Principles of Combustion

Combustion

Highly complex chemical process requiring oxygen, heat, and fuel; Fire is the resulting chemical reaction of those elements and can't start or continue if any one of them aren't present; Things to remember: Heat is a form of energy, oxygen is the most common oxidizer, fuel is most commonly hydrocarbon-based; The simplest combustion process: Fuel + Oxygen = Carbon Dioxide + Water

Fire (in the cremation process)

Occurs when combustible material, in combination with sufficient quantity of oxygen, is exposed to external ignition source above the flash point for fuel and oxidizer mix; molecules rearrange, either releasing or absorbing energy in process known as oxidation; to sustain flame, a chemical reaction must be able to maintain a rate of rapid oxidation that produces a self-sustaining, exothermic chemical chain reaction; process will continue without the need for an additional external ignition source until all available fuel or oxidant is consumed, removed, or the temperature reduced by cooling

Stoichiometric Combustion

The theoretical point where the ratio of fuel to oxygen results in complete combustion, when all the carbon in the fuel forms CO2 and all the hydrogen forms H2O; intended to maximize efficiency and reduce emissions into the environment as the result of incomplete combustion

Incomplete Combustion

Occurs when complete combustion of a combustible material doesn't happen; Lack of oxygen results in one atom of carbon uniting with only one atom of oxygen, creating carbon monoxide (CO) rather than carbon dioxide (CO2)

Process of Combustion

Occurs when heat doesn't dissipate faster than created; To achieve complete combustion, it is vital to control amounts of air introduced to combustion process to ensure burning of all fuel; Adjustment of combustion process so that proper level of excess air is present is key; Cremator operators make adjustments based on: type of fuel supply (i.e., fuel + cremation container + remains), changes in atmospheric pressure, time of day; Combustion spreads from ignition source to adjacent layer of gas mixture. In turn, each point of burning layer serves as ignition for the next one; When total heat energies of reactants and total heat energies of products reach equilibrium, combustion ends.

Excess Air

The amount of air necessary to make certain all energy is retrieved in the cremation process

Reactants

The substances that take part in and undergo change during a chemical reaction

Incineration (for cremation purposes)

Application of controlled combustion in the cremation process; Rapid oxidation process in which hydrocarbons react with oxygen to release a great deal of heat and light; A chemical reaction of combustible waste materials with air for the primary purpose of destruction and reduction in volume and weight

Primary Incineration

Drying of combustible material to drive off moisture, volatilization of the vapors and gasses which occur as the temperature of the material rises, combustion of the charge, and burnout of the solids

Secondary Incineration

Combustion of the vapors, gases, and particulates driven off during the primary process

Crematory Incinerators

In 1968, the Incinerator Institute of America defined and classified varying incinerator standards and the types of waste produced by them; Defined as 'Class VI Crematory and pathological incinerators suitable for Type 4 waste'

Type 4 Waste

Pathological. 100% human and animal tissue, organs, and solid organize wastes from hospitals, laboratories, animal pounds, farms, abattoirs, etc.; containing up to 85% moisture by weight. Average heat value 1,000-2,000 BTU/lb.

Cremation of human pathological waste

Permissible in most states; Crematories that accept pathological waste for cremation must consider the legalities of engaging in such business practices; Materials some states will not cremate: Municipal solid waste, Medical/infectious waste (other than pathological waste), Radioactive waste, Hazardous waste, Fiberglass and plastic, narcotics

Cremator/Retort

Type of incinerator that provides a controlled combustion process to reduce human remains to CO2, water vapor, and small non-combustible residues; a multiple-chambered retort that is specifically designed for the incineration of human remains; two designs: In-line hearth (Combustion gases flow vertically), Retort hearth (Flow of combustion gases is directed sideways through a secondary chamber adjacent to the primary combustion chamber. The most efficient design for cases weighing less than 750 lbs)

Operation of Cremator/Retort

The case is placed into the primary chamber, and the door is closed and locked. The afterburner is ignited to heat the secondary chamber, followed by the primary burner. The body is dried, ignited, and combusted by direct flame and radiant heat. Moisture and volatile gases vaporize and pass into the secondary chamber, where they are ignited and fully combusted. Gases exit through the stack. Once combustion is complete, burners shut off, the chamber cools, and the remains are removed

Maximizing Incinerator Combustion

Cremation must be tailored to specific conditions, with three key factors affecting efficiency: Time—sufficient retention in the chamber ensures complete combustion; Temperature—ideal range is 1,400°-1,800°F, with some states requiring 1,800°F in the secondary chamber to ensure full incineration; temperatures above this can reduce retention time, causing visible emissions

Turbulence

Increases combustion efficiency; Created in the cremator's exhaust system by having the gasses change directions through perforated walls and baffle systems; Aids in the mixture of gases, driving off and consuming any products of combustion before release into the atmosphere

Chemical Composition of the Human Body

The average human body is between 60-90% moisture; For this reason, retorts are designed with a fixed hearth with a raised edge at the door to prevent liquids from spilling during charging; Average heat value as fired of a human body is 800-3,600 BTU/lb; The body of a healthy, lean adult male is composed of about 62% water, 17% fat, 16% protein, 6% minerals, and less than 1% carbohydrates; Healthy, lean adult females carry about 22% fat and slightly fewer other chemical components

Emissions

Major constituents of air emissions from cremation include: Organic compounds, Nitrogen oxides, Sulfur dioxides, Carbon monoxide, Particulate matter; Emission rates depend on the design of the cremator, combustion temperatures, gas retention time, hot air duct design, stack temperature, and any control devices

Particulate Matter Emissions

Consists of tiny solid particles and/or water droplets made up of acids, organic compounds, metals, and dust. It forms during cremation from incomplete combustion and is controlled through proper equipment design and operation. PM is measured in grains per cubic foot, with a typical crematory limit of 0.08 grains per dry standard cubic foot of flue gas

Visible Emissions

Visible emissions, or opacity, indicate poor combustion and high particulate levels. Opacity measures how much light is blocked by the exhaust—0% means the exhaust is fully transparent. Cremators may have stack-mounted opacity monitors, or operators can visually assess emissions with the sun behind them. Maintaining proper combustion temperatures helps minimize visible smoke

Wet Scrubbers

Devices used to trap suspended particles by direct contact with a spray of water or another liquid; Sometimes called flue gas washers; Washes fly ash, odorous compounds, and other objectionable materials out of the airstream as they're carried along by droplets in the spray; Not commonly used in the U.S. on cremation equipment due to high costs

Baghouse/Fabric Filtration

Removes suspended particulates using an assembly of fabric-filter bags; Dust-laden air is blown upward by fans that trap the particulates inside; Offers high resistance to air flow, leading to substantial energy use to the system; Uncommon usage due to high costs

Visible Emissions

Visible emissions, or opacity, indicate poor combustion and high particulate levels. Opacity measures how much light is blocked by the exhaust—0% means the exhaust is fully transparent. Cremators may have stack-mounted opacity monitors, or operators can visually assess emissions with the sun behind them. Maintaining proper combustion temperatures helps minimize visible smoke

Wet Scrubbers