ABE 482 Environmental Engineering in Biosystems. November 14 Lecture 30

Transcription

1 ABE 482 Environmental Engineering in Biosystems November 14 Lecture 30

2 Today Air quality issues Odours Greenhouse gases (GHG) Dispersion of gases Measuring source strength Calculating gas emissions Emission reducing technologies Biofiltration Gas scrubbing Management techniques

3 Air Quality Issues Air quality affects human and plant health and quality of life Criteria pollutants for EPA PM, SO 2, NO 2, CO, O 3 Saskatchewan air pollution index O 3, CO, SO 2, NO 2 Saskatoon air quality rated as good to excellent from 1998 to 2002 Odours GHGs have regional and global impacts

4 Air Quality Issues Odour Why measure? Odour complaints from communities can prevent industry growth Validation of claims Claims of odour presence and odour reduction! Problem sources in SK Landfills Upgraders/refineries Livestock industry

5 Air Quality Issues Odour What is odour? Interaction of over 165 compounds Ketones Sulphur compounds (hydrogen sulphide) Volatile fatty acids Aldehydes Volatile organic compounds Phenols Nitrogen compounds (ammonia) Why is it so difficult to measure? Effect of interaction>actual concentration No single odour compound is a reliable indicator

6 Air Quality Issues GHGs Why measure? Kyoto protocol, climate change, carbon credits/targets/etc. Problem sources in SK What are GHGs? CO 2 CH 4 N 2 O Others Oil and Gas Industry Electricity Agriculture Transportation Other Industry Residential Commercial Waste

7 Dispersion of Gases Atmospheric dispersion depends on Source strength Weather conditions Wind speed Wind direction Atmospheric stability Topography Stack height and orientation Dispersion modelling

8 Assessing Odours Frequency Strength Duration Offensiveness Persistence Character Intensity (number on a scale of 0-5) Concentration (odour units or OU/m 3 )

9 Assessing Odours Frequency Hedonic Tone Scale How often an odour occurs Rating Description 1 Dislike extremely # times/year, for example 2 Dislike very much Duration 3 Dislike moderately How long an odour lasts 4 Dislike slightly # hrs/day, for example 5 Neutral Offensiveness 6 Like slightly How pleasant/unpleasant an odour is 7 Like moderately AKA Hedonic Tone 8 Like very much Subjective value, depends on scale used Character What an odour smells like 9 Like extremely Descriptor words: earthy, burning, sewer, flowery, etc.

10 Assessing Odours Threshold/Detection Recognition Annoyance

11 Assessing Odour Concentration Number of dilutions required to reach threshold High number of dilutions = high concentration Number of dilutions ~ number of odour units Olfactometry panel Sample collected, analyzed by a specialized olfactometer (auto diluter) and 4-8 trained panelists Field olfactometer Hand-held unit, dilutions-tothreshold reading directly in the field Hedonic tone is also assessed using the olfactometer

12 Assessing Odour Intensity Relative strength of odour above threshold Standard intensity scale established by ASTM N-butanol, concentrations increase exponentially from 0 ppm to 20,000 ppm Intensity Level Description No odour Very faint odour Faint odour Moderate odour Strong odour n-butanol in water (ppm) ,250 6,750 Trained odour assessors (Nasal Rangers ) rate odour (in field) on a scale of 0 to 5 5 Very strong odour 20,250

13 Assessing Odour Persistence A measure of how an odor's intensity changes related to its concentration Determined from intensity measurements of an odour at full strength and at several dilution levels above threshold Persistence is the value of the exponent of the power relationship on the log-log plot of odour intensity vs dilution ratio Varies between and -1.86

14 Developing an Odour Index Concentration, intensity, pleasantness and persistence are not necessarily intrinsically related Concentration: OU/m 3 Intensity: number on a scale (unitless) Persistence: value of an exponent (unitless) Pleasantness: number on a scale (unitless) How do you determine if an odour is acceptable or not? Which of the following scenarios would you be OK with? An extremely unpleasant odour that is very weak A strong odour that is mildly unpleasant A mildly unpleasant odour, weak, but very persistent A slightly pleasant odour that is very strong What about duration and frequency?

15 Developing an Odour Index Take three odour measures: concentration, offensiveness, persistence Break down typical persistence, offensiveness and concentration values into 4 or 5 categories Concentration (OU/m 3 ) Offensiveness Persistence (absolute value) < > , >10, <0.9 4

16 Developing and Odour Index Weight each measure according to how they affect the perception of an odour Persistence = 3 Offensiveness = 1 Concentration = 2 Concentration Offensiveness Persistence (absolute value) < > , >10, <

17 Application of the Odour Index Measure the concentration, persistence and offensiveness of an odour and calculate its odour index value Acceptable all the time Sometimes acceptable Never acceptable Index < >15 Example: Concentration = 958 OU/m 3, hedonic tone of 3 and a persistence of (all values can be measured using an olfactometer).

18 Assessing GHGs Measure concentration of each compound separately CO 2, CH 4, N 2 O Gas chromatography, specific gas analyzers, sensors, etc. Global warming potential of each gas CO 2 = 1 CO 2 CH 4 = 21 CO 2 N 2 O = 310 CO 2

19 Collecting Representative Samples Need to collect and transport air samples to measure GHG concentration Odour concentration, offensiveness, persistence Samples must be representative of the source Point (i.e.: stack, exhaust fan) Non-point (i.e.: lagoon, landfill, field)

20 Collecting Samples from Point Sources Syringe? Jar and lid? Vacuum? Sampling lung principle Other considerations Representative over time Continuous or intermittent? Sample container must be inert Sample storage time/conditions

21 Collecting Samples from Non-Point Sources Big jar and lid? Multiple point-source samples? chamber technique Simple, effective, good for small scale sources and treatments Problematic for some gases, labour-intensive, no measure of spatial or temporal variation micrometeorology Accurate, effective, good for small and large scale sources, provides measure of spatial and temporal variation Costly, complex equipment and analysis

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23 Calculating Emission Rates For point sources GHG: unit of mass per unit time Odour: odour unit per unit time Requires measure of ventilation rate or volumetric output of a stack, etc. For non-point sources GHG: unit of mass per unit area per unit time Odour: odour unit per unit area per unit time For chambers: need volume, surface area, airflow (if dynamic) and time of deployment (if static) For micromet: need area of site from which samples are captured

24 Emission Reducing Technologies Filtration or electrostatic precipitation Good for PM Biofiltration Layer or bed of organism-rich material Organisms consume odourous compounds Gas scrubbing Wet scrubbers Convert gaseous components into liquid phase Dry scrubbers Absorb gas particles by use of sorbents

25 Dual Throat Air Scrubber

26 Multiple Venturi Air Scrubber

27 Packed Bed Air Scrubber

28 Gas Scrubbers Parameters to consider when designing/operating a gas scrubber? System efficiency type of liquid solubility of gases temperature of liquid ph of liquid surface area of liquid in contact with gas stream gas to liquid ratio residence time liquid/sorbent regeneration Other considerations used liquid will require treatment before disposal may need to dehumidify output gas to prevent fogging minimize pressure drop to minimize power requirements PM control for dry scrubber reactants

29 Next Day Course evaluations Air quality guest lecture