A moving-bed gasifier may be designed on the basis of characteristic design parameters such as specific grate gasification rate, hearth load, and space velocity.

Specific grate gasification rate is the mass of fuel gasified per unit of cross­section area in unit time. The hearth load of a gasifier may be expressed in terms of the fuel gasified, the volume of gas that is produced, or the energy throughput.

2 4 Mass of fuel gasified Hearth load (kg/s • m2) =

Hearth cross-sectional area

TT, , W„T 3 Volumetric gas production rate

Hearth load (Nm3/s • m2) =

Hearth cross-sectional area

or

Heart, load (MW/m-)= Energy throughpu’ In product gas (6 28)

Hearth cross-sectional area

The hearth load in volume flow rate of gas per unit of cross-section area is also known as superficial gas velocity or space velocity, as it has the unit of velocity (at reference temperature and pressure).

The following section discusses type-specific design considerations.

Updraft Gasifier

Updraft gasifiers are one of the simplest and most common types of gasifier for biomass. The maximum temperature increases when the feed of air or oxygen increases. Thus, the amount of oxygen feed for the combustion reaction is carefully controlled such that the temperature of the combustion zone does not reach the slagging temperature of the ash, causing operational problems. The gasification temperature may be controlled by mixing steam and/or flue gas with the gasification medium.

The hearth load of an updraft gasifier is generally limited to 2.8 MW/m2 or 150 kg/m2/h for biomass (Overend, 2004). For coal it might be higher. In an oxygen-based coal gasifier, for example, the hearth load of a moving bed can be greater than 10 MW/m2. A higher hearth load increases the space velocity of gas through the hearth, fluidizing finer particles in the bed. Probstein and Hicks (2006) quote space velocities for coal on the order of 0.5 m/h for steam — air gasification and 5.0 m/h for steam-oxygen gasification. Excessive heat generation in such a tightly designed gasifier may cause slagging. Based on the characteristics of some commercial updraft coal gasifiers, Rao et al. (2004) suggest a specific grate gasification rate as 100 to 200 kg fuel/m2h for RDF pellets, with the gas-to-fuel ratio in the range 2.5 to 3.0. Carlos (2005) obtained a rate of 745 to 916 kg/m2h with air-steam and air preheat at temperatures of 350 and 830 °C, respectively.

For an updraft gasifier, the height of the moving bed is generally greater than its diameter. Usually, the height-to-diameter ratio is more than 3 : 1 (Chakraverty et al., 2003). If the diameter of a moving bed is too large, there may be a material flow problem, so it should be limited to 3 to 4 m in diameter (Overend, 2004).