5. Interferences
5.1 This procedure produces biomass samples with a particle size range. The NREL
procedures for biomass compositional analysis have been optimized for samples with
this particle size range. Deviation to a smaller particle size may result in a low bias in
carbohydrate content (and consequent high lignin bias) due to excessive carbohydrate
degradation. Deviation to a larger particle size may also result in a low bias in
carbohydrate content (and consequent high lignin bias) due to incomplete hydrolysis of
polymeric sugars to monomeric sugars.
5.2 This procedure produces samples with moisture contents below 10%. The NREL
procedures for biomass compositional analysis have been optimized for samples with
low moisture contents. Higher moisture content in biomass samples will alter the
effective acid concentration in the concentrated acid hydrolysis steps. Lowering the acid
concentration may result in a low bias in carbohydrate content due to incomplete
hydrolysis of polymeric sugars to monomeric sugars. Incomplete hydrolysis leaves
oligomeric sugars, which are not soluble in 4% acid and are incorrectly counted as acid
insoluble residue in these procedures, introducing a consequent high lignin bias.
5.3 This procedure assumes that chemical fractionation does not occur during the optional
sieving steps and that the chemical composition of any fines removed from the sample is
substantially similar to the composition of the bulk sample. Sieving can be performed if
the ash content according to LAP “Determination of Ash in Biomass” is high. Sieving
may also be performed if homogeneous particle size is critical. However, if the entire
biomass sample needs to be analyzed, sieving can frequently cause fractionation
and should not be performed.
6. Apparatus
6.1 Large table or drying rack for air drying biomass (method A only).
6.2 Convection oven capable of maintaining 45 ±5ºC (method B only).
6.3 Freeze-Drier - System with vacuum chamber and pump capable of maintaining a
pressure of <1 torr and a cold finger in the chamber capable of maintaining a temperature
of -50°C (method C only).
6.4 Balance, sensitive to 0.1 g.
6.5 Standard laboratory knife mill with 2 mm screen. A Wiley Mill, size No. 4 with a 2-mm
screen, is suitable for samples >20 g, and the intermediate model Wiley Mill, with 1-mm
screen, is suitable for samples <20 g that will not be sieved. Equivalent knife mills are
acceptable.
6.6 Sieve Shaker that provides motion in both horizontal and vertical axes- for optional
sieving step
6.6.1 Sieve Set, No. 20 (850 µm), No. 80 (180 µm) stackable sieves with lid and bottom
pan. Sieves and bottom pan should be 8.9 cm (31/2 in.) in height. Sieves should
conform to ASTM Specification E 11.
6.6.2 Riffle Sampler with Pans –– A manual sample divider that splits the milled biomass
into two or more equivalent sub-samples. Riffle divisions should be between 6.4
mm and 12.7 mm (1/4 to 1/2 in.) with at least twenty-four riffle openings. The feed
chute and riffles should have a slope of at least 60°. Collection pans, one to pour the
sample into the riffler, and two or more to collect the sub–samples.
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