11. Types of Biomass Furnaces [14].

Fuel feeding method	Combustion Technique	Size range	Fuel used (typical size)	Ash content	Moisture content
Manual (batch-fired)	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	understoker furnaces	20 kW-2.5 MW	wood chips, wood residues	<2%	(5-50)%
	moving grate furnaces	150 kW-15 MW	all wood fuels and 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 burners	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 fluidized bed	5 MW-15 MW	various biomass, d<10mm	<50%	(5-60)%
	circulating fluidized bed	15 MW-100 MW	various biomass, d<10mm	<50%	(5-60)%
	dust combustor, entrained flow	5 MW-10 MW	various biomass, d<5mm	<5%	20%
Co-firing (biomass covers less than 10% of the total fuel input)	stationary fluidized bed	50 MW-150 MW	various biomass, d<10mm	<50%	(5-60)%
	circulating fluidized bed	100 MW-300 MW	various biomass, d<10mm	<50%	(5-60)%
	cigar burners	5 MW-20 MW	straw bales	<5%	20%
	dust combustor in coal boilers	100 MW-1GW	various biomass, d<(2-5)mm	<5%	20%

The biomass furnaces used for domestic application may be classified by used fuel types according to Fig. 5.

figure 5. Classification of small combustion biomass appliances.

The combustion technologies may be classified by flow conditions in: fixed-bed, fluidized-bed and entrained flow or dust combustion (Fig. 6).

as biomass combustion in small appliances occurs with higher unburned emissions and lower efficiency the downdraft combustion technology has been developed (Fig. 7).

The batch fired boilers should be coupled with heat storage tank to avoid boiler running at low load due to unfavourable combustion conditions that lead to high emissions of unburned pollutant.

Boilers using wood pellets have automatic fuel feeding and good combustion quality due to controlled quality of pellets.

Fixed bed Fluidised bed pulverised combustion

Figure 6. Biomass combustion technologies (fixed bed, fluidized bed, dust combustion) [2].

Figure 7. Downdraft combustion technology of biomass.

Wood chips and similar fuels with low ash content are used in under stoker furnaces (Fig. 8).

Because biomass combustion has high particulates emissions a new combustion technology is developing (Fig. 9). The particulates from complete biomass combustion are formed from ash components in biomass with salts, especially with Kcl. During the cooling of flue gas inside the wood combustion boiler the aerosol s are formed by nucleation, coagulation and condensation. The aerosols are mainly salt of potassium. This salt can be oxidized at high temperature if oxygen is present. Potassium oxides have low devolatilization temperature and they are vaporized into the flue gas. The new technology called „low particle” uses the following measures [14]:

• reduced temperature in the glow bed to reduces evaporation of ash constituents and formation of salts (KCl) in the flue gas;

• reduced gas velocity through the fuel bed to reduce entrainment of fuel particles that enter the hot section in the flame, where released vapours from ash constituents remain in the flue gas;

• reduced oxygen content in the fuel bed to reduce the tendency of oxides formation from ash constituents;

• high fuel bed to act as a filter for initially released particles.

Figure 8. Understoker combustion technology of biomass.

Figure 9. “Low particle” combustion technology [14].

Solid biomass fuels with high ash and water content are used in grate furnaces (Fig. 10).

Figure 10. Grate combustion technology.

Straws are used in conventional grate furnaces (Fig. 11) or special furnaces that operate with whole bales such as cigar burners (Fig. 12).

Figure 11. Straw bale furnace.

F igure 12. Cigar burner [Source: Babcock & Wilcox Vølund A/S].

Wood pellets are used in residential heating boilers equipped with pellets storage tank and automatic pellets feeding system (Fig. 13).

Figure 13. Wood pellets boiler.

Waste wood and wastes from pulp and paper industry can be used in large boilers with stationary, bubbling (Fig. 14) and/or circulating fluidized bed (Fig. 15). Boilers with circulating fluidized bed offer good conditions for complete combustion and relatively low NO_x emissions.

Figure 14. Bubbling fluidized bed [15].

Figure 15. Circulating fluidized bed [16].

Biomass gasification and pyrolysis

Different useful products can be obtained by biomass gasification and pyrolysis. These products can be converted in fuels and chemicals (Fig. 16).

Gasification is the thermochemical conversion process of a solid or liquid carbon-based material at high temperature into combustible gas by using a gasification agent (steam, oxygen, air). The combustible gas contains CO₂, CO, H₂, CH₄, H₂O, and trace amounts of higher hydrocarbons, inert gases present in the gasification agent, various contaminants such as small char particles, ash and tars [17].

when an oxidant gasification agent is used for partial oxidation of feedstock to keep the high temperature of process, the gasification is called direct gasification (Fig. 17). If no oxidizing agent is used, but external energy source is used, the process is called indirect gasification. The most used indirect gasification agent is steam because it is easily produced and increases the hydrogen content in the combustible gas [17].

The entire gasification process includes drying up to 120°C, devolatilisation up to 700°C and gasification above 800°C.

Pyrolysis is the indirect gasification process that uses inert gases as gasification agent.

Figure 16. Gasification and pyrolysis processes and their products [17].

The products of direct gasification are combustible gas and ash and the products of indirect gasification and pyrolysis are combustible gas, tars and oils, char and inert materials present in feedstock.

Figure 17. Gasification and pyrolysis processes.

The liquid fraction (tars and oils) may be upgraded to refined fuels. The combustible gas has high content of hydrocarbons with high calorific values. The produced char can be used as a fuel [18].

The gasification rate depends on the following parameters: feedstock composition, size, shape and structure; gasification environment; flow of gasification environment; heating rate and temperature [18].

When the feedstock has smaller size the heat transfer is better and the temperature is uniform in the gasification environment.

The biomass feedstock should be pelletised or cut into small chips with size less than 2.5cm in order to obtain uniform rate of gasification and uniform composition of producer gas.

The biomass with high porosity allows facile diffusion of reactant leading to uniform temperature and uniform composition of generated gases. The low biomass porosity leads to a non-uniform temperature and composition of gases.

When reactive environment (air, oxygen) is used complete gasification occurs and inert environment favours devolatilization (pyrolysis) phase with more char production.

The char produced by biomass pyrolysis may undergo secondary degradation leading to smaller amount of char than the fixed carbon present in biomass. To avoid this degradation the devolatilized gases should be seep away by inert gases (nitrogen)

Fast pyrolysis (heating rates up to 1000°C) at temperature below 650°C and with suddenly cooling of liquid allows intermediate products of pyrolysis condenses without further breaking down higher molecular weight species into gaseous products and leads to the minimized char production. If the desired end product is combustible gas the fast heating rate at high temperature is preferred and if liquid hydrocarbon is desired, the fast pyrolysis is preferred. Slow pyrolysis with low maximum temperature (<500°C) is preferred to produce char.

There are two temperature levels used in gasification and pyrolysis processes. Temperatures lower than 500°C are used for pyrolysis to obtain maximum amount of char with inert medium flowing. For gasification the temperatures above 500°C with air flowing are used. Those higher than 500°C are used for reduction of carbon dioxide by carbon to carbon monoxide.

Mineral matter in ash can catalyze or inhibit thermal degradation of biomass. Metal oxides presented in biomass have catalytic effect on gasification.

The main reactions involved in gasification are [21]:

1. Water–gas reaction (partial oxidation of carbon by steam)

- 131.38kJ/mol carbon

2. Boudouard reaction

- 172.58 kJ/mol carbon

3. Water – gas shift conversion

- 41.98 kJ/mol carbon

4. Methanation

+ 74.90 kJ/mol carbon

As composition of the gas generated by gasification depends on many parameters, it is difficult to predict it. Assumption of chemical equilibrium of the gas components offers good values to start research.

Heat capacity of combustible gas depends on the used gasification agent (Table X). It has lower values when nitrogen is present in gasification agent.