An incinerator manufacturer (?) describes the pollutants to be controlled
from a medwaste incinerator.
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Air-pollution-control Methods--Part 1: Waste-burning Basics
8/17/99 This--the first of a
multi-part--presentation introduces some concepts
basic to understanding the devices used for
controlling emissions from
medical-waste incinerators.
By: Isaac Ray
The medical-incinerator control-system designer
must comply with existing
air-pollution-control requirements, limit
nuisance emissions, and grasp that even
an occasional emission-control failure will
probably engender severe public and
regulatory reaction.
Also the designer must be mindful that many
non-regulated components should
be removed from the incinerator's combustion
stream to safeguard downstream
equipment from excessive erosion (physical wear)
and/or corrosion (chemical
degradation).
Fortunately, excellent technology exists for the
consistent removal of large,
small, and extremely fine particles and acid
gases from discharge gas streams.
Incineration has been the major technology for
disposing of hospital and
infectious waste for more than 10 years.
The incineration equipment for this application
normally is small, in a range of 250
to 3000-lb/hr capacity, and operated in a batch
mode. An example is a
microprocessor-controlled, air fixed-hearth,
two-chamber, prepackaged unit.
Regulatory agencies tend to classify
pathological waste, infectious waste, and
medical laboratory waste together under the
general heading of hospital waste.
These wastes, however, have different properties
(see Table 1) and require
different combustion conditions for optimum
oxidation.
The type of waste produced depends on the nature
of medicine that is practiced
at the hospital, the extent to which lab work is
done, the number of beds, the
presence of special (such as radioisotope)
facilities, and the hospital's philosophy
about what goes to the incinerator.
Trends
In general, the plastic content of hospital
waste continues to grow, from 10% to
more than 30% during the last 20 years. Many of
the plastics contain chlorine
and generate hydrogen chloride emissions during
incineration.
In addition, most hospitals tend to route more
and more and their waste to the
incinerator and less to municipal
solid-waste-disposal facilities. As a result, the
paper content of the waste today is greater than
that in Table 1, which reflects
conditions prior to hospitals' installing their
incinerators.
Table 1
Hospital Waste Composition
Component
Higher
Heating
Value on
Dry Basis,
103Btu/lb
Bulk
Density
as Fired,
lb/cu ft
Moisture
Wt, %
Heating
Value
as
Fired,
103Btu/lb
Human,
Anatomical
8 - 12
50 - 75
70 - 90
0.8 -
3.6
Plastics
Swabs
Adsorbents
14 - 20
5 - 144
0 - 1
13.9
- 20
Animal
Infected
Anatomical
11 - 14
5 - 62
0 - 30
5.6 - 12
Animal
Infected
Anatomical
9 - 16
48 - 62
0 - 0.2
11 - 14
Glass
0
175 - 225
0
0
Bedding,
Shaving
Paper, Fecal
Matter
8 - 9
20 - 45
10 - 50
4 - 8.1
Gauze Pads,
Swabs
Garments,
Paper
Cellulose
8 - 12
5 - 62
0 - 30
5.6 - 12
Plastics,
PVC,
Syringes
9.7 - 20
5 - 144
0 - 1
9.6 - 20
Sharps,
Needles
60
450 - 500
0 - 1
0.06
Fluids,
Residuals
0 -10
62 - 63
80 - 100
0 - 8
Pathological wastes such as animal carcasses and
human body parts are
usually handled separately by burning on a batch
basis in a dedicated
incineration facility. Such waste has very high
moisture content and produces no
acid-gas emissions and particulate emissions are
relatively harmless oxides of
calcium.
Gaseous Emission Control
Many types and sizes of air-pollution-control
equipment are on the market,
designed to clean gas streams of solid, liquid,
and gaseous pollutants. The most
common types are the fiber filter (bag house),
the wet scrubber, the dry
electrostatic precipitator (ESP), the wet
electrostatic precipitator (WESP), and
the CONDENSING WESP.
The Nature of Pollutants
The primary requirement when selecting the best
gas-cleaning method is a sound
knowledge of the particulates to be
removed--principally, particle size distribution,
concentration, and chemical composition.
The pollutants from incineration are a function
of many factors, including the
composition of the waste, charging rate, method
of charge, furnace type and
design, burn conditions (temperature,
turbulence, time), and excess air.
Particulate matter (including heavy metals,
dioxins, furans, and mercury),
chlorides, and sulfur oxides are the most common
pollutants in the incinerator
gas stream requiring substantial removal. Other
pollutants, if present, often will be
removed by the same control arrangements.
Particulate Type and Size
Particulate size is measured in microns
(micrometers). Particles are classified
according to their mode of formation as dust,
fumes, smoke, mists, or sprays.
Dust, fumes, and smoke are solid particles. Dust
particles normally are larger
than one micron. Breaking up larger
particles--as by crushing or grinding--creates
smaller dust particles.
Fumes are fine, solid particles formed by the
condensation of vapors of solid
materials, such as heavy metals and their
oxides. Fume particles normally are
smaller than one micron.
Smoke comprises fine (less than one micron)
solid particles resulting from
incomplete combustion of organic materials.
Mists and sprays are not products of
incineration but of the incinerator's
pollution-control equipment. Mists are liquid
droplets, generally smaller than 10
microns, generated in a gas scrubber. Sprays are
larger liquid droplets, ranging
from 10 to 400 microns.
An aerosol is an assembly of small particles,
solid or liquid, suspended in a gas.
The diameter of the particles may vary from 100
microns down to 0.01 micron or
smaller.
Particulate Concentration
The particulate concentration is a measure of
the total mass of all the particles
suspended in a given volume of gas. It is
usually measured as grains (gr) per
cubic ft (one lb = 7000 gr) or as grams per
cubic meter.
Air-pollution-control systems are classified
according to the particulate
concentration of gas to be cleaned. For
hospital-waste incineration, total
particulate loading can be as high as 0.3 gr/scf.
Particle-size Distribution
Particulate matter that originates in the
incinerator varies considerably in particle
size as well as concentration. Knowledge of
particle-size distribution therefore
can be important. The particle-size distribution
of a typical incineration sample is
illustrated in Table 2.
Table 2A
Particulate Characteristics of a Typical
Incineration Sample --
Concentration
LOCATION 1*
LOCATION 2**
grains/dcf
1.88
0.01
lbs/hr
217.01
1.48
lbs/wet ton
52.08
0.36
Table 2B
Particulate Characteristics of a Typical
Incineration Sample -- Size
Distribution
(Percent by Weight Less than
Indicated Size)
Size, microns
LOCATION 1*
LOCATION 2**
18.7
37.9
100.0
11.7
30.6
98.0
8.0
16.4
94.9
5.4
6.6
93.4
3.5
2.6
92.8
1.8
1.6
83.3
1.1
0.9
67.7
0.76
0.1
54.6
*Measured at incinerator outlet, w/out controls
burning 100 tons/day wet sludge
cake"
**Measured after venturi scrubber with a
total pressure drop of 30-in. wc.
Source: C.R. Brunner, Incineration ,Systems:
Selection and Design (New York: Van Nostrand
Reinhold, 1984).
Particle Settling Velocity
Particles suspended in relatively calm
atmosphere and regions of low gas velocity
surrender to gravity and slowly settle.
Gravitation causes a particle to move
downward at increasing velocity, but buoyancy
force and drag resist the
downward motion. The particle will fall at a
constant or settling velocity when the
gravitational force equals the combined buoyant
and drag forces.
The smaller the particle, the less its response
to gravity and inertial forces. For
example, a 100-micron water droplet has a
settling velocity of 59.2 fpm, the
settling velocity of a 10-micron droplet is 059
fpm, for 1 micron, it's 0.007 fpm,
and for 0.1 micron, it is 0.00007 fpm. In other
words, fine and extremely fine
particles essentially never settle by gravity.
About the author: Isaac Ray is a vice president
with Croll-Reynolds Clean Air
Technologies, 751 Central Avenue, PO Westfield,
NJ 07091-0668. Tel:
908-232-4200; Fax: 908-232-2146.
--
Neil TANGRI