The burning of wood and other solid biomass is the oldest energy technology used by man. Combustion is a well-established commercial technology with applications in most industrialised and developing countries, and development is concentrated on resolving environmental problems, improving the overall performance with multi-fuel operation and increasing the efficiency of the power and -where ever possible- the heat cycles.
Devices overview

The devices used for direct combustion of solid biomass fuels range from small domestic stoves (1 to 10 kW) to the largest boilers used in power and CHP plants (>5 MW). Intermediate devices cover small boilers (10 to 50 kW) used in single family houses heating, medium-sized boilers (50 to 150 kW) used for multi-family house or building heating and large boilers (150 to over 1 MW) used for district heating. Co-firing in fossil fired power stations enables the advantages of large size plants (>100 MWe) that are not applicable for dedicated biomass combustion due to limited local biomass availability.

Most frequently used furnaces for biomass combustion

Application

Type

Typical size range

Fuels

Ash

Water content

Manual

Wood stoves

2 kW – 10 kW

dry wood logs

<2%

5%-20%

Log wood boilers

5 kW – 50 kW

log wood, sticky wood residues

<2%

5%-30%

Pellets

Pellet stoves and boilers

2 kW – 25 kW

wood pellets

<2%

8%-10%

Automatic

Unterstoker furnaces

20 kW – 2.5 MW

wood chips, wood residues

<2%

5%-50%

Moving grate furnaces

150 kW – 15 MW

all wood fuels, most biomass

<50%

5%-60%

Pre oven with grate

20 kW – 1.5 MW

dry wood (residues)

<5%

5%-35%

Understoker with rotating grate

2 MW – 5 MW

wood chips, high water content

<50%

40%-65%

Cigar burner

3 MW – 5 MW

straw bales

<5%

20%

Whole bale furnaces

3 MW – 5 MW

whole bales

<5%

20%

Straw furnaces

100 kW – 5 MW

straw bales with bale cutter

<5%

20%

Stationary fluidised bed

5 MW – 15 MW

various biomass, d < 10 mm

<50%

5%-60%

Circulating fluidised bed

15 MW – 100 MW

various biomass, d < 10 mm

<50%

5%-60%

Dust combustor, entrained flow

5 MW – 10 MW

various biomass, d < 5 mm

<5%

<20%

Co-firing*

Stationary fluidised bed

total 50 MW – 150 MW

various biomass, d < 10 mm

<50%

5%-60%

Circulating fluidised bed

total 100 – 300 MW

various biomass, d < 10 mm

<50%

5%-60%

Cigar burner Dust

straw 5 MW – 20 MW

straw bales

<5%

20%

combustor in coal boilers

total 100 MW – 1 GW

various biomass, d < 2 – 5 mm

<5%

<20%

*: biomass covers typically less than 10% of the fuel input
Source: Nussbaumer, 2002

To achieve complete burnout and high efficiencies in small-scale combustion, downdraft boilers with inverse flow have been introduced which apply the two-stage combustion principle. An operation at very low load should be avoided as it can lead to high emissions. Hence it is recommended to couple log wood boilers to a heat storage tank. Since wood pellets are well suited for automatic heating at small heat outputs, as needed for nowadays buildings, pellet furnaces are an interesting application with growing propagation. Thanks to the well-defined fuel at low water content, pellet furnaces can easily achieve high combustion quality. They are applied both as stoves and as boilers.

Understoker furnaces are mostly used for wood chips and similar fuel with relatively low ash content, while grate furnaces can also be applied for high ash and water content. Special types of furnaces have been developed for straw that has very low density and is usually stored in bales. Beside conventional grate furnaces operated with whole bales, cigar burners and other specific furnaces are in operation. Stationary or bubbling fluidised bed (SFB) as well as circulating fluidised bed (CFB) boilers are applied for large-scale applications and often used for waste wood or mixtures of wood and industrial wastes e.g. from the pulp and paper industry. In fluidised bed boilers, nearly homogeneous conditions of temperature and concentrations can be ascertained thus enabling high burnout quality at low excess air. The choice of different bed materials in CFB offers additional opportunities of catalytic effects. Further, the option of heat removal from the bed allows controlling the combustion temperature and hence enables an operation at low excess air without excessive ash sintering. Since similar conditions for nitrogen conversion as by air and fuel staging are attained, relatively low NOX emissions are achieved.