Pyrolysis and gasification reactivity of brown coal-algaeblends

Kawnish Kirtania, Luguang Chen, Sankar Bhattacharya 1

Department of Chemical Engineering, Monash University, Clayton, Victoria - 3800, Australia

Abstract

Combustion and gasification of coal-algae blends have the potential of reducing the carbon footprintfrom combustion and gasification of coal alone. This paper presents results from an experimental studyon pyrolysis and gasification of a Victorian brown coal and a marine algae, first these fuels alone andthen as blends of 90:10 to 50:50 by mass. These experiments are conducted in a Thermogravimetricanalyzer with steam-injection capability. In general, algae were found to be more reactive than Victorianbrown coal during pyrolysis while coal was more reactive than algae during gasification. The reactivityof the coal-algae blends was found to be lower relative to coal alone during steam gasification. But asthe pyrolysis and gasification characteristics remains in between the behavior of algae and coal, smallpercentage of algae can be blended with coal for gasification purposes.

Key words: Brown coal, Algae, Pyrolysis, Steam Gasification, Reactivity.

1 Introduction

Coal fuels almost 22% of global power supply. It is a reliable source of fuel for several countries.In Australia, 22 % of the electricity generated in 2007-08 was from Victorian brown coals [1].Victorian brown coal has large reserves of coal of over 500 years at current rate of usage.

However, CO2 emission from brown coal combustion is high. Therefore, gasification of browncoal is eventually investigated for power generation at a higher efficiency and for chemicalproduction. Biomass has the potential to reduce the carbon footprint from coal combustion,when used as coal-biomass blends. Biomass which does not compete with food crops for land ispreferred. One biomass that can be produced periodically with high yield and flexibility of spaceis algae [2–8]. That is why algae is attracting increasing attention for research and development.

Email address: Sankar.Bhattacharya@monash.edu (Sankar Bhattacharya).1 The author to whom all correspondence should be addressed

Algae is a kind of biomass which will practically recycle theCO2 produced from combustion byusing the CO2 during its growth. Algae has several advantages over the other biomass in that itcan be grown in a pond rather than occupying land. Its oil production rate is 90,000 l per hectare[9,10] which is almost 15 times higher than palm oil yield [11]. Also it has some unknowncatalytic effect on the combustion as it contains different metal ions [12]. Li et al. [12] alsoshowed that the coal-algae slurry showed better performance in combustion than the coal-waterslurry due to its pseudo-plastic behavior. Tang et al.[13] found that the N2/O2 atmosphere wasbetter than CO2/O2 atmosphere for algae and waste blends. The replacement of N2 by CO2,resulted in incomplete burnout, and thus some improvement measures, such as an increase inoxygen concentration, are required under CO2/O2 atmosphere to achieve the same combustionperformances to air [13].

Three kinds of macro-algae were studied by Li et al.[14] to understand their pyrolytic char-acteristic and kinetics. More discussion on marine macro-algae as raw material for biodieselproduction were completed by Maceiras et al. [15]. The economics of algae production and al-gal oil were discussed by Lee [16] and Norsker [17]. Demirbas [18,19], Brennan and Owende[20] gave an extensive review on the existing technology to produce energy from algae thoughtheir emphasis was only on oil production from algae. But the residue of the algae after extract-ing the oil have significant amount of carbohydrate and protein which also contains carbon andhydrogen. Gasification can use up the useful energy in the algae properly.

Thermochemical processing of coal and biomass blends have been studied extensively in smallscale. Thermogravimetric analysis on the blends of coal and wheat straw was carried out byWang et al. [21]. Co-combustion characteristics of low quality lignite and biomass has beeninvestigated by Varol et al. [22]. Also coal with pinewood sawdust was studied thermogravimet-rically by Ulloa et al. [23]. Co-pyrolysis characteristics of sawdust and coal blend were deter-mined in a TGA and a fixed bed reactor by Park et al. [24]. However, studies on thermochemicalanalysis of coal-algae blend has so far been non-existent. Our work is the first reported workinvolving thermochemical conversion of coal-algae blends. This paper is organized as follows.Section 2 presents the description of experiments. In section 3, the results from the pyrolysisand gasification experiments are interpreted.

2 Experimental

To understand the pyrolysis and gasification behavior of the algae and coal, they were analyzedindividually first and then as coal-algae blends. Both coal and algae samples were dried at105°C for 8 hours to remove moisture. They were then ground and sieved to obtain a size rangeof 106 to 150 µm. The samples were then analyzed for their elemental composition. Threesamples of the same species were analyzed to have an average elemental composition. Thepyrolysis and gasification experiments were carried out in a Thermo-Gravimetric Analyzer withsteam injection facility. For all the pyrolysis, two steps of heating were used at two heatingrates. At first, the temperature was raised to 200°C at a heating rate of 5 K/min. Then thetemperature was raised up to 1100°C at a constant heating rate of 10 K/min. N2 flow was kept

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Table 1Ultimate analysis of algae and Victorian brown coal (% db)

Carbon Hydrogen Nitrogen Sulfur Oxygen Ash

Algae 33.16 % 5.58 % 4.8 % 2.42 % 27.24 % 26.8 %

Coal 60.7 % 5.95 % 1.93 % 1.62 % 27.6 % 2.2 %

constant at 20 ml/min in all pyrolysis experiments. Gasification contains an extra step to normalpyrolysis. Steam was injected into the system after pyrolysis was complete. The steam injectiontemperature was decided to be 800°C from pyrolysis experiments. The nitrogen to steam ratioduring gasification was kept at 80:20.

After the pyrolysis and gasification experiments of algae and coal individually, blends at differ-ent compositions were analyzed. The (coal/algae) ratio were varied at 90:10, 70:30, 50:50. Theinstantaneous reactivity for the weight loss of any sample is defined as -

Ri =1

wi

(dw

dt

)i

(1)

where wi is the weight of the sample at time ti and (dwdt

)i is the instantaneous rate of decompo-sition at that time. Ri for all the samples was determined to identify the stages where the ratechanges during pyrolysis and gasification.

3 Results and Discussion

The results obtained from the experiment are organized and discussed according to the orderof the experiments. The results from one experiment is used to identify the required conditionfor the later experiment. The experimental results revealed the behavior of coal-algae and theirblends in both pyrolysis and gasification condition. In this section, the results are analyzed andsome general findings are noted from the experimental results.

3.1 Ultimate Analysis

The ultimate analysis of the algae and coal samples are shown in Table 1. The carbon contentof the marine algae is around 33 % which is almost 27 %-point less than the coal sample. Thismeans that coal would take more time to devolatilize than algae. Hydrogen content of both algaeand coal are almost same but the nitrogen content in algae is almost double the amount in coal.Also the sulfur content of the algae is higher than coal by 1 %-point. The ash content is muchhigher in case of algae than the coal.

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(a) Comparative weight loss (b) Comparative reactivity

Fig. 1. Comparative pyrolysis behavior of coal, algae and blends

3.2 Pyrolysis

Marine algae being a biomass, has high volatile content which are known to release at a lowertemperature than coal. The weight loss curve during pyrolysis of coal and algae are shown inFigure 1. The total weight loss was higher in case algae than that of coal. As evident from Figure1(a), coal has approximately 50% volatile content where algae has 58%.

The comparative reactivity of algae, coal and the three blends is shown in Figure 1(b) . It wasobserved that the decomposition of algae started around 200°C. After 500°C, the rate of lossbecame eventually constant. So the range of pyrolysis for algae was in between 200°C and500°C. In case of coal, pyrolysis started around 200°C and completed after 750°C. The pyrolysisregion was also observed to be broader for coal.

The reactivity was quite high in case of marine algae initially and became stagnant after 350°C.After 450°C the reactivity attained the same rate as before and continued to the end of thepyrolysis. Similar results were observed by Li et al. [14] for the case of three red algae. Anas-tasakis et al. [25] studied the pyrolysis behavior of main carbohydrates of brown macro-algaeseparately and found that different components of marine algae decomposed at different tem-peratures which complies with the pyrolysis behavior in the current study. However, this is anissue that needs further investigation.

The reactivity of the coal decreased after 500°C. Reactivity increased with the increased per-centage of algae in the blends. The change of reactivity was quite distinct for the case of 90:10and 50:50 blends. The reactivity of 70:30 was in between them. But there was a distinct step for50:50 blend which was quite similar to the algae reactivity. Among the three blends, the 50:50blend of coal and algae has the highest rate of decomposition as it reached to a steady statebefore the other two blends. The pyrolysis rate of the 50:50 blend was the highest in this casedue to high percentage of algae whereas the 90:10 blend was the slowest.

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From the experimental findings from the pyrolysis of algae and coal, it was obvious that theblend pyrolysis behavior would depend on the percentage of the algae in the blends. For a coal-algae blend of 90:10 ratio show the coal dominated characteristics for pyrolysis whereas a 50:50blend show equal contribution.

From figure 1(b) it is apparent that small amount, around 10%, of algae with coal will not affectthe pyrolysis behavior of coal much except slight increased reactivity which may be desirable.

3.3 Gasification

Gasification was carried out by injecting steam into the system to assess the effect of steamgasification on the blends distinctly from pyrolysis. As the pyrolysis of both coal and algaewas complete after 800°C, it was selected to be the temperature for steam injection. The puremarine algae and coal were first gasified to assess their individual behavior. Figure 2(a) showsthat coal lost about 43% of its weight during gasification whereas the algae lost only 12.3%. Butduring the pyrolysis, algae lost 6.75%-point more weight than coal. This indicates that most ofthe volatile of algae devolatilized during pyrolysis and small amount of weight loss observedduring gasification. This also reflects higher level of ash in the algae relative to coal.

As algae has ash content of around 27%, no more weight loss was observed after 68%. In case ofVictorian brown coal, more than 93% weight loss took place through pyrolysis and gasification.For both coal and algae, the remaining at the end of gasification was higher than the ash contentlisted in Table 1. This meant that there might remain a small amount of unburned char by about4% of the total weight.

As in the case of pyrolysis, three different coal and algae blends were tested through steamgasification. The weight loss increased during pyrolysis and reduced during gasification withincreasing percentage of the algae. So for a higher percentage of algae, total weight loss wasless and ash content was higher. This behavior is observed in Figure 2(b).

The instantaneous reactivity of coal, algae and the three blends during steam gasification arepresented in Figure 3. The reactivity curves indicate that the gasification of Victorian browncoal and marine algae was completed at about the same temperature. So the blend of any amountwithin the investigated range, would unlikely change the completion point of gasification. Figure3(a) shows reactivity of coal is much higher than the reactivity of algae in the temperature range900°C to 1040°C during gasification. The reactivity was same for coal and algae below 900°Cand above 1040°C for gasification.

Figure 3(b) illustrates that the reactivity decreased with the increased percentage of algae forgasification in the temperature range of 900°C - 1040°C. The overall reactivity of gasificationwas much higher than that of pyrolysis for blends.

At the end of gasification, an unremovable solid layer was found to have formed at the bottomof the alumina crucible with increased amount of algae in blends. This layer was not found in

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(a) (b)

Fig. 2. Comparative pyrolysis and steam gasification behavior of coal, algae and blends

(a) (b)

Fig. 3. Comparative pyrolysis and steam gasification reactivity behavior of coal, algae and blends

case of pure algae or pure coal gasification. This indicates interaction of the mineral matter fromcoal and algae, which will require detailed investigation.

4 Concluding Comments

A preliminary study on the pyrolysis and gasification behavior of a Victorian brown coal andmarine algae blends was carried out using thermogravimetric analysis. Algae has higher levelof volatiles and ash and much lower level of carbon to that of coal. Pyrolysis and gasificationcharacteristics of the blends was found to be proportional to the algae content. With increasedpercentage of algae in blends, the reactivity during pyrolysis increased; gasification reactivitydecreased within the temperature range. Unconverted carbon was observed to remain in the

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residual sample after the steam gasification of coal, algae and blends. Small percentage of algaein blend did not appreciably alter the pyrolysis and gasification behavior. It appears high yieldingbiomass like marine algae can potentially be blended with coal for reducing the carbon footprintof coal gasification. However, detail studies on gasification and agglomeration characteristicsare necessary; some of these are currently in progress.

Acknowledgements

This research is financially supported by the State government of Victoria through its EnergyTechnology Innovation Strategy programme, Loy Yang Power, International Power Loy YangB, Tru Energy, International Power Hazelwood and Monash University.

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