To burn 800 lbs of biomass per hour we need 80cf * 800# / 60minutes=1066 cfm flue gas fan. 
To generate 3 million btu requires approximately 500 lbs per hour fuel input - 80cf * 500# / 60minutes=666 cfm
To generate 6 million btu requires approximately 1,000 lbs per hour fuel input - 80cf * 1000# / 60minutes=1,333 cfm
The text recommends 50% additional air to ensure complete combustion.

http://www.engineeringtoolbox.com/9_170.html

Fuels and Values of Combustion Air and Flue Gases

Combustion air and flue gases for several fuels as coke, oil, wood, natural gas ..


Fuel

Theoretical air for combustion
Volume at standard pressure

Theoretical flue gas
Volume at standard pressure

m3/kg

ft3/lb

m3/kg

ft3/lb

Anthracite

9,4

150

9,5

152

Bituminous coal

6,9

110

7

112

Charcoal

8,4

134

8,4

135

Coke

8,4

134

8

135

Lignite

5,7

92

8,4

93

Peat

5,7

92

5,9

94

Semi anthracite

8,4

135

8,6

137

Wood (dry)

4,4

70

5

80
         
 

m3 air/
m3 fuel

ft3 air/
ft3 fuel

m3 air/
m3 fuel

ft3 air/
ft3 fuel

Butan C4H10

31

31

27

27

Natural gas

9,5

9,5

8,5

8,5

Propan C3H8

24

24

22

22

Town gas

4

4

3,8

3,8
         
 

m3 air/
litre fuel

ft3 air/
gal fuel

m3 air/
litre fuel

ft3 air/
gal fuel

Gas oil

9,8

1570

10,4

1670

Heavy fuel oil

10,8

1730

11,6

1860