Combustion and Energy Recovery

waste-to-energy

continues to be a v**iable alternative to landfilling.**

**British thermal unit (Btu)**

* defined as that **amount of energy necessary to heat one pound of water one degree Fahrenheit**
* **One of the earliest measures of heat energy**

Joule.

internationally accepted unit of energy

Ultimate analysis

**uses the chemical makeup of the fuel to approximate its heat value.**

DuLong equation

* **most popular method using ultimate analysis**
* originally **was developed for estimating the heat value of coal**

elemental analysis

**can be conducted using standard methods published by the American Society for Testing and Materials (ASTM).**

proximate analysis,

it is **assumed that the fuel is composed of two types of materials**: volatiles and fixed carbon

Calorimetry

the **referee method** of determining the heat value of mixed fuels

Thermogram.

a **plot of temperature (T) versus time (t)**

higher heating ; lower heating

An i**mportant aspect of calorimetric heat values** is the distinction between ________value and ______ value.

gross calorific energy,

The **higher heating value (HHV)**

net calorific energy.

lower heating value (LHV)

carbon dioxide and water.

products of combustion

photosynthesis

The **energy from the sun is stored using the process** of __________ in organic molecules, and this energy is slowly released as the organic materials decompose.

stoichiometric oxygen.

**amount of oxygen necessary to oxidize some hydrocarbon**

Energy

when expressed in common units, can be pictured as a **quantity that flows**, and thus, it is possible to analyze energy flows using the same concepts used for materials flows and balances

black box

process or **operation into which certain flows enter and others leave**

1\. Ash must not exceed a percent combustible level.

2\. Exhaust gas in the stack must be within a predetermined temperature range.

These __**two criteria ensure complete combustion of the solid waste and recovery of the heat**__, and both criteria can be easily monitored.

1. have a ***storage pit*** **for storing and sorting the incoming refuse**
2. ***crane*** for charging the combustion box
3. ***combustion chamber*** consisting of bottom grates on which the combustion occurs
4. ***furnace or combustion chamber***
5. ***heat recovery system*** of pipes in which water is turned to steam
6. ***ash-handling system***
7. ***air-pollution control system***

components of a typical solid waste combustor

chamber

heart of the combustion process

grates

its function is to **provide turbulence** so that the MSW can be thoroughly burned, to **move the refuse down and through the combustion chamber,** and finally, to **provide underfire air** to the refuse through openings in the grates

control of underfire air

**most important variable in maintaining a desired operating temperature** in the combustion chamber

over fire air

* **air blown into the combustion chamber above the refuse**
* its purpose is to **provide the oxygen necessary for combustion** as well as to **enhance the turbulence in the combustion chamber.**

rotary kiln,

* **modification of the combustion chamber**
* **provide the most turbulence of any grate system** and thereby enhance the rate and completion of combustion.

water wall

furnace walls of modern combustors are lined with metal tubing through which water is circulated.

reciprocating, (c) rocking, (d) traveling

three types of grates

water tubes

**protect the combustion chamber housing** by transferring the heat into the water.

superheated steam

**allows the use of multistage high-efficiency turbine generators**

modular starved-air combustor

these units are **characterized by a two-stage combustion system**, with the **first stage** being ***operated in a starved-air mode***, producing a large quantity of suspended carbon which is then ***burned using a fossil fuel in the second stage.***

Modular combustors

**useful in cases where the waste has to be combusted but the quantity is insufficient** to warrant the construction of a large refractorylined or water-wall combustor.

Modular combustors

One of the most **widely used applications of these units is in the destruction of some hazardous materials**, such as biohazards waste from hospitals.

Pyrolysis

**destructive distillation** or combustion in the absence of oxygen.

true pyrolysis

In ____________, **heat is added to the complex organic feed.**

gasification,

**modification of pyrolysis** in which a **limited quantity of oxygen is introduced as pure oxygen** or as air, and the resulting oxidation produces enough heat to make the system self-sustaining.

Slow pyrolysis

Intermediate pyrolysis

Rapid pyrolysis

Flash pyrolysis

Four General Modes of Operation for Manipulation of Pyrolysis

Slow pyrolysis

**proceeds at a very slow rate of temperature increase**, generally less than one degree C per second, and the final ***temperature range is between 500 and 750ºC.***

Intermediate pyrolysis

**takes place at a more rapid temperature rise,** of 5 to 100ºC/sec and reaches ***temperatures of between 750 and 1000ºC.***

Rapid pyrolysis

**occurs when the temperature rise is fast**, beween 500 and 106 degrees C per second. The temperatures ***reached with this process are over 1000ºC.***

Flash pyrolysis

**takes place when the temperature rise is essentially instantaneous,** of over 106 degrees C per second. The ***temperatures attained in this process exceed 1200ºC.***

mass burn units; refuse-derived fuel (RDF)

**Combustion systems are characterized** as either ________ or __________ units

mass burn unit

**has no pre-processing of the solid waste prior to being fed** into the combustion unit.

RDF system

**solid waste is processed prior to combustion to remove noncombustible items and to reduce the size of the combustible fraction,** thus producing a more uniform fuel at a higher heat value.

semi-suspension firing

**combustion takes place above the grate** with the remaining combustion occurring on the grate.

suspension firing

* If **no grate exists and all combustion occurs in the air,**
* not generally applicable to RDF.

RDF-1

**mixed refuse without any processing,**

RDF-2

**refuse that has been shredded**

RDF-3

**shredded refuse from which most of the inorganic materials have been removed.**

RDF-4

Further shredding into a **fluff**

RDF-5

**pelletized** into dog-food-sized pellets

RDF-6 and 7

**have been tried on a pilot basis but have not been found to be successful** at full-scale plant

RDF-6

Organic fraction of the waste that has been **further processed into a liquid fuel**, such as oil.

RDF-7

Organic waste **processed into a gaseous fuel.**

MSW

**low-grade fuel** that can be used for the production of steam

MSW

This steam is **sufficiently useful for driving turbines**, but the remaining steam has little industrial use

blowdown

(fresh water added to the recycle)

Bottom ash

**recovered from the combustion chamber and consists of the inorganic material** as well as some unburned organics,

fly ash

**particulates removed from the gaseous emissions**.

• Road base material

• Structural fill

• Gravel drainage ditches

• Capping strip mines

• Mixing with cement to make building blocks

Some of the uses of ash

Clean Air Act

**required all large and small plants to be retrofit to meet Maximum Achievable Control Technology (MACT)** by 2000 and 2005, respectively

gases or particulates

**air pollutants of concern in municipal-waste incineration** can be classified as _______

Primary pollutants

**products of the combustion process that can be shown to be harmfu**l in the form they are emitted

Secondary pollutants

those that are f**ormed in the atmosphere as a direct result of the emission of primary pollutants.**

sulfuric acid

produced in the reaction of chlorine compounds

nitric acid

produces acid rain.

Ozone

**reacts with hydrocarbons to form a series of compounds** that includes aldehydes, organic acids, and epoxy compounds.

formation of photochemical smog

**dynamic process** that begins with the production of nitrogen oxides from automobiles, industrial facilities, and MSW combustion.

Mercury

especially **difficult to control**, because i**t volatilizes so readily** and escapes with the gaseous emissions

albedo

**earth acts as a reflector to the sun’s ray**s, receiving the radiation from the sun, **reflecting some of it into space**

methane (CH4) and carbon dioxide ( CO2

**adsorb radiation** at wavelengths approximately the same as the heat radiation trying to find its way back to space.

greenhouse gases,

**gases that adsorb the heat energy radiation**

Methane

17 times more potent as a greenhouse gas than carbon dioxide.

settling chambers

**simplest devices for controlling particulates** consisting of nothing more than wide places in the exhaust flue where larger particles can settle out, usually with a baffle to slow the emission stream.

cyclone

**Possibly the most popula**r, economical, and effective means of controlling particulates from many industrial sources

scrubber

* another **method for removing large particulates**
* **used in MSW combustion mostly for the removal of gaseous pollutants**, but they also help in the removal of particulates.

Dry scrubbers

* i**nject a chemical slurry** such as lime. This type of scrubber **does not produce a visible plume,** and the waste is a powder—not a liquid.
* very effective in controlling sulfur oxides

Electrostatic precipitators

**widely used in power plants**, mainly because power is readily available.

Wet scrubbers

**can be used for partial particulate removal**, can also remove gaseous pollutants by simply dissolving them in the water.

Waste-to-energy combustors

**also are a source of nitrogen oxides that can lead to the formation of photochemical smog** and contribute to acid rain

selective non-catalytic reduction (SNCR) process

nvolves the **injection of ammonia and steam** into the furnace.

selective catalytic reduction (SCR) process,

a **catalyst is used to achieve higher reductions** than are possible with a SNCR process.

Dioxin

**combination of many members of a family of organic compounds** called *polychlorinated dibenzodioxins (PCDD).*

polychlorinated dibenzofurans (PCDF)

have a similar structure except that the **two benzene rings are connected by only one oxygen**

* in the waste and are not combusted in the furnace
* de novo dioxins that are created during combustion

Dioxins emitted from waste-to-energy facilities come from **two sources**