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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 kilogram
1 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


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