Cassava Stem Powder as an Additive in Biomass Fuel Pellet

Article pubs.acs.org/EF

Cassava Stem Powder as an Additive in Biomass Fuel Pellet Production Sylvia Larsson,* Oscar Lockneus, Shaojun Xiong, and Robert Samuelsson

Downloaded by GEORGETOWN UNIV on August 26, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.energyfuels.5b01418

Department of Forest Biomaterials and Technology, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden ABSTRACT: In biomass fuel pelletizing, some raw materials with less favorable binding properties require a binding enhancing additive for enhancing required pellet quality. Starch is commonly used as a binder in industrial fuel pelletizing but is a refined product that adds substantially to pellet production costs. In this study, finely milled cassava stems, a starch-rich, cheap, and underutilized byproduct from food production, were used and compared to refined starch as an additive in biofuel pellet production. The evaluation was performed in an experimental design with the factors cassava stem/starch content, moisture content, and material temperature. Measured responses were pellet bulk density, mechanical durability, amount of fines, pelletizer motor current, coefficient of variance for pelletizer motor current, CVA (a measure of process stability), pellet temperature, die temperature, and pellet moisture. Each response was modeled by multiple linear regression (MLR). Cassava stem addition gave similar effects as starch addition by increasing pellet durability and reducing the amount of fines, particularly at low moisture contents (MC ∼ 11%). The highest pellet durability in the study was achieved at a low moisture content (11% MC) when using cassava stem as an additive. Combustion properties of the pellets were determined in a residential pellet burner. Low emissions and no ash fouling were obtained for both of the additive types. In conclusion, cassava stem powder is a good additive substitute for refined starch to increase fuel pellet quality.

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INTRODUCTION To produce biomass fuel pellets that fulfill quality specifications according to the current standard, EN 14961-1:2010,1 some raw materials with less favorable binding properties require a binding enhancing additive to reach a high enough pellet durability. Examples of feedstock that give rise to low pellet durability are eucalyptus,2 poultry litter,3 corn stover,4 and fresh pinewood.5 Durability enhancing additives for fuel pellet production are lignin,6,7 starch,6−11 proteins,9 and caustic soda.10 Of those, starch is the most commonly used additive. Starch is tightly bound in granules that are built by crystalline and amorphous areas that are “arranged radially in concentric layers”.12 To open up the granules, starch needs to be modified, for which heating is the simplest method. When heating potato starch, its initial gelatinization temperature is constant at 62 °C when the water content is above 0.6 g of water/g of dry starch. If the water content is decreased to 0.2 g/g of dry starch, the initial gelatinization temperature rises rapidly to 97 °C.12 Assuming a linear relationship between the water content and gelatinization temperature at low moisture contents, an initial gelatinization temperature of about 100 °C is attained at the raw material moisture contents commonly used in biomass fuel pelletizing. Because of friction, such pellet temperatures are often achieved in the pelletizing process. Higher temperatures start a solubilization, and when starch is set to cool, it reassociates into aggregates and once again forms a gel.13 Through this behavior, starch acts as a glue that improves particle binding. However, pure starch addition is a costly additive that has negative impact on the pellet production economy. Alternative starch sources were suggested by Kuokkanen et al.,6 who showed that potato flour and potato peel starch-containing industrial wastes increased pellet durability and were © XXXX American Chemical Society

economically and environmentally favorable to refined products. Another starch-rich crop is cassava (Manihot esculenta Crantz). The worldwide annual production of cassava roots for food is approximately 256 billion tons, and stem residues amount to approximately 34 billion tons of dry mass.16 About 10−20% of the cassava stems is used for propagation, but the rest is often burned or left at the cropping site, being considered as waste.14,15 The starch content in cassava stems can be as high as 30% of the dry mass.15 Consequently, milled cassava stems could be a suitable low-price starch-based additive for pellet fuel production. The objective of this study was to compare cassava stem powder and refined starch as additives in biofuel pellets with respect to pellet quality, pelletizing performance, and combustion performance. The study was carried out according to an experimental design, varying the factors cassava stem/ starch content, moisture content, and material temperature and with pellet quality and pelletizing performance parameters as responses. In addition, combustion properties of pellets from both assortments were evaluated to determine if pellets with cassava stem additive are functional as fuel in residential appliances.

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EXPERIMENTAL SECTION

Biomass. The bulk raw material in the pelletizing experiments was a sawdust blend of Scots pine (Pinus sylvestris L.) and Norwegian spruce [Picea abies (L.) H. Karst] delivered by Neova in 250 kg bags. Fresh pinewood, known to give rise to low pellet quality,5 mixed with spruce (75:25), was used as model material to which the starch and Received: June 24, 2015 Revised: August 18, 2015

A

DOI: 10.1021/acs.energyfuels.5b01418 Energy Fuels XXXX, XXX, XXX−XXX

a

0 0 0 0 2.5 2.5 2.5 5 5 5 5 0.5 0.5 0.5 1 1 1 1

Cas/St1 Cas/St10 Cas/St5 Cas/St8 Cas6 Cas4 Cas9 Cas3 Cas7 Cas2 Cas11 St14 St18 St15 St13 St16b St12 St17b

11.3 11 14 13.7 12.7 12.7 14.3 11.4 11.3 13.5 13.9 12.4 12.7 12.5 12.2 11.2 13.9 13.9

raw material moisture content (%) 23.1 46.2 23.5 50.5 31 39.4 28.1 24.7 46.5 24.7 46.9 28.1 29.1 38.3 24.6 52 23.3 55

material temperature (°C) 602 557 499 449 559 533 536 595 560 496 449 557 526 507 599 492 529 489 (12)

(20) (7) (7) (15) (4) (10) (13) (16) (14) (5) (1) (4) (11) (8)

pellet bulk density (kg/m3)

Standard deviation in parentheses (n = 3). bn = 1; N/A = missing values.

cassava/starch content

sample name

Table 1. Mean of Achieved Factor Values and Measured Response Valuesa

84.1 83.9 69.9 64.9 87.4 83.5 87.5 91.7 93.1 81.9 77.7 89.0 87.7 85.6 92.1 88.2 91.3 85.4 (1.4)

(4.9) (2.1) (1.8) (0.7) (1.1) (2.6) (1.2) (2.0) (3.0) (1.5) (0.8) (1.0) (2.8) (1.6)

pellet durability (%) 2.1 1.8 3.6 8.0 1.2 1.6 1.2 0.8 0.8 1.5 2.7 1.0 1.8 1.1 0.9 0.7 0.8 1.3 (0.21)

(0.78) (0.31) (1.15) (0.21) (0.20) (0.21) (0.12) (0.17) (0.46) (0.25) (0.00) (0.99) (0.42) (0.17)

fines (%) 27.5 25.6 (0.39) 27.5 (0.49) 25.7 (0.34) 27.3 (0.19) 27.7 (0.20) 27.3 (0.51) 29.0 (1.6) 25.2 (0.36) 25.4 (0.63) 26.0 (0.27) 28.7 (0.05) 26.8 (1.4) 27.6 (1.4) 28.9 (1.3) N/A 27.1 (0.90) N/A

pelletizer motor current (A) 15 11 11 12 10 13 10 13 8 12 12 10 8 7 10 N/A 7 N/A

CVA for motor current (%)


96.3 N/A 97.7 (1.2) 98.8 (1.6) 98.6 (0.2) 101.0 (0.1) 96.31 97.8 (0.37) 102.8 (0.7) 96.8 (0.5) 97.9 (1.7) 96.0 (2.5) 103.5 (1.2) 92.11 105.6 (0.3) N/A 96.6 (1.5) N/A

pellet temperature (°C) 61.9 69.8 (2.7) 61.8 (4.8) 77.2 (0.44) 69.3 (0.50) 70.2 (0.44) 57.8 (2.7) 67.6 (1.5) 76.2 (3.0) 67.8 (0.93) 74.5 (0.46) 70.1 (0.81) 81.5 (1.3) 59.1 (5.5) 73.4 (1.3) 73 58.5 (7.3) N/A

die temperature (°C) 9.4 10.1 10.4 11.3 9.7 9.8 10.4 8.4 9.0 10.8 11.7 9.6 9.8 10.7 8.4 10.9 10.7 10.7

(0.00)

(0.49) (0.67) (0.06) (0.12) (0.06) (0.10) (0.15) (0.12) (0.17) (0.21) (0.06) (0.07) (0.25) (0.12)

pellet moisture content (%)

Energy & Fuels Article

B


Article


Energy & Fuels cassava powders were added. The conifer blend that was hammermilled with a screen size of 4 mm (Hammer Mill Vertica DFZK-1, Bühler AG, Switzerland) had a moisture content of ∼10% after milling. Additives. The cassava stem powder was made from dried cassava of approximately 8% moisture content that was delivered in bales and knife-milled down to a particle size of 0.5, and the difference between R2 and Q2 is 0.1 is considered as significant. The number of factors used in the models was determined by optimization of Q2.

Table 2. Model Performance Indicators for Cassava Stem Additiona model descriptor number of observations R2 Q2 degrees of freedom


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RESULTS AND DISCUSSION Pelletizing Process and Pellet Quality. Achieved factor values and measured responses for all runs are shown in Table 1. Values for the moisture content were in good agreement with the settings, except for the midpoint Cas9, which achieved a high level value of the design, whereas the material temperature was more difficult to control, especially for the midpoints. Because of low durability and bulk density values, none of the produced pellet batches fulfilled the standard for class A1, A2, or B pellets. Pellet durability ranged from 65 to 92% and bulk density ranged from 449 to 602 kg/m3. The amount of fines varied from 0.8 to 3.6%. When setting up an experimental screening design, factors are varied outside the optimal process window, and thereby, pellet quality responses will fall outside their normal ranges. In hindsight, a longer die channel length should have been chosen to reach a higher overall pellet quality for the experimental series. Because of material temperature inconsistency, no replicates could be recognized by the statistical software and, consequently, no test for lack of fit could be performed. However, variations in midpoint settings made it possible to add squared terms in the models, even though the chosen designs were screening designs. Pelletizer motor current varied from 25.2 to 29.0 A, where the current required for idle running was 17.0 A. No good model could be obtained for the pelletizer current (Q2 = 0.13 for cassava, and Q2 = 0.37 for starch). This was probably caused by the large variability in the current compared to the variation range. Cassava stem addition gave models for pellet temperature, die temperature, and pellet moisture with Q2 values of 0.86, 0.91, and 0.96, respectively, whereas no significant models were achieved for these responses with starch addition. Model statistics using cassava stem and starch as additives are shown in Tables 2 and 3, respectively. Responses modeled were pellet bulk density, mechanical durability, and fines. Because of a non-normal distribution, data for fines was log-transformed before analysis. Except for bulk density with starch as an additive, the correlation (R2) and prediction (Q2) coefficients were high (>0.74), indicating high predictive power for all models. For both additives, raw material moisture content and material temperature had the largest influence on bulk density, with high density at low moisture content and low temperature. As reported earlier,5 a decrease in the moisture content usually results in an increase in bulk density as a result of a higher friction in the die channels. Because material temperature is controlled by steam addition, an increase in the moisture content of the material is a consequence of steam treatment, and therefore, it is impossible to judge whether the effect from material temperature on bulk density is caused by a change in the temperature or moisture content. To obtain high mechanical durability and low amount of fines, low moisture content, low material temperature, and high

constant moisture content (m) cassava content (c) material temperature (t) m×m c×c m×t

bulk density (kg/m3)

durability (%)

11

11

11

0.984 0.905 5

0.984 0.939 5

0.996 0.982 4

fines (%)

Coefficients 555 (0.00) 86.6 (0.00) −60.3 (0.00) −8.73 (0.00)

0.106 (0.01) 0.243 (0.00)

1.96 (0.58b)

5.95 (0.00)

−0.224 (0.00)

−32.6 (0.00)

−2.35 (0.02)

0.106 (0.00)

25.1 (0.02) −45.2 (0.00) nsc

7.03 (0.00) −10.6 (0.00) ns

−0.160 (0.00) 0.302 (0.00) 0.094 (0.00)

a

p values are given in parentheses and indicate probability to obtain the displayed value for the coefficient when assuming that the coefficient has a true value of zero. bInsignificant p values at a 95% confidence level that are kept in the model for maximizing Q2. cns denotes insignificant factors excluded from the model.

Table 3. Model Performance Indicators for Starch Additiona model descriptor number of observations R2 Q2 degrees of freedom constant moisture content (m) starch content (s) material temperature (t) m×m s×s m×s m×t

bulk density (kg/m3)

durability (%)

11

11

11

0.801 0.564 8

0.993 0.956 5

0.948 0.794 5

fines (%)

Coefficients 525 (0.00) 86.4 (0.00) −40.1 (0.00) −5.26 (0.00)

0.139 (0.07b) 0.223 (0.00)

nsc −48.0 (0.00)

7.44 (0.00) −3.43 (0.00)

−0.343 (0.00) 0.142 (0.04)

ns ns ns ns

ns −3.98 (0.00) 3.74 (0.00) ns

−0.387 (0.04) 0.354 (0.03) ns ns

a

p values are given in parentheses and indicate probability to obtain the displayed value for the coefficient when assuming that the coefficient has a true value of zero. bInsignificant p values at a 95% confidence level that are kept in the model for maximizing Q2. cns denotes insignificant factors excluded from the model.

amounts of additive was favorable, for both cassava and starch addition. All three factors were significant and had nonlinear behaviors for both durability and fines. However, as for bulk density, it is not possible to separate the effects from material moisture content and material temperature, because the increased moisture content as a result of steam treatment will influence these responses in the same manner as an increase in the moisture content of the pellet raw material. A distinct negative correlation between durability and fines was also observed, which is typical. D


Article

Energy & Fuels Response surface plots for pellet quality responses, bulk density, mechanical durability, and fines, are shown in Figures 1−3 (cassava addition) and Figures 4−6 (starch addition).


Figure 4. Response surface plot for bulk density (kg/m3) when potato starch is used as an additive.

Figure 1. Response surface plot for pellet bulk density (kg/m3) when cassava stem is used as an additive.

Figure 5. Response surface plot for durability (%) when potato starch is used as an additive.

Figure 2. Response surface plot for pellet durability (%) when cassava stem is used as an additive.

Figure 6. Response surface plot for fines (%) when potato starch is used as an additive.

quality was found at the lowest moisture content. In the starch case, bulk density was found to be independent of starch addition, while optimum values for durability and fines were achieved at 0.71 and 0.74% potato starch addition, corresponding to 0.59 and 0.61% of pure starch. Thus, 1.5 times more pure starch is needed in the form of cassava stem powder compared to when using potato starch. The difference in binding effectivity between the starches can be due to the difference in amylose/amylopectin content because starch with a high amylopectin content exhibits an initial gelatinization temperature that is significantly higher than starches with a high amylose content when the water content is about 30%.25

Figure 3. Response surface plot for fines (%) when cassava stem is used as an additive.

For cassava, optimum values for all three responses were obtained at the lowest moisture content level (11% MC). Maximum bulk density was achieved at 2.5% cassava stem addition. The highest durability and lowest amount of fines was achieved when adding 3.2 and 3.4%, respectively, of cassava stems. With a pure starch content of 27% in the cassava stem powder, these additive levels correspond to 0.68, 0.86, and 0.92% of pure starch. Also, for starch addition, optimum pellet E


Article

Energy & Fuels Table 4. Results from Combustion Experiments assortment

reference assortmenta

unit

Cas1, 0% sampleb

Cas7, 5% cassava

St17, 1% starch

6.9 0.29 39.0

8.7 0.38 40.3

9.5 0.58 28.9

10.8 0.44 42.6

0.77 0.59 0.17 0.6 0.00 2

0.96 0.67 0.29 3.1 0.00 2

0.84 0.53 0.31 2.7 0.01 2

1.32 1.08 0.24 4.0 0.01 2

10.1 290 0.20 33

10.9 240 0.20 36

10.9 540 0.20 62

10.6 220 0.20 21

Fuel moisture content of pellets ash content in pellets amount of fuel

% % kg of DM

combustion residual unburned material ash, pure sintered material sintered material category of sintered material

% % % % %

O2 CO CO2 dust

% mg/Nm3 (10% O2), geometrical mean kg/Nm3 (10% O2) mg/Nm3 dry gas (10% O2)

Bottom Ash of of of of of

fuel fuel fuel ash fuel


Emissions

a

Mean of four experiments. bMean of two experiments.

thus a very interesting resource that could be used as an additive in biofuel pelletizing.

Assuming that the difference observed at a high moisture content also remains at a lower water content, this difference of about 10 °C might be critical, because the pellet temperature in the experiments is of the same order as the initial gelatinization temperature of the starches. Modeled values for bulk density and mechanical durability are much higher than the highest values obtained in the experiments. The reason for this is that the optimum point is quite distant from the nearest experimental point in the design. To obtain a more reliable optimum value, a new design surrounding this point should be carried out. Combustion Process. In Table 4, results from the combustion experiments are summarized. Assortments with additives produced small amounts of non-problematic sintered ash and did not show any significant increased tendency for sintering in comparison to the reference sample with no additives. The higher amount of ash in the Cas7 assortment is due to the comparably higher ash content in the cassava stem additive. However, this increase in ash content did not result in any increased sintering tendency. The ash content in the combustion residual was lower than that of the original fuel, partly because of the loss of potassium in the fly ash. Low amounts of unburned material indicate effective combustion. A slightly elevated level of the emission of dust was observed for the cassava additive assortment, which is in accordance with previous results by Diaz-Ramirez et al.26 for the same cassava species (denotated CassW instead of SC205). The higher emission level can be due to a higher concentration of potassium in cassava. Pellets with 5% cassava stem additive passed the European EN 303-5 requirements set at 3000 mg/ Nm3 CO and 150 mg/Nm3 dust at 10 vol % O2 for boilers of nominal output

Cassava Stem Powder as an Additive in Biomass Fuel Pellet

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