Grate burners (Fig. 5.15) achieve a higher combustion velocity and efficiency, with respect to pile burners, because the solid fuel is evenly spread (not piled) on a grate by a dedicated device called a stoker, and this allows a better mixing between air and fuel. Moreover the fuel moves across the grate, from the inlet section to the ash discharge section, mechanically driven by the movement of the grate or by gravity; this increases the mixing of fuel and air and it facilitates the disruption of the solid char, which then burns more quickly. Moreover the movement of the fuel inside the combustion chamber allows a certain degree of differentiation in the air supply to different areas of the combustion chamber, which can then be adjusted according to the typical heating-drying-pyrolysis-oxidation sequence.

A scheme of a modern grate-fired boiler is composed of: a fuel feeding system, a grate assembly, a secondary air (including over-fire air or OFA) system and an ash discharge system (Yin etal., 2008).

Typical fuel feeding and distribution systems are represented by mechanical stokers and spreader stokers which continuously propel fuel into the combustion chamber and above the grate. The smallest particles burn in suspension (if the fuel is very fine, i. e. 30% or higher then mass fraction has dimensions smaller than a few millimeters) while bigger pieces fall into the grate and form the fuel bed. The fuel feeding system regulates the amount of fuel which is fed to the furnace while the distributor evenly spreads the fuel on the grate. Typical feeding systems are positioned under a hopper and are either a conveyor belt, a piston, a screw conveyor or a rotary vane feeder, whose speed may be varied to regulate the fuel flow (dosing system). The rotary vane system provides also a physical isolation of the hopper from the combustion chamber preventing backfire and may also be used in combination with the other systems. From the feeding systems

the solid fuel reaches the distributors which can be either mechanical or pneumatic (Fig. 5.16). In the first case a revolving device throws the fuel into the furnace while in the second case a pulsating high pressure air flow blows the fuel inside the combustion chamber and on the grate.

Capacities of grate-fired boilers range from 4 to 300 MWt and are mainly grouped around 20­50 MWt in biomass fired CHP plants. The heat release per grate area can be around 4 MWt/m2 (U. S. Environmental Protection Agency, 2007). The grate that is at the bottom of the combustion chamber has two main functions: to guarantee a homogeneous distribution of the fuel and of the bed of embers over the whole grate furnace, and equal distribution of air entering from beneath over the whole grate areas.

As far as the primary and secondary air supply systems are concerned, they play a very important role in the efficient and complete combustion of biomass. For grate-firing the overall excess air is about 25-50% (Nor’West-Pacific Corporation, 1981) and it can be considered that excess air equals the moisture content of the fuel. The ratio between primary air and secondary air tends to be 40/60. Primary air supply must be divided into sections in order to operate the grate furnaces at partial loads (down to about 25% of the nominal furnace load) and control the primary air ratio needed (to secure a reducing atmosphere in the primary combustion chamber necessary for low NOx operation).

The fuel bed is composed of solid particles that are piled up with a characteristic porosity and it is heated by over-bed flames and refractory furnace walls until it ignites on the top surface of the fuel bed. The accepted combustion mechanism of cross current units considers that after ignition a reaction front propagates from the surface of the bed to the grate against the direction of primary air even if this traditional pathway may not be observed, depending on fuel properties and operating conditions (Saastamoinen et al., 2000; Thunman and Leckner, 2001; Zhou et al., 2005; Yang et al., 2004). A reaction front propagating from the grate upwards to the surface of the bed was also reported. The reaction front then moves downward against the primary air flux and a char and has layer remains in the surface. When the reaction reaches the surface of the grate the availability of oxygen increases and char can be oxidized as well; glowing combustion takes the place of flaming combustion.

Inhomogeneous air supply may cause slagging and higher fly-ash amounts and may increase the excess oxygen needed for a complete combustion, resulting in boiler heat losses. To avoid

BIOMASS , FEED

AIR

DISTRIBUTOR

ROTATING DAMPER
FOR PULSATING
AIRFLOW

Figure 5.16. Working principle of a mechanical (a) and pneumatic (b) spreader-stoker.

this problem continuously moving grates, a height control system of the bed of embers (e. g. by infrared beams) and frequency-controlled primary air fans for the various grate sections can be used. Gases released by biomass conversion in the grate and a small amount of entrained fuel particle continue to burn over the bed and secondary air plays an important role in mixing, burnout and emissions.

An advanced secondary air supply system is one of the most important elements in the opti­mization of the gas combustion in the freeboard and it is an important retro-fit to improve burnout in old grate-fired boilers and reduce pollutant emissions.

Grates can be water-cooled (more indicated for dry biomass fuels with low ash-sintering temperatures) or air-cooled (more indicated for wet bark, saw dust and wood chips). Grate burners may be divided in two main categories (see Table 5.7) (Yin et al., 2008; Van Loo and Kopperjan, 2002):

• stationary grates, where the grate is fixed with respect to the combustion chamber and fuel motion is driven by gravity; they are also called sloping grates;

• moving grates, where the grate moves with respect to the combustion chamber and the mechan­ical motion induced (vibration, alternate or translational) moves the solid fuel. Depending on the motion they are called vibrating grates, reciprocating grates and traveling grates.

In stationary sloping grates fuel is introduced on the top of the slope with mechanical or pneumatic distributors and it tumbles down the slope, which angle may be constant or varying according to the different velocity of combustion required at different stages. The grates consists

of steel cast alloy blocks, which may be water-cooled, and evenly spaced to form either a flat surface or a staircase. Water cooling may be used to increase the life length of the grate and also to reduce the local temperature where ash is formed to avoid ash melting and slagging problems.

Primary combustion air is provided in excess with respect to stoichiometric conditions through pin holes in the grate blocks and may represents up to 75% of the overall combustion air. Alternative solutions for air supply consider integral fins where holes are protected by deflector plates welded to the fins to prevent fines from plugging the holes and to direct the air down the slope and help the advancement of ashes towards the ash pit. Stationary grates (Fig. 5.17) require periodical manual cleaning of the grate from ashes, since the slope and air jets are not sufficient to move all the ashes to the pit, however, much less than in pile burners. The remaining 25% of combustion air is provided as secondary over-fire air from ports in the brick wall.

Moving grates have a different configuration according to the different mechanical principle that moves the bars of the grate.

In vibrating grates a rapid oscillatory motion of the elements facilitates the movement of the solids throughout the combustion chamber and the sliding across the grate which is usually a slope with a fixed angle. The compacting action of the vibrations allows a wide range of fuels to be burnt with a maximum thermal load around 2500 kW/m2.

In reciprocating grates it is the alternate motion of the moving segments, either forward- backwards on sloping grates or an angular tilting on horizontal grates, that pushes solids throughout the grate and also the ashes to the ash pit. The moving bars are positioned between fixed ones and are attached to a frame which is activated by a hydraulic system slowly and inter­mittently. The frequency and width of the strokes allow adjustment of the fuel layer to optimize combustion and emissions. Under-fire primary air is provided through the gaps between movable and fixed bars.

In a traveling grate the elements (Fig. 5.18) are linked together to form a chain like structure which is continuously moved in one direction around running wheels of which one is motive and the other is idle. It enters one side of the combustion chamber, where it receives the fuels, it runs inside the combustion chamber carrying the fuel bed until it reaches the far end where ashes are

Figure 5.19.

discarded and eventually it exits the chamber to renter from the other side, after traveling the whole distance below and outside the combustion chamber. A proper grate tension is essential to guarantee the proper functioning, among the different solutions a movable shaft of the idle wheel and the free hanging of the return side of the grate to form a catenary are used.