USC Chemical Engineering Student Research Annual 2010

Calorific Values and Proximate Analysis of

Sargassum spp. and Ulva spp.

Carlo S. Alburo, Radzwell H. Conje, Maria Gracelda B. Pino,

Engr. Patrick U. Tan*Department of Chemical Engineering, University of San Carlos, 6000 Cebu City, Philippines

*Corresponding Author:

Abstract

The potential of seaweeds, Sargassum spp. and Ulva spp., as biomass for energy production was investigated based on their calorific values. Further, calorific values were correlated with the proximate analysis, which covers moisture, ash, volatile matter and fixed carbon contents of the seaweeds, using multiple linear regression. The calorific value was measured using bomb calorimetry while the proximate analysis was conducted using gravimetric method. The seaweed samples were taken from three different spots in Mactan Island, namely, Cordova, Buaya, and Maribago. It was found that the calorific values of Sargassum spp. range from 8495.21 kJ/kg to 12944.40 kJ/kg while Ulva spp. have a mean calorific value of 13477.41kJ/kg on dry basis. This shows that the seaweeds have comparable calorific values with those of the conventional biomass fuels like bagasse, rice husks and corn cobs. Correlation between calorific values and proximate analysis was finally established in this study as:

with R2 = 0.80.

Keywords: Calorific Values, Proximate Analysis, Seaweeds, Multiple Linear Correlations

1. Introduction

1.1. Seaweeds as Biofuels

The Philippines has an abundant supply of biomass resources which include agricultural crops (rice

hulls, coconut husk and meat, cocoa, cassava and bagasse), saw mill (forest logs) and furniture residues

(sawn timber). But these biomass resources readily compete with food production and agricultural land

[1]. To address these constraints, a potentially viable alternative is to use aquatic biomass, such as algae,

as the feedstock for second-generation biofuel and bioenergy production.

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Using macroalgae (or seaweeds) as biofuel has the advantage of higher growth rates compared to

terrestrial crops and and avoids competing with the agricultural land. Moreover, seaweeds can mitigate

greenhouse gases emission since CO2 from flue gas can be utilized as carbon source in algae growth.

Aside from that, wastewater can be used to provide nutrients for seaweed growth [2].

Past attempts in utilizing seaweed as source of fuel started in 1970’s oil crisis. Among the programs

started to compensate for the energy crisis, the Giant Kelp Program of United States used the brown

seaweed Macrocystis pyrifera as energy crop. However the program was discontinued when it was

thought that the crisis was over [3]. But with the serious problems posed by global warming and the rapid

depletion of oil reserves today, the use of aquatic biomass energy is now being reconsidered as a means of

CO2 mitigation. Processes that are now considered for energy generation from aquatic biomass include

direct combustion, anaerobic digestion, fermentation to alcohol, thermal liquefaction, thermal

gasification, and pyrolysis [4]. In Germany, studies are ongoing for the closed cycle Solar Oxygen Fuel

Turbine (SOFT) in which dried seaweeds (specifically Ulva spp.) are combusted in a fluidized bed boiler

in Rankine cycle and the combusted products are returned to the cultivation ponds as algae nutrients

source [5]. According to Bruton et al. [6], in France, annual green tides generate about 60,000 tons of wet

Ulva spp, (or 8,000 tons dry weight) which have been considered to be utilized for local energy

production.

Seaweed species suitable for biomass energy should display high productivity in terms of standing

crop and biomass yield. The species should be easily cultivated and harvested. Also, chemical

composition of seaweeds should be accounted since it determines the fuel quality and dictates the process

of energy production [4]. Seaweeds that are considered as potential energy crops include Macrocystis

pyrifera, Laminaria, Sargassum and Ulva [6,4,3]. These seaweeds are said to have high productivity and

biomass yield.

Among the five species mentioned above, Sargassum and Ulva species are chosen in this study

because of their local abundance and little economic value. Sargassum is a dominant genus in tropical and

subtropical waters in terms of standing crop, percent cover and height.

Ulva species are common in the intertidal zones of the Philippines, but, at certain times, could over-

proliferate, producing blooms or ‘green tide’ in some protected bays. Common Ulva species that

proliferate in Cebu province are Ulva lactuca, Ulva fasciata and Ulva reticulata [7]. In Mactan Island

(Cebu), central Philippines, at least two species constitute the Ulva population, either as free-living or

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attached form. Ulva lactuca mainly consists of free-living population while the species referred to as Ulva

reticulata consists mainly of attached population [7]. Ulva species are frequently found in nutrient-rich

saline waters, attached to the bottom of the waters [8].

1.2 The Calorific Values

Good knowledge of the heating or calorific value of a fuel and its ash compositions is needed for

control of ash-related conditions such as slagging, fouling, or erosion, management of environmental

emissions, heat rate calculations, and modifying operating parameters [9,10]. The calorific value of a

material is the amount of heat released by a material during combustion. It is affected by the ash and

moisture contents of the biomass. The net calorific value or lower heating value is defined as the heat to

be removed from the reaction products to obtain a final temperature equal to the initial temperature of the

reactants, assuming that the reaction products remain in gaseous phase, i.e. that the heat of condensation

of water is not available. On the other hand, the reference state of the water for the gross calorific value

(higher heating value) is liquid state.

The ash (inorganic) content of a fuel lowers down the calorific value and may cause major problems at

high temperature combustion (melting of ash and clogging of reactors) [9,10]. Weber and Zygarlicke [10]

reported that ash-related problems, including slagging, agglomeration, corrosion, and erosion, can cause

frequent unscheduled shutdowns, decreasing the availability and reliability of the energy source.

The calorific value decreases with increasing moisture content. The presence of moisture in fuel lowers

its effective heating value since a portion of the heat of combustion is utilized in evaporating the

contained moisture, hence decreasing the calorific value. It is highly desirable, therefore, that the moisture

content be kept as low as possible [9].

 

The fixed carbon also affects the heating value of the fuel in such a way that the higher the fixed

carbon, the higher the thermal value of fuel. In general, an increase in the ash content corresponds to a

decrease in the fixed carbon content and hence a decrease in heating value [11]. Table 1 shows the

heating values of commonly used biomass fuels. 

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Table 1. Typical characteristics of different biomass fuels used commercially for energy generation

Type Calorific Value (MJ/kg) % moisture % ashFruit Stems[1] 5 63 -Oil-palm husks[3] 7-8 55 5Oil-palm fibers[3] 7-8 55 10Bagasse[1] 7.7-8 40-60 1.7-3.8Fibers[1] 11 40 -Phaeophyta[2] 9-11 - 25-45Chlorophyta[2] 8-13 - 24-40Rice husks[1] 14 9 19Maize Cobs[1] 13-15 10-20 2Coffee husks[1] 16 10 0.6Cocoa husks[1] 13-16 7-9 7-14Groundnut shells[3] 16.7 3-10 4-14Wood[1] 8.4-17 10-60 0.25-1.7Coconut shell[1] 18 8 4Wheat straw and husk[3] 17-19 7-15 8-9Switch grass[3] 18-20 8-15 6Charcoal[1] 25-32 1-10 0.5-6

Very few studies have been done to establish the calorific values of seaweeds, much less those of

endemic seaweeds in the Philippines. At the international level, Lamare and Wing [12] reported the

calorific values of 28 species in New Zealand. Their report showed that for the macroalgae species, the

calorific value varies from species to species depending on the type of storage products that range from

high-energy polysaccharide starch to mannitol and glycerol. The constituents that contribute to the

calorific value of the seaweeds are carbohydrates, proteins and fats, which are all found to differ from

species to species. It was reported that Chlorophyta (green algae), Rhodophyta (red algae) and

Phaeophyta (brown algae) have different calorific values with Chlorophyta having the highest mean

calorific content and the Rhodophyta having the lowest as shown in Table 1. In a similar study, Littler

and Murray [13] reported that Chlorophyta had the highest average ash-free calorific content followed by

Rhodophyta and lastly Phaeophyta.

1.3 The Calorific Value Determination

The calorific value can be determined using proximate analysis or adiabatic calorimetry [14].

However, obtaining data from bomb calorimetry is usually tedious and expensive. For that reason several

correlations are devised to predict the higher heating value from the proximate analysis data. Parikh et al.

[15] used 450 different types of biomass and presented the correlation:

HHV = 0.3536FC + 0.1559VM – 0.0078ASH [Equation 1]

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where HHV is the higher heating value in MJ/kg, FC is the fixed carbon content, VM is the volatile

matter content and ASH is the ash content of the sample content in weight % on dry basis.

Demirbas [16] calculated the calorific values (higher heating values, HHV) of 16 biomass samples

experimentally from both ultimate and proximate analyses. The HHV (MJ kg−1) of the biomass samples

as a function of fixed carbon was calculated from the following equation:

HHV (MJ/kg) = 0.196(FC) + 14.119 (R2= 0.9997) [Equation 2]

Sheng et al. [17] gave new correlation between the HHV and dry ash content of biomass (in weight

percent) as follows:

HHV (MJ/kg) = 19.914–0.2324 ASH [Equation 3]

Lamare and Wing (2001)

The major advantage of these correlations is that they allow the determination of HHV of fuels simply

from their proximate analysis and thereby provide a useful tool for modelling of combustion processes.

These can also be used in examining old/new data for probable errors when experimental results lie much

outside the predicted results of HHV [15].

This study sought to explore the possibility of using locally available seaweeds as biomass fuel by

pitting its heating values against those of the conventional biomass outlined in table 1. The study also

sought to establish relation between heating values and moisture contents of the seaweeds while being

sun-dried and finally correlations between heating values and the proximate analyses were established for

Ulva and Sargassum species.

2. Materials and Methods

2.1 Sampling

Algal samples were taken from different areas around Mactan Island. The places chosen were

dependent primarily on the abundance of seaweeds in one particular place. Ortiz and Trono [18] reported

that 72 species of Sargassum are found in the Philippines. Table 2 gives the standing crop of Sargassum

beds in Central Visayas.

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Table 2. Standing crop of dominant Sargassum spp. in Central Visayas [18]

Site Standing Crop(kg wet weight per m2 )

Dominant Sargassum species

Tongo, Mactan Is. 2.5 S. siliquosumCordova, Mactan Is. 3.4 S. binderiMaribago, Mactan Is. 2.7 S. polycystum , S. oligocystumCaubyan Is. 4.9 S. polycystumDanajon Reef 6.7 S. polycystumBilangbilangan Is. 2.6 S. polycystum, S. oligocystumOlango Is. 1.28 S. polycystum, S. oligocystum

and S. siliquosum

In this study, Sargassum polycystum seaweeds were taken from the shores of Barangay Buagsong,

Cordova while Ulva lactuca was taken from Barangay Buaya. Both Sargassum oligocystum and Ulva

reticulata were taken from the shores of Maribago. The seaweeds were collected during low tides. One

liter of seawater samples was also taken during the collection of the species to determine its DO, pH,

temperature and salinity.

The seaweeds were collected by thallus. Three thalli were collected for Sargassum polycystum,

Sargassum oligocystum and Ulva lactuca. The thalli would serve as the samples of each species. There

were three samples for each kind of species. And since Ulva reticulata thrive in continuous mats, this

species was obtained by composite sampling. The collection of seaweeds was done by hand. Specimens

adhering to the seaweeds were removed using knife and the collected species were wrapped in a

newspaper and placed in an ice bucket.

Water samples were analyzed within 24 hours after they were collected. pH was determined using the

pH meter and the salinity was measured using the hand refractometer. The temperature of the seawater

and its dissolved oxygen were measured using the DO meter.

The seaweed samples were brought to the Marine Biology Department for identification. The

identification of the species was based on their external morphology, color and shape. Pictures of the

macroalgae aided the determination of species.

2.2 Preparation of Specimens for Analyses

The seaweed samples were cleaned with water and the epiphytes were removed. Excess water from

the samples was then removed by gently pressing the algae against tissue paper.

All the collected seaweed samples were sun dried. To determine relationship between calorific value

and moisture content during sun-drying of seaweeds, one whole thallus of each species, Sargassum

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polycystum and Ulva reticulata, was sampled for moisture content and calorific values analyses at

different intervals of time for three days. A total of seven samples were taken from the whole thallus of

each species during the sun-drying period and each sample had three replicates.

After all the seaweeds were sun-dried, they were stored and cut into small pieces for the proximate

analyses and calorific value determination. The whole experiment was conducted in two trials, one

month apart.

2.3 Proximate Analysis

Proximate analysis is an indirect method of measuring the calorific value of a sample. It converts the

component weights of proteins, fats and carbohydrates in the sample to their equivalent heating values.

Proximate analysis requires the values of the ash content, moisture content, volatile combustible matter

content, and the fixed carbon in order to determine the heating value of the sample. The Proximate

analysis method used in this study was based on ASTM D3172 (Standard Practice for Proximate Analysis

of Coal and Coke) but the temperatures were adjusted since proximate analysis of biomass is limited to

600ºC only [19].

2.4 Calorific Value Determination

In adiabatic calorimetry, direct combustion of the samples is done in a temperature-controlled bomb.

This jacket is maintained at the temperature of the bomb throughout the combustion process to eliminate

the heat-leakage. The calorific value of the sample obtained from calorimetry is said to be more accurate

than that obtained from proximate analysis [14].

The calorific value was determined using Parr 1108 Oxygen Bomb Calorimeter based on ISO

1928:1995 (Determination of gross calorific value by the bomb calorimetric method, and calculation of

net calorific value).

3. Results and Discussion

3.1. Calorific Values of Different Seaweed Species from Different Places (Maribago, Cordova and

Buaya)

A preliminary run was done to investigate available seaweed species in Maribago, Cordova and

Buaya. Table 3 shows the seawater conditions when the seaweed samples were obtained.

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Table 3. Seawater conditions of sampling locations

Parameter Maribago Buaya Cordova

T (oC) 30.8 30 30

DO (g/ml) 7.17 3.14 3.12

Salinity (%) 29 26 36

pH 7.99 8.31 8.34

In Maribago, the most abundant species were S. polycystum and U. reticulata. In Cordova, the most

abundant specie was S. oligocystum. Figure 1 shows the calorific values of different seaweed species

harvested.

Figure 1. Calorific Values of Different Seaweed Species from Maribago, Cordova and Buaya

From the figure above, it can be seen that the calorific values of Sargassum spp. range from 8495.21

kJ/kg to 12944.40 kJ/kg while Ulva spp. have a mean calorific value of 13477.41 kJ/kg on dry basis

which is in agreement with values reported by Lamare and Wing [12].

The results also show that the local species of Ulva and Sargassum collected in Mactan have

comparable calorific values with those of the conventional biomass fuels like bagasse, rice husks and corn

cobs as cited in Table 2.

3.2 . Calorific value as a function of moisture content during sun drying

The most serious disadvantage of biomass fuel is that they often have high moisture content which

readily inhibits combustion [19]. In general, aquatic biomass contains 80 to 90% water [20]. The best way

to recover the lost energy because of high moisture content is through drying of biomass. In the study,

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samples of Sargassum polycystum and Ulva reticulata were sun-dried to minimize the moisture content. It

was observed that when the moisture content was still higher than 40%, the calorific value of the seaweed

samples could not be determined by bomb calorimetry since no temperature change is observed during

ignition.

Figures 2 and 3 below show the relation between moisture content of the seaweed and its calorific

value for Sargassum polycystum and Ulva reticulata samples, respectively.

In both figures, it can be clearly seen that at moisture content below 40%, the calorific values have

negative linear correlations with moisture content consistently in the two experimental trials done.

However, Figure 2 shows that the two experimental trials produced different correlations despite

involving the same species. On the other hand, figure 3 shows that the two trials produced almost

identical correlations. It is also seen that at moisture content 15% (or below), the calorific value of

seaweeds is at par with those of the conventional biomass fuels. Since the two trials produced two

different correlations, only correlation of Figure 3 is thus developed as:

[Equation 4]

3.3 Correlation between the calorific values of seaweeds and proximate analysis data

After two to three days of sun-drying, the moisture content of the seaweeds lowered to 10 to 15% by

weight. Essentially all seaweed samples have negligible fixed carbon content (~0%) and have high

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Figure 3. Calorific Value of Ulva reticulata versus

moisture content

Figure 2. Calorific Value of Sargassum polycystum

versus moisture content

volatile matter (40-75%) and ash content (15-45%) on dry basis. The combustibles of a solid fuel are the

volatile matter and the fixed carbon. Since the fixed carbon content is negligible, the volatile matter is the

only contributor to the energy of seaweeds.

Ash content is the remaining incombustible minerals of a fuel. Consequently, the calorific value

decreases with increasing ash content. In general, fuels should have ash content less than 5% [9]. But

seaweeds in this study have been found to have high ash content which would then restrict the direct

combustion of seaweeds as biofuels.

Figure 4, 5 and 6 are plots of calorific values against moisture contents, volatile matter contents, and

ash contents, respectively, covering all the sampled species of Ulva and Sargassum. It can be seen in

Figure 4 that the moisture content has a poor correlation with the calorific value, while volatile matter

content and ash content each have good correlations with the calorific value (R 2 = 0.739 and R2 = 0.723,

respectively).

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Figure 5. Calorific value versus volatile matter contentFigure 4. Calorific value versus moisture content

By multiple linear regression analysis, moisture, volatile matter and ash contents were correlated with

the calorific value. Constraints in the use of this correlation include 10 to 20% moisture content, 40 t0

75% volatile matter and 15 to 45% ash content.

[Equation 5]

To measure how well the correlation fits the data, coefficient of determination (R2) was computed.

The R2 computed was 0.80. This means that 80% of the variation of the calorific values of the seaweed

samples has been explained by the linear regression equation established here.

4. Conclusion and Recommendation

Seaweeds have high potential to be used as solid biofuel since it has comparable calorific values to the

conventional biomass like bagasse, rice husks and corn cobs. However, seaweeds had to be dried from

about 87% to 15% moisture content in order to obtain calorific values close to those of the conventional

biomass (9000 – 12000 kJ/kg). In the case of Ulva reticulata species, there was a highly negative

correlation between moisture content and calorific values. On the bigger picture, Equation 5 can be used

to predict calorific values of Ulva and Sargassum species.

In the case of sun-drying of Sargassum species, more experiments should be conducted to establish a

consistent correlation between moisture contents and calorific values. To improve R2, more species of

Sargassum should be included.

5. References

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Figure 6. Calorific value versus ash content

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