Appendix 2 - Design calculation of downdraught gasifier

Table of conversion factors and symbols (used in this manual)

The gasifier will be designed for operation in conjunction with the engine from Annex I.

A. Biomass consumption gasifier

The thermal efficiency of the gasifier is taken at 70 per cent

Thermal power consumption (full load): Pg/0.7 = 116.6 kW

Heating value of biomass (14% moisture content): 17000 kJ/kg

Biomass consumption gasifier: 116.6/17000 = 0.0069 kg/s = 24.7 kg/in

So the installation under consideration uses 24.7/18.3 = 1.35 kg biomass to produce 1 kWh electricity.

B. Reactor design

For a double throat gasifier (see Chapter IV) holds:  S = surface area "throat" = 68 cm² = 1/4 x (dthroat)² x dthroat = 9.3

The turn down ratio of this type of gasifier can be estimated at a factor 3 (see Chapter 3).

So at loads below 6.1 kVA, tar production of this installation can be expected.

C. Further gasifies dimensions

Once the throat diameter has been fixed, further important gasifier dimensions can be derived from figures 3.3. and 3.4 (Chapter 3).

1. Height h of the nozzle plane above the smallest cross-section of the throat (dthroat = dt).

h/dt = 1.15
h = 1.15 x 9.3 = 10.7 cm

2. Diameter (dr) of the fire box.

dr/dt = 3.0
dr = 3.1 x 9.3 = 29 cm

3. Diameter (drl) of nozzle top ring

drl/dt = 2.3
drl = 2.3 x 9.3 = 21.4 cm

4. Nozzle diameter (dn)

Assumption: gasifier to be equipped with 5 nozzles

Total nozzle area (An): 5 x 1/4 x p x dn² dn = 10.4 mm

Table of conversion factors and symbols (used in this manual)

 Energy Units J = Joule kJ = 1000 J GJ = 1000000000 J kWh = 3600000J TWh = 3.6 x 1015 J

 Power Units W = Watt kW = 1000 W MW = 1000000 W HP = Horse Power = 745.7 W kVA = kilo Volt-Ampere

 Force Units N = Newton kgf = force kilogram1 kgf = 9.8067 N t = ton = 1000 kgf

 Pressure Units Pa = Pascal = N/m² kgf/cm² = 98067 Pa mm Wg = 9.8067 Pa cm Wg = 98.067 Pa atm =1.0133 x 105 Pa

 Length Units m = meter cm = 0.1 m mm = 0.001 m km = 1000 m m m = 0.000.001 m

 Surface Units m²= square meter cm = 0.01 m

 Mass Units kg = kilogram mg = 0.000.001 kg g = 0.001 kg

 Time Units s = second min = minute = 60 s h = hour = 3600 s yr = year

 Volume Units m³ = cubic meter dm³ = 0.001 m³ mm³ = 0.000000001 m³ 1 = litre = 1 dm³

 Temperature Units K = Kelvin °C = degree Celsius 1° C = 1 K

 Miscellaneous Nm³ = normal cubic metre (at 1 atm and 0°C) mol = quantity of matter unit gram atom = atomic weight expressed in grams gram molecule = molecular weight expressed in grams ppm = parts per million rpm = revolutions per minute ° = angular degrees person-km = one person transported one km ton-km = one ton transported one km man-yr = one man working one year

 Other Units Hz = Hertz (frequency) V = Volt

 Currencies US\$ = U.S.A. dollars G = Guaranies Hfl = Dutch florin