watts, etc.) m = mass flow (lb/hr, kg/hr, etc.) C p = Constant pressure specific heat (Btu/lb-⁰F, J/kg-⁰K, etc.) DT = Temperature difference between actual temperature and reference temperature (T-T ref ) (⁰F, ⁰C, etc.) Specific heat varies based on temperature and is tabulated for commonly encountered gases in many reference books. It is also frequently presented as a polynomial function of temperature. C p = a + bt +ct 2 A very accurate mean Cp can be obtained by integrating this polynomial across the temperature range. C pmean = [a + bt +ct 2 ]/(T-T ref ) [a(t-t ref )+(1/2)b(T 2 -T ref2 )+(1/3)c(T 3 -T ref3 )]/(T-T ref )

26 Radiation Losses Shell losses are a function of numerous variables: Shell temperature Wind velocity Shell color Shell area Emissivity Furnace Radiation Losses 1 Furnace Rate (MBtu/hr) Radiation Losses (%) < > Handbook of Incineration Systems, Brunner, C. R., 1991 McGraw-Hill

27 Mass & Energy Balance Conservation of Mass & Energy m stream1 + m stream2 + m stream3 =m stream4 Q stream1 +Q stream2 +Q stream3 =Q stream4 + Thermal Losses

28 Converting Mass to Volume When calculating process emissions, we routinely need to convert between mass & volume. Avogadro s Law: Equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules. Definitions & Conversions: lb-mol = mass (lbs) MW 1 lb-mol = ft 70 ⁰F and psi g-mol = mass (grams) MW 1 g-mol = ⁰C and 760 mm Hg ppmv = 10 6 * Volume of Component Overall Volume

29 Converting Mass to Volume Volume is temperature & pressure dependent. It is common to normalize volume to a standard temperature, to facilitate mass/volume conversions Nm 3 /hr & m 3 /hr m 3 /hr = Nm 3 /hr * (273 + T)/(273 +T ref ) T = Process Temp. (⁰C) T ref = Reference Temp. (⁰C) usually 0 ⁰C SCFM & ACFM ACFM = SCFM *(460+ T)/(460 +T ref ) T = Process Temp. (⁰F) T ref = Reference Temp. (⁰F) Be aware, different reference temps are used for SCFM 60 ⁰F, 68 ⁰F, 70⁰F Nm 3 /hr * = SCFM (@ 70 ⁰F)

30 Converting Mass to Volume I. Volume is also effected by pressure, including pressure due to elevation changes: I. SCFM is referenced to standard atmospheric pressure at sea - level = psi (406.8 inches H 2 O) II. Nm 3 /hr is referenced to 760 mm Hg. III. When pressure effects must be accounted for: I. ACFM =SCFM *(406.8/ P actual )*(460+T)/530 II. m 3 /hr = Nm 3 /hr * (760/ Pactual )*(273+T)/273

31 Combustion Air Composition of Dry Atmospheric Air Gas Volume Nitrogen % Oxygen % Argon % Carbon Dioxide % Neon % Helium % Methane % Typical C.A. Constituency (Assumed) % Weight % Volume Nitrogen Oxygen

32 Excess Air Excess Air: The air remaining after a fuel has been completely burned or that air supplied in addition to the amount required for stoichiometric combustion. Increased O 2 content can enhance combustion, or it can lead to the formation of problematic compounds (i.e., SO 3, CO 2, NO 2 ) Excess air may be required to control exothermic temperature rise or flammability levels. Regulatory emission limits are typically referenced to a specific O 2 level and a correction factor must frequently be applied to actual data. ppmv corrected = ppmv test * [21-%O 2base ]/[21-O 2test ]

33 Particulate Matter Particulate Matter (PM) can be solid, or liquid aerosol. Can include condensables Particle Size Distribution (PSD) is important. Will drive APC Technology Selection Determined by source testing. Units for PM Micrograms per dry cubic meter (μg/m 3 ) Grains per standard cubic ft ( gr/scf). 7,000 gr = 1 lb Measurements are dry basis

34 QUESTIONS?