Nicholas P. Cheremisinoff. Handbook of Solid Waste Management and Waste Minimization Technologies

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Figure 7. RDF manufacturing process scheme.

There are two main types of RDF: coarse-RDF (c-RDF) and densified pellet or

briquette (d-RDF). RDF is produced by processing waste into coarse or densified

RDF after first mechanically removing the noncombustible materials, such as

metal and glass. A RFD plant with a capacity of approximately 100,000 tons of

waste per year may occupy up to 0.6 hectares or an area equivalent to a soccer

field for the fuel production facility. An extra hectare is required for the

combustion facility. The vent height would be between 10 and 15 m high. Land

requirements are approximately circa 5000 to 6000 m

for the RDF production

facility and 1000 m

for the combustion facility of a plant 90,000 to 100,000 tons

per annum (approximate size). Plants require a revolving screen (trommel), a

FUEL

STORAGE

FUEL

SHREDDER

RESIDUE FERROUS METAL

MAGNETIC

SEPARATION

SCREEN/ROTATING

TROMMEL

BLOW-OFF PANEL

SOLID

WASTE

RECEIVING

STATION

RDF is often used to replace traditional fuels in industrial processes or power

stations.

shredder, and a boiler or power plant. Figure 7 illustrates a process scheme for a

RDF manufacturing facility. There are a number of variations for these

processes. In addition to the process scheme illustrated in Fig. 6, other dry

processing schemes first shred the waste and then send the material to an air

classifier that separates out the light organic material from metals and other

heavy organic and inorganic materials. The light material is then sent through a

rotating screen (or trommel) that removes abrasive fine sand, glass, and grit.

Heavy materials from the classifier and trommel then move to a magnetic

separator that recovers ferrous material. Some facilities also attempt to recover

aluminum, glass, and mixed nonferrous metals. Refer to Figure 8.

WASTE

PRIMARY

SHREDDER

LIGHTS

AIR

CLASSIFIER

HEAVIES TO

MATERIALS

RECOVERY AND

LANDFILL

GRIT

Figure 8. Dry processing scheme for making RDF.

DENSIFIER

DENSIFIED

RDF

POWDERED

RDF

FLUFF

RDF

BALL

MILL

EMBRITTLING

AGENT

TROMMEL

SECONDARY

SHREDDER

The process illustrated in Fig. 8 shows the light organic material from the

trammel being sent to a secondary shredder that farther reduces the size of the

particulates. The intermediate product that is formed at this stage is referred to as

fluff RDF. The fluff RDF can be passed through as palletizing or briquetting

machine to produce densified RDF. Densification of the product improves

storage, handling, and stoker-furnace characteristics. As an alternative, the light

output from the trommel can be treated with a chemical embrittling agent and

then ground into a fine powder in a ball mill. This results in a material that is a

dust or powdered RDF. Typical particle sizes are around 0.15 mm.

Another process known as the wet RDF method makes use of a machine called a

hydropulper. A hydropulper is essentially a large blender. In this method raw

refuse is fed to the hydropulper where high-speed rotating cutters chop the waste

in a water slurry. Large items are removed while the remaining suspension is

pumped into a liquid cyclone separator which removes the smaller, heavier

materials. The water is drained off leaving a wet RDF which has between 20 and

50%

moisture content. This material can be burned alone or be employed as a

supplemental fael with coal depending upon the water content.

The wet method has several advantages over the dry method of RDF production.

First, sewage sludge can be mixed with the wet pulp prior to dewatering and the

resultant mixture can be burned as a method of codisposal. Second, this method

reduces the risk of explosion due to spontaneous combustion. In the dry process,

shredding is a stage of the operation that is prone to conditions for spontaneous

combustion depending on the fineness of the material being processed. Since this

step is eliminated in the wet method, the potential hazard is mitigated.

A third advantage is that some organic fiber can be recovered from the wet

process, and although the quality of the fiber is too poor to be used in the

manufacture of recycled paper products, it has found application as a reinforcing

material in roofing profiles.

A major disadvantage of the wet method is that it requires significantly higher

operating costs than the dry technology. This comes from the fact that the wet

product or pulp must be dried to an acceptable level for combustion. Usually

thermal methods are required beyond simple drainage.

BIOMASS-DERIVED RDF

Biomass-derived fuels have been highly successful in Europe. Finland as an

example has hundreds of plants which produce heat and power from various

types of biomass fuels. Because of the wide variety of fuels, the fael handling

systems are designed to be versatile. The raw feedstock for these systems is

typically bark, peat, wood chips, sludge, industrial wood, and various types of

combustible waste, including MSW.

A typical biomass handling system is illustrated in Fig. 9. The simplified scheme

provides an overall identification of the major operations involved.

Figure. 9. Schematic for biomass handling operation.

The objective of developing cost-effective and efficient fuel drying technology is

to increase the thermal efficiency of the RDF. Plants that generate heat from the

burning of biomass-derived RDF are referred to as biomass-fired energy plants.

ASH SILO SYSTEM

BOTTOM

ASH

FUEL SILO

SYSTEM

BOILER

FLY ASH

REMOVAL

SYSTEM

STORAGE

SILOS

FUELRECEIVING

STATION

MECHANICAL

CONVEYORS

CRUSHING AND

SCREENING

A conceptualized schematic for a process is illustrated in Fig. 10. These systems

are highly efficient and can be operated both in small-scale modular units and as

large-scale single units depending upon the application. There are both municipal

and industrial applications for these designs, although MSW has been the most

prevalent area of use.

Figure 10. Schematic of bed mixing dryer.

In order to burn biomass a wide range of combustion methods can be chosen on

the power-plant scale. In Europe the most commonly used methods are grate

firing and fluidized-bed combustion. From an economic and applicability point of

view, the combustion of biomass often takes place in multifiiel boilers, which

additionally use natural gas, oil, or coal. Biomass combustion is based on several

combustion methods, including pulverized fuel combustion, grate combustion,

fluidized-bed combustion, and gasification/gas combustion. Comments on each of

these technologies are given below.

Non-Condensing Gases

Hot Bed Material

Fuel + Bed

Material

Fuel

Tank

Dryer

Pulverized combustion is a rarely used alternative for wood residues. The plants

are almost always too small for feasible pulverized combustion. Pulverized

combustion is used in large peat and coal fired plants.

Grate combustion has been the most usual form of combustion in past, but is

seldom used any more in new large-scale boiler investments, because of higher

investment costs, greater emissions, and limited availability for multi-fuel use.

However, in smaller-capacity boilers, below 20 MW

, and for wet fuel, such as

bark residue in sawmills, grate firing methods offer competitive solutions with

minor fuel prehandling investments and low emissions. Improved grate-firing

makes it possible to use very moist fuels, such as sawdust. Sawmills and other

mechanical wood-processing plants produce wood fuel, which can be used to

generate heat for drying sawn timber. The approach is cost-effective if excess

heat can be sold to a district heating network or utilized for drying or as process

steam. Fuel conversion of old oil-fired heating boilers (of up to 1 MWJ

represents a considerable niche potential for biomass. Such a conversion can be

implemented by retrofitting the boiler with a fixed-bed gasifier or with a special

burner. Both technologies are commercial.

An alternative still in the laboratory stage is replacement of fuel oil by biomass-

derived pyrolysis oil. Production and utilization of pyrolysis oil is investigated

and technologies developed in many countries.

Small-scale grate boilers (<500 kWJ are usually over-fire or under-fire boilers

as illustrated by the diagrams in Fig. 11. In an over-fire boiler, fuel combustion

takes place in the whole fuel batch at the same time. It is normally equipped with

a primary air inlet, the grate and secondary air inlet over the fuel batch, in the

gas combustion zone. They are hand-fired using natural draft. Over-fire boilers

are usually connected to heat accumulators having volume 1 to 5 m

. The most

common fuel is chopped firewood. In under-fire boilers gasification and partial

combustion take place in only a small amount of fuel and final combustion in a

separate combustion chamber. Usual fuels in under-fire boilers are wood chips,

chopped wood, and sod peat.

Among latest innovations are down-draft boilers. Flue gases are forced to flow

down through holes in a ceramic grate. Secondary combustion air is introduced at

this point. After this fuel gases flow along ceramic tunnels where final

combustion takes place in a high-temperature environment. Traditional under-

feed stokers are used for combustion of sawdust, wood chips, and wood residues

from sawmills.

A special kind of horizontal stoker burner, illustrated in Fig. 12, is suitable for

the combustion of wood biomass has been on the market for about 20 years in

Europe. Most of the devices have been designed for output range 20 to 40 kW

and they are used for heating of detached houses and farms.

Note that there are many varieties of these systems marketed throughout the

world. They generally tend to be more popular and economical in Western

Europe, quite often because of their small scale. They have not penetrated the

Eastern European market which seems to be an ideal candidate for wide-scale

applications, but in general solid waste management and WTE technologies have

not been given serious attention in that part of the world. For the most part, the

small-scale technologies have never been popular in the United States.

Over-fire boiler

Under-fire boiler

Figure 11. Illustrates an over-fire and under-fire boiler for wood fuels.

STOKER BURNER WITH

FUEL STORAGE

BURNER

ASH

Figure 12. Stoker burner with preheating of combustion air for wood-

chip and biomass RDF burning.

Fluidized-bed combustion (FBC) technology has made it possible to significantly

increase the utilization of various biomasses and wastes in power and heat

generation. The pulp and paper industry served as a forerunner because of the

adequate biomass and waste fuel supply and energy demand on site. The range of

available biomasses and wastes includes wood-based fuels and residues, such as

bark, wood chips, and sawdust, sludge from paper mills and de-inking plants,

and other wastes, such as municipal sludges, paper waste, packaging waste, and

RDF.

New environmental regulations and taxation of fossil fuels have recently

further increased interest in the use of biomasses and wastes in energy

generation. In many cases disposal by burning is the only available method of

waste treatment. A promising way to fulfill these targets and to utilize energy is

to apply co-combustion or gasification of different fuels and wastes. Fluidized-

bed combustion has mainly been used in new plants and many grate-fired boilers

have been converted to fluidized-bed boilers. Early concepts were based on

bubbling fluidized beds (BFB). Since the beginning of the 1970s the development

of circulating fluidized-bed technology (CFB) has been very intensive and

successful.

BIOMASS

LCV GAS

GASIFIER

COAL

BOILER

BOTTOM

ASH

FLY ASH

Figure 13. Coutilization of gasified biomass, RDF, and coal.

Fluidized-bed technology has a commercially proven track record in its

superiority in the efficient, economic and environmentally sound combustion of a

large variety of fuels starting from very high-ash fuels, such as oil shale, and

very moist fuels, such as bark and sludges up to high-grade fuels, such as low-

sulfiir bituminous coals. BFB units are offered up to 100 MW

and CFB units up

to 400 to 600 MW

111

CFB boilers have proven feasibility to burn about 70 different fuels alone or in

cocombustion mode. BFB boilers have proven their feasibility for biomass and

waste fuels with similar characteristics especially in lower capacities, starting

from 5 MW

with well-preprocessed fuel. CFB gasification technology has been

developed for various biomasses since the 1970s. In gasification air and steam

are used to gasify the fuel. The product gas is burned in a boiler equipped with a

gas burner. Steam can be used to control the consistency of the ash. The moisture

content of the fuel may be up to 50%. The process is not suitable for pulverized

fuels.

Refer to Fig. 13 for an example of a process scheme utilizing an updraft

gasifier. Commercial applications ranging from 15 to 35 MW

were delivered

before the mid-1980s and applied in sulfate pulp mills for firing low-grade

product gas produced from various biomasses in lime-reburning kilns. Despite

this reliable operation, the economy of atmospheric gasification became poor in

the mid-1980s with the lowering price of oil. More advanced fuel drying and

gasification technologies combined with environmental regulations have recently

improved the feasibility of gasification.

PYROLYSIS

The technology of pyrolysis is a form of incineration that chemically decomposes

organic materials by heat in the absence of oxygen. Pyrolysis typically occurs

under pressure and at operating temperatures above 430

C (800

F). In practice,

it is not possible to achieve a completely oxygen-free atmosphere. Because some

oxygen is present in any pyrolysis system, a small amount of oxidation occurs. If

volatile or semivolatile materials are present in the waste, thermal desorption

takes place.

During pyrolysis organic matter is transformed into gases, small quantities of

liquid, and a solid residue containing carbon and ash. Off-gases are generally

treated in a secondary thermal oxidation unit. Particulate removal equipment such

as electrostatic precipitators is also employed.

There are several variations of pyrolysis systems, including the rotary kiln,

rotary hearth furnace, and fluidized-bed furnace. Unit designs are similar to

incinerators except that they operate at lower temperatures and with less air

supply.