Lignocellulosic composition and higher heating values

The experimental lignocellulosic composition of agricultural straw can be determined using the modified NREL LAP method for "Determination of Structural Carbohydrates and Lignin in Biomass" (Table 1) (Adapa et al., 2011; Sluiter et al., 2008). This procedure uses a two-step acid hydrolysis to fractionate the biomass into forms that are more easily quantified. During this process, the lignin fractionates into acid insoluble material and acid soluble material, while the polymeric carbohydrates are hydrolyzed into the monomeric forms, which are soluble in the hydrolysis liquid and subsequently can be measured using HPLC. The Percentage cellulose in the samples can be measured using the percentage glucan content, while the percentage hemicelluloses can be measured by adding the percentage mannose, galactose, xylose and arabinose content in the biomass samples.

Table 1 shows the lignocellulosic composition and higher heating values of non-treated and steam exploded barley, canola, oat and wheat straw samples. In general, the cellulose, hemicelluloses and lignin content of steam exploded straw was higher than non-treated straw. This may be due to other components (soluble lignin, loosely-bound sugars) being washed away during steam explosion, thereby leaving the proportion of insoluble lignin, cellulose and hemicellulose in the resulting dried sample higher than for the non-treated samples (i. e. higher percent of dry mass).

Properties

of Barley Straw

Canola Straw

Oat Straw

Wheat Straw

Biomass

NT

SE

NT

SE

NT

SE

NT

SE

Composition (% of dry matter)

Celluloseb

22.7 ± 0.9а

25.3 ± 1.8

22.4 ± 0.8

27.5 ± 1.1

25.4 ± 1.0

27.4 ± 2.4

27.1 ± 1.0

29.9 ± 1.4

Hemicellulosec

21.2 ± 0.5

21.0 ± 1.4

16.9 ± 0.5

20.2 ± 0.7

21.7 ± 0.9

18.8 ± 1.2

21.1 ± 0.5

19.7 ± 0.9

Galactose

0.9 ± 0.0

0.7 ± 0.0

1.0 ± 0.0

0.9 ± 0.1

0.8 ± 0.0

0.7 ± 0.0

0.8 ± 0.0

0.9 ± 0.1

Mannose

1.6 ± 0.2

1.5 ± 0.0

2.3 ± 0.1

1.9 ± 0.4

1.4 ± 0.0

1.7 ± 0.1

1.6 ± 0.1

2.8 ± 0.2

Xylose

14.4 ± 0.3

15.3 ± 1.0

11.5 ± 0.5

14.3 ± 0.2

15.1 ± 0.8

13.3 ± 1.0

14.9 ± 0.4

13.5 ± 0.4

Arabinose

4.4 ± 0.2

3.5 ± 0.5

2.0 ± 0.1

3.2 ± 0.0

4.4 ± 0.2

3.1 ± 0.2

3.9 ± 0.1

2.6 ± 0.2

Total Lignind

21.0 ± 0.6

21.6 ± 0.6

19.6 ± 0.6

22.3 ± 0.2

19.5 ± 0.6

23.7 ± 0.2

22.5 ± 0.7

24.2 ± 0.3

Soluble Lignin

1.6 ± 0.1

1.4 ± 0.1

1.6 ± 0.1

1.2 ± 0.1

1.5 ± 0.1

1.3 ± 0.1

1.4 ± 0.0

1.0 ± 0.1

Insoluble Lignin

19.4 ± 0.6

20.2 ± 0.6

18.0 ± 0.6

21.1 ± 0.1

17.9 ± 0.7

22.4 ± 0.1

21.0 ± 0.7

23.3 ± 0.4

Higher Heating Values (MJ/kg of dry matter)

HHV (MJ/kg)

16.4±0.3ff

17.4±0.1

16.7±0.3

18.3±0.0

16.4±0.1

17.8±0.0

17.0±0.2

17.8±0.0

DM — Dry Matter; NT — Non-Treated; SE — Steam Exploded; a Average and standard deviation of 3 replicates at 95% confidence interval; b%Cellulose = %glucan; c%Hemicellulose = %(mannose + galactose + xylose + arabinose);

d%Total Lignin = %(soluble lignin + insoluble lignin); HHV — Higher Heating Values (measured using Parr 1281 Bomb Calorimeter); ф3 replicates; f 95% confidence interval

Table 1. Lignocellulosic composition and higher heating values of non-treated and steam exploded agricultural straw (Adapa et al., 2011)

The calorific (heating) value of biomass feedstocks are indicative of the energy they possess as potential fuels. The gross calorific value (higher heating value, HHV) and the net calorific value (lower heating value, LHV) at constant pressure measures the enthalpy change of combustion with and without water condensed, respectively (Demirba§, 2007). A bomb calorimeter can be used to determine the HHV of the non-treated and steam exploded straw in MJ/kg. In addition, the ASTM Standard D5865-03 (ASTM, 2003) test method for gross calorific value of coal and coke, can be used as a guideline for heating value testing (Table 1).

Cellulose, hemicelluloses and lignin are major components of a plant biomass. Therefore, a change in composition could potentially lead to change in HHV of the biomass (Adapa et al., 2010a). The Net combined percentage change of cellulose, hemicelluloses and lignin in steam exploded barley, canola, oat and wheat straw is 5%, 19%, 5% and 4% higher than non-treated straw, respectively. As a result, the average HHV of steam exploded barley, canola, oat and wheat straw was 6%, 10%, 9% and 5% higher than non-treated straw, respectively (Table 1).

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