The effect of excess air ratio on syngas produced by gasification of agricultural residues briquettes

DANIELA TASMA, KRISZTINA UZUNEANU, TĂNASE PANAITThermal Systems and Environmental Engineering Department

“Dunărea de Jos” University of Galati47 Domneasca St., 800008, Galati

ROMANIAdtasma@ugal.ro, kuzuneanu@ugal.ro, tpanait@ugal.ro

Abstract: - In this paper were investigated the effects of the excess air ratio on the syngas obtained by gasification of agricultural residues briquettes. The syngas composition and syngas heating value are strongly influenced by the excess air ratio. The results indicate that the concentrations of H2 and CO decrease with the increase in excess air ratio, while the concentrations of N2, CO2 and CH4 increase. At a lowerexcess air ratio, a syngas with a high heating value was obtained. The optimum excess air ratio for gasification of briquettes made from agricultural residues was evaluated to be an approximately 0.17–0.20.

Key-Words: - gasification, syngas, excess air ratio, agricultural residues

1 IntroductionBiomass, as a source of renewable energy which has sustainable and mitigating global warming characteristics is getting greater attention. The thermochemical gasification is one of the effective methods for obtaining energy from biomass [1].

Gasification of solid biomass converts it to gas often referred to as producer gas or syngas which is mainly composed of CO, CO2, CH4, H2 and N2 (if the gasification agent is air). The useful gas or combustible gas components are CO, CH4 and H2

[2].Stoichiometric combustion occurs when all the carbon in the fuel is converted to CO2 and there is no excess O2 left over. The basis of gasification is to supply less oxidant than would be required for stoichiometric combustion of a solid fuel. The energy value of the useful gas is typically 75% of the chemical heating value of the original solid fuel. The syngas temperature will be substantially higher than the original solid fuel due to the gasification process [3].

The excess air ratio is one of the most important operational variables in biomass gasification with air. In practice, it defines the temperatures of the bed and of the freeboard, the tar yield, and the composition and calorific value of the fuel gas [4].

Wang [1] observed that an increase of the gasification air ratio from 0.16 to 0.26 led to increases of the higher heating value and of the

concentration of H2 and CO in the gas produced.Doherty [3] observed that H2 and CO reach a

maximum concentration at an excess air ratio of 0.35, and syngas heating value decreases with increasing excess air ratio.

With an increase of the excess air ratio from 0.19 to 0.27, Lv [5] observed that hydrogen content varied little while the lower heating value first increased and then decreased. The optimal value of the excess air ratio was found to be 0.23.

On increasing the excess air ratio from 0.20 to 0.45, Narvaez [4] found that the heating value of the syngas decreases, the gas yield increases and the amount of syngas (H2, CO, CH4, and C2H2) decreases.

In this paper is presented the effect of excess air ratio on the syngas composition resulted from the gasification of the briquettes made from agricultural residues. Agricultural residues have acquired considerable importance as biofuels for domestic cooking, industrial process heating, electrical power generation, and are used directly as well as in briquetted form for a variety of energy end uses [6].

The results indicate that the concentrations of H2 and CO decrease with the increase in excess air ratio, while the concentrations of N2, CO2 and CH4 increase. The syngas composition is found to be similar to those reported by other researchers [7], [8], [9].

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2 Experimental procedureThe biomass materials used in this experiment were briquettes made from agricultural residues (Table 1). The agent gasification used was air. All experiments were carried out at ambient temperature and at a pressure of 1 bar.

Table 1: Fuel properties

Fuel sample

Reed briquettes

Sawdust briquettes

Sawdust 50%+

corn stalk 50%

briquettes

Sawdust 50%+ wheat

straw 50%briquettes

Ultimate analysis (% of dry fuel with ash)C 52.4 53.3 49.41 50.63

H 5.91 6.28 5.90 6.12

N 0.65 1.91 0.43 0.53

O 33.55 35.75 40.73 38.08

A 7.85 2.77 3.54 4.64

Proximate analysis (% of wet fuel)Fixed

Carbon29 24.85 22.60 20.30

Volatile matter

56.70 66.55 67.40 70.10

Ash 7.30 2.60 3.30 4.40

Moisture 7 6 6.70 5.20

The gasification process consists of drying, pyrolysis or devolatilization, combustion or oxidation and gasification or reduction [2].These processes are largely dependent on the parent biomass composition, moisture content, local stoichiometry, reactivity of biomass, and the gasifier design [10].

Fig. 1 - The downdraft fixed bed gasifier [11]

The choice of gasifier type depends on the type of fuel to be gasified and end use of the gas produced. In a downdraft fixed bed gasifier(Fig.1), the biomass is fed in from the top, the gasifying agent is introduced at the sides above the grate and the producer gas is withdrawn under the grate Fixed bed reactors are relatively simpler, reliable, amenable to gasify different kinds of feedstock, other lower particulate concentration in product gases, and can achieve higher efficiencies than other reactors [12].

The excess air ratio is a measure of the air flowrate. The excess air ratio is defined as the ratio of the actual air to fuel ratio divided by the stoichiometric air to fuel ratio required for complete combustion [13]. In biomass gasification it varies from 0.20 to 0.40 [4]. Air flow influences the syngas in different ways. Air supplies the O2 for combustion and effects the residence time. By varying amount of the O2 supply, air flow rate controls the degree of combustion wich in turn, affects the gasification temperature. Higher airflow rate results in higher temperature wich leads to higher biomass conversion and a higher quality of fuel. An excess degree of combustion results in decreased energy content of the syngas produced because a part of biomass energy is spent during combustion [14].

In this paper, the lower heating value of the syngas is reported as an indicator of the gasifier efficiency. The LHV can be calculated from the following equation:

4422 CHLHVYLHVYLHVYLHV CHHHCOCOg .

where Y is the mole fraction of each gas species. The lower heating values of the gas species are [15]: LHVCO = 13.1 MJ/Nm3, LHVH2 =11.2 MJ/Nm3, LHVCH4 = 37.1 MJ/Nm3.

3 Results and discussionTo obtain the desired syngas composition and to increase the energy conversion efficiency, the gasification operating conditions must be optimized. This section describes the effect of the excess air ratio on the syngas composition produced by gasification of briquettes made from agricultural residues.

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Fig. 2: Effect of excess air ratio on the composition of syngas from different agricultural residues briquettes

Fig. 2 depicts the syngas composition resulting by gasification using air, a gasification agent, as a function of the excess air ratio. The increase in the excess air ratio means that the feeding air increases, which will cause more oxidations reactions, normally to increase the CO2 and H2O concentrations at the expense of CO and H2.

The excess air ratio for the gasification of agricultural residues briquettes is found to be in range 0.16 - 0.33.

It has been found that the syngas composition for sawdust briquettes with an excess air ratio of 0.21was about 37.35%, 27.10%, 0.14%, 0.39%, 34.56% for CO, H2, CH4, CO2, N2, respectively, in a moisture free basis.

It can be observed that the molar fraction of CH4

is very low. This increases with the increase of the excess air ratio. The concentrations of H2 and CO in the syngas decrease with increase of excess air ratio. The molar fraction of N2 increases linearly with the excess air ratio.

The effect of the excess air ratio variations

on the LHV of the syngas produced is shown in Fig. 3. It is evident that the lower heating value decreases with increasing of excess air ratio. The LHV of the syngas ranges from 5.4 MJ/Nm3 to 8.5 MJ/Nm3. The value of LHV is high for low excess air ratios, mainly due to the high concentration of H2 and CO.

Fig. 3 – The variation of lower heating value of syngas with the excess air ratio

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Fig. 4 presents the variation of the H2

concentration with the excess air ratio for all agricultural residues briquettes used in the gasification process. A large decrease in H2 at a higher excess air ratio was obtained from the oxidation of H2 at a higher excess air ratio.

Fig. 4: Variation of the molar fractions of H2 with the excess air ratio

The maximum concentration of H2 (30%) was obtained by gasification of sawdust 50% + wheat straw 50% briquettes at the minimum excess air ratio (0.17).

The variation of the molar fractions of CO2

with excess air ratio is presented in Fig. 5. It can be observed that the increase in excess air ratio increases the CO2 content in the syngas.

Fig. 5 - Variation of the molar fractions of CO2 with the excess air ratio

The highest contents of the CO2 in the syngas were obtained by gasification of briquettes made from sawdust 50% + corn stalk 50%.

Fig. 6 shows the variation of the CO molar fractions with excess air ratio. The increase in

Fig. 6 - Variation of the molar fractions of CO with the excess air ratio

excess air ratio decreases the CO content in the syngas. The highest content of the CO in the syngas was obtained by gasification of briquettes made from sawdust 50% + corn stalk 50% at the minimum excess air ratio (0.17).

In Fig. 7 is presented the variation of CH4

with excess air ratio. The molar fraction of CH4

is very low and this increases with the increase of the excess air ratio.

Fig. 7 - Variation of the molar fractions of CH4 with the excess air ratio

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The highest concentration of CH4 was obtained for gasification of sawdust 50%+wheat straw 50% briquettes at an excess air ratio of 0.30.

4 ConclusionGasification was conducted in a temperature range of 700–900 °C and excess air ratio of 0.16- 0.33 for air as a gasification agent. The syngas was composed mainly of CO, H2, CH4, CO2, H2O, and N2, as in typical gasification process.

The syngas composition is strongly influenced by the excess air ratio. As the excess air ratio increased, the concentration of CO2

increased sharply, whereas the concentration of H2 strongly decreased.

At a lower excess air ratio, a syngas with a highheating value was obtained. At an excess air ratio of 0.16, a syngas with a LHV of 8.5 MJ/Nm3 was obtained.

The optimum excess air ratio for gasification of briquettes made from agricultural residues was evaluated to be an approximately 0.17–0.20.

AcknowledgmentsThe work of this paper was supported by Project SOP HRD – EFICIENT 61445/2009.

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