Technological Background

Contents

General Need............................................................................................................2

Background...............................................................................................................2

Biomass and Residue Conversion Processes...............................................................3

Combustion...............................................................................................................5

Gasification...............................................................................................................7

Fluidized Bed Technology..........................................................................................9

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General NeedFossil fuels continue to be the indispensable mainstay of our energy supply, a situation unlikely to change in the near term. Forecasts based on anticipated population growth and the economic catching-up process in developing and threshold countries project that global demand for energy will have risen 40% to 60% by 2020. Given the growing scarcity of resources and unabated pollution, this is an extremely problematic development as fossil fuels provide 90% of the energy consumed worldwide.

One promising approach to defusing these two problems is the recovery of energy from biomass and residues originating in agriculture (culm products, husks, coconut shells, palm oil fruit cakes), forestry (timber), industry (plastics, composites) and municipalities (sewage sludge, residential waste). The recovery of energy simultaneously eliminates any landfilling or disposal of residual materials and thus such problems as groundwater contamination or the development of noxious gases.

The wide variety of sources is reflected in its varying material composition. Its high degree of heterogeneity and potentially high water content as well as the potential presence of varying contaminants raise the requirements on the technological imple-mentation of thermo-chemical conversion processes to generate power, heat and cold.

Given the distinctive characteristics of waste, biomass and residues, fluidized bed technology appears to be the best option to achieve the goal of maximizing the energy recovered while minimizing the emission of noxious gases. Fluidized bed technology has significant advantages over widespread conventional grate firing. Its optimal capacity range virtually predestines it for decentralized small and medium sized plants (stationary fluidized beds fired with wood and producing 1-20 MW) � a factor that particularly facilitates development in rural regions and encourages companies and plants to develop internal power supplies.

BackgroundFundamental research and industrial applications of the fluidiszed bed technology has a long tradition in Germany. Since its first invention by Winkler in 1922, the technology has evolved and has been applied in various fields of process engineering, starting from coal gasification, catalytic cracking of mineral oils or drying processes.

In recent years the fluidized bed technology is applied more and more in power plant engineering. State of the art applications are now able to utilise high varieties of fuels e.g. sewage sludge, different biomasses, wood, plastic or rubber residues and many oth-ers. The advancement in science and engineering in recent years, e.g. ORC process (Organic-Rankine-Cycle to turn small sources of thermal energy efficiently into electrical energy) lead to possibilities of highly efficient industrial applications (combined heat and power, ) which can be commercially applied even on small scales up to 10 MW.

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The Fraunhofer Institute for Factory Operation and Automation IFF in Magdeburg�s and its research partners have long been developing methods to recover energy from biomass and residues and implementing their findings in the engineering of distributed industrial-scale plants of small and medium capacity. Applications of fluidized bed technology are a primary focus of its work. With an eye on the competition and demand for energy resources and the resultant global economic and ecological impacts, the aim of the proposed program is to make the expertise on the fluidized bed technology available in SE Asia. Its significant advantages under specific application conditions in terms of scale, efficiency, possible fuels and emission avoidance makes it an suitable technology to support sustainable development on an economic scale.

Biomass and Residue Conversion ProcessesBiomass and residues of different provenances can be utilized as renewable energy sources in a variety of ways (see Fig. B1). Different conversion processes are employed depending on the product desired.

Figure B1: Energy sources, conversion processes and products

While extraction and distillation are chiefly employed to physiochemically convert fruits of oil plants (palm oil, rape) into fuels and bioproducts, fermentation is used to biochemically convert culmiferous biomass and animal excrement into heat, power and fuel. Neither process fully converts the source material into the desired product. A considerable share of the source material accumulates as converted or unconverted residue, often with high energy content (e.g. 67 mass % of rape fruit is processed into

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rapeseed cake). By contrast, thermochemical conversion processes nearly fully process the source material to produce heat, power and fuel.

Consequently, the two conversion processes of combustion and gasification are primarily utilized to recover energy thermochemically from biomass and residues. Figure B2 presents different methods of generating power and heat.

Figure B2: Methods for recovering power and heat from biomass and residues

Combustion fully converts a solid into hot flue gas. When transferred into a cycle, the thermal heat can power steam turbines and thus generators to generate electricity.

Gasification on the other hand converts fuel partially. As a result, the fuel gas produced contains a large proportion of volatiles, which downstream engines (gas turbines, gas engines, fuel cells) can ultimately convert into power, heat and cold.

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flue gas

dustfiring

grate firing

fluidized bed combustion

special process(cycloid, rotary kiln)

combustion

Biomass/Residues

indirect

combustion chamber

indirect

boiler

direct

Stirlingengine

IGCC process

ORC process

steam turbine

steam engine

generator

Electricity + Heat

gas turbine

gas engine

pilot injectionengine

gas treatment

direct

fuel cell

reforming

entrained flowgasification

fixed bed gasification

fluidized bed gasification

gasification

special process(slag tap gasifier)

fuel gasflue gasflue gas

dustfiring

grate firing

fluidized bed combustion

special process(cycloid, rotary kiln)

combustion

dustfiring

grate firing

fluidized bed combustion

special process(cycloid, rotary kiln)

combustion

Biomass/Residues

indirect

combustion chamber

indirect

combustion chamber

indirect

boiler

indirect

boiler

direct

Stirlingengine

direct

Stirlingengine

IGCC processIGCC process

ORC process

steam turbine

steam engine

generator

ORC process

steam turbine

steam engine

generator

Electricity + HeatElectricity + Heat

gas turbine

gas engine

pilot injectionengine

gas treatment

direct

gas turbine

gas engine

pilot injectionengine

gas treatment

direct

fuel cell

reforming

fuel cell

reforming

entrained flowgasification

fixed bed gasification

fluidized bed gasification

gasification

special process(slag tap gasifier)

entrained flowgasification

fixed bed gasification

fluidized bed gasification

gasification

special process(slag tap gasifier)

fuel gasfuel gas

CombustionFiring is the oldest and easiest way to recover energy from biomass and residues. In order to ensure combustion is complete and emissions are low as well as to allow for ash content and the composition, shape and particle size of solids, different types of firing have been developed for plants of a wide variety of sizes (see Figure B3). The basic difference between them is the type of solids processing and charging and the type of combustion chamber. In addition, solids and plant parameters influence the way gas is utilized to generate power and heat.

Figure B3: Combustion process chain

Figure B4 depicts the combustion process in a cogeneration plant with fluidized bed firing and an ORC module together with the dominant material and gas flows.1

Figure B4: The combustion process

1 The Organic Rankine Cycle employs organic oil instead of water, thus making it possible to increase the efficiency of small and medium-sized power plants from 11% to 16%. It is typically an element f a CHP.

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Air

Biomass

Ash

Additives

Fly Ash

Thermo Oil Sorbent

Flue Gas

Fluidized bedCombustion Module

Boiler Module Flue GasCleaning Module

ORC Module

Heat(80/60°C)

Power

Air

Biomass

AshAir

Biomass

Ash

Additives

Fly Ash

Additives

Fly Ash

Thermo Oil Sorbent

Flue Gas

Fluidized bedCombustion Module

Boiler Module Flue GasCleaning Module

ORC Module

Heat(80/60°C)

Power

The Fraunhofer IFF applied this principle to design and engineer a distributed biomass combustion plant with a thermal firing capacity of 3.7 MW that generates power (500 kWel) and heat (2.4 MWth). It also provided support during the official approval process and coordinated the construction work. At present, the Fraunhofer IFF is commissioning the plant before handing it over to the operator. The successful initial operation and the hand over of the turn key plant to the customer were accomplished by the end of 2007.

Figure B5: A biomass combustion plant in plan and reality (during construction)

The Fraunhofer IFF developed and integrated technical innovations � fuel charger, air baffle, compact fluidized bed plant, cyclone and ash discharge system � for this particular plant.

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GasificationJust as in a combustion plant, the solids parameters (ash content, composition, shape, particle size) and plant parameters (logistic, output, distributed/centralized) as well as the objective of maximizing the yield of solids while minimizing pollution provide the basis for selecting the individual components of a gasification plant.

Figure B6: Gasification process chain

The main advantages of gasification combined with combustion engines are:• Higher electric efficiency than conventional steam power processes,

• Lower investments in a small performance range than steam power processes,

• Lower operating costs in small performance range than steam power processes,

• More cost effective fuel gas treatment than in pure combustion plants because of smaller gas volumetric flows and

• Higher degree of biomass utilization than fermentation plants.

Chiefly small and medium-sized distributed gasification plants convert biomass into fuel gas to ultimately produce power and heat. Such plants are predestined for integration in the wood processing industry or operation in agricultural, waste disposal and municipal sectors. Gasification plants have a rated thermal output of 1 to 5 MW. Classic energy conversion processes and newer developments are instrumental in the uncoupling of electrical energy and decentralized energy supplies. As fuel gas is being optimized, secondary treatment processes are also being developed. Energy conversion in gas engines requires corresponding frontend gas treatment. The fuel gas generated can also be used in small gas turbines to convert energy. Furthermore, it is possible to use the gas in a combustion chamber with a downstream waste heat boiler to generate steam and thus operate small turbines or steam engines (e.g. Spilling engines). The latter concept has the advantage of a lower volumetric flow of fuel gas, which can be treated in a compact system preceding the combustion chamber. The fuel gas can be subsequently fed to a motor. Figure B7 depicts a simplified gasification process together with the material and heat flows.

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Figure B7: The gasification process

The Fraunhofer IFF applied this principle to design and engineer a distributed biomass gasification plant with a thermal firing capacity of 1 MW that generates power and heat. It also provided support during the official approval process and coordinated the construction work. At present, the Fraunhofer IFF is commissioning the plant before handing it over to the operator.

Figure B8: A biomass gasification plant in plan and reality (during construction)

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Fluidized Bed TechnologyGrate firing and solid bed gasification systems are used for mechanically transported resting solids. Fluidized bed combustion and gasification systems employ moving, i.e. fluidized solids. In dust firing or entrained-flow gasification, the gas velocity in the system is greater than the solid particles� sedimentation velocity (see Table C1).

Reactor TypePacked Bed Fluidized Bed Flue Dust

Above bed flow

Through-bed flow

Stationary Circulating

Typical reactor elements

Muffle, multiple hearth roaster, rotary kiln, belt drier

Vertical kiln, Moving grate

Fluidized bed roaster, multiple fluidized bed

Circulating fluidized bed, Venturi fluidized bed

Flash drier, melting cyclone, burner

Solids motion Mechanical Gravity Mechanical

GravityGas flow

GravityGas flow

Particle size Small � very large

Medium � very large

Medium Very small � small

Very small

Solids residence time

Hours � days Hours Minutes Seconds - millisec.

Gas residence time

Seconds Seconds Milliseconds

Heat & mass transfers

Very low Low � average High Very high Very high

Temperature control

Average � good

Poor � average Good Very good Average � good

Space-time efficiency

Very low � average

Average Average � high

High Very high

Suitable solids Fine � coarse material, even with high ash content

Pulverized solids,extremely variable moisture, ash content and calorific value

Dust, shavings

Table C1: Reactor types categorized according to motion state of the solid [Dreyh]

Their easy availability and advanced development have made combustion grates a widespread technology in plants fired with coal, waste and biomass. However, grate firing has certain disadvantages: moving mechanical parts in high temperature combustion chambers, the relatively irregular loading of the grate and the resultant

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uneven airflow above the grate cross section. This results in relatively uneven combustion and the appearance of layers of uncombusted carbon monoxide and hydrocarbons. Fluidized bed technology has significant advantages over grate firing:

• A wide range of fuel types and lumpiness can be utilized,• Fuel moisture content may vary widely,• In-situ and secondary optimizations directly benefit the process.• Processes can be actively optimized,• Process parameters can be adjusted quickly• Conditions for mass and heat transfer are typically excellent,• Solids are intensively mixed radially and axially and• Solids and temperature are uniformly distributed.

Given the distinctive characteristics of biomass and residues, fluidized bed technology appears to be the best option to achieve the goal of maximizing the energy recovered while minimizing the emission of noxious gases.

Fritz Winkler invented the fluidized bed in 1921 to convert lignite into fuel gas (coal gasification) [Wink22]. The stationary fluidized bed reactor operating at atmospheric pressure was followed by developments leading to reactors with circulating atmospheric and stationary pressure-charged fluidized beds (see Table C1 and C2).

Stationary atmospheric FBF

Circulating atmospheric FBF

Stationary pressure-charged FBF

Velocity 1 to 3 m/s 5 to 8 m/s 1 to 2.5 m/s

Inert material grading 2 to 3 mm 0.1 to 0.3 mm 2 to 3 mm

Cross-sectional loading 1 to 2 MW/m² 4 to 12 MW/m² 1 to 20 MW/m²

Part load performance 100% to 50% 100% to 30% 100% to 30%

Combustion efficiency 90% to 95% 95% to 99% 90% to 99%

NOx emissions 300 to 600 mg/m³ 50 to 300 mg/m³ 200 to 400 mg/m³

Applications Industry, cogene-ration plants (CHP)

Industry, cogene-ration plants (CHP)

(heat and) power plant

Power range � coal 0.5 to 200 MW 30 to 500 MW up to 1000 MW

Power range � wood 1 to 20 MW 10 to 100 MW �

Table C2: Technical characteristics of fluidized bed firing [Dreyh]

Different developments of their hydrodynamic, thermodynamic and energy characteristics place special demands on the solids being converted and require varying optimal plant sizes.

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Its capacity range (1-20 MW for stationary fluidized bed firing) virtually predestines fluidized bed technology for distributed small and medium-sized plants � a factor that particularly facilitates development in rural regions and encourages companies and plants to develop internal power supplies.

Apart from its original use in thermochemical plants, fluidized bed technology is also utilized in many other domains of processing. Applications range from drying and coating with any fluidizable material up through the production of pills in the pharmaceutical industry.

Literature

Winkl: Winkler, F.: Verfahren zur Herstellung von Wassergas.

Patern number: 437970, 1922

Dreyh: Dreyhaupt, Franz-Joseph: VDI-Lexikon Umwelttechnik, Duesseldorf, VDI-Verlag,

1994

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