Characterisation and empirical correlation analysis for the woody biomass.

Introduction

Biomass gasfiers are being developed around the world today to produce C[O.sub.2] neutral energy. Gasification is the thermal process where biomass is converted into combustible producer gas [1]. The main components in producer gas are [N.sub.2], [H.sub.2],CO,C[O.sub.2]and C[H.sub.4]. Solidfuels are gasified in a closed chamber using air as the gasifing medium under a slightly negative or positive pressure with respect to the atmosphere. [2]

The thermo chemical process involved is complex and the gasifiers have four separate zones like Drying, Pyrolysis, Oxidisation, and Reduction. The gasification stages takes place at the same time in four different zones of the gasifier. Figure 1 illustrates the various gasification stages with products and the temperature characteristics in different phase.

[FIGURE 1 OMITTED]

Drying

Generally the combustible biomass may have the moisture content varies from 5-40 percent at temperatures in excess of 100 oC, the water content is removed and converted into vapour. The biomasses do not undergo any change in chemical composition.

Pyrolysis

Pyrolysis is the process of thermal decomposition of the biomass fuels in the absence of oxygen [1, 2, 3] Figure 2 illustrates during the pyrolysis the products released are solids, liquids and gases. The products proportions are depend on the chemical composition of the fuel and the working conditions. The lower heating value (LHV) of the gas produced by a process of pyrolysis is low.

[FIGURE 2 OMITTED]

The pyrolysis rate depends on several factors such as

1. Composition, shape and size

2. Heating rate

3. Residence time

4. Temperature level

5. Pressure.

Slow pyrolysis is adopted to maximise solid char while fast pyrolysis is used for getting more of liquid and gaseous products [1.4]

Regardless of the type of gasifier, there will be always being a low temperature zone where pyrolysis takes place, generating condensable hydro carbons. [2]

Oxidation

The air is fed through the oxidation zone contains not only oxygen and water vapour, but also inert gases such as nitrogen and argon. [2.5]oxidation takes place at temperatures 700[degrees]C -2000[degrees]C and the gases do not take part in the chemical reactions with the constituents of the fuel.

C+[O.sub.2] = C[O.sub.2] +406MJ/kmol

Heterogeneous reactions take place between the oxygen in the air and the carbonised solid fuel and the main product of a complete oxidation process is carbon dioxide. The hydrogen content in the fuel reacts with oxygen in the air, producing the vapour.

[H.sub.2]+ 1/2 [O.sub.2] = [H.sub.2]O +406MJ/kmol

Reduction

In the reduction zone the high temperature chemical reactions taken place in the absence of oxygen. The following are the main reactions [1, 2, 5 6 and 7]

The Boudouard reaction

C[O.sub.2] +C = 2CO -172MJ/mol

The water gas reaction

C+ [H.sub.2]O =CO+ [H.sub.2] -131MJ/mol

The vapour producing reaction

C[O.sub.2] + [H.sub.2]=CO+ [H.sub.2]O +41.MJ/mol

The methane producing reaction

C+2 [H.sub.2] =C [H.sub.4]+75 MJ/mol

The heat is needed in the reduction processes, so the temperature is reduced in this phase. If the gasification is complete, all the carbon is burnt or reduced to carbon monoxide. As a result it gives combustible gas and char and ash residues. The heating value is depending on the composition of the hydrogen carbon monoxide, carbon dioxide and methane in the gaseous mixture.

Dry product gas low heating value is obtained by the following equation LHV (kJ/[Nm.sup.3])[7]

LHV = (30.0 CO +25.7 [H.sub.2] + 85.4 C[H.sub.4]+151.3 Cn Hn)*4.2(kJ/ [Nm.sup.3])

Where CO, [H.sub.2], etc are the product gas concentrations.

The present study lies in its focus on the selected Eucalyptus woody biomass for calculating the LHV as a function of the process temperature.

Calculation procedure

The procedure is illustrated considering eucalyptus as the biomass to gasify, but it can be extended directly to other commonly used materials, for which only the final results of the study are reported here.

Eucalyptus is wood and it's having lots of application apart from using as a fire wood. The availability of wood in the southern part of India is more so it can be used as a fuel for the biomass gasfiers for producing electricity as well as thermal applications such as process heating and cooking.

The present experimental work was carried out in the 20kW thermal mode biomass gasfier supplied by M/s associated engineering works, Tanku. Hyderabad, India and erected in the campus of Periyar Maniammai University, vallam, Tamilnadu for the institutional hostel cooking purposes.

For various temperatures the 10 gas samples were taken and analysed the average composition is tabulated in the table 1 and the table 2 illustrates the ultimate analysis of the wood.

[FIGURE 3 OMITTED]

From the gasification data the chemical composition of the gas depends on the maximum temperature of the process. These results can be given in the form of graph, thus emphasising how, within the temperature range considered, the composition changes quite dramatically as a result of the chemical reactions that govern the phenomenon (Fig 3)

The volumetric composition of the gas is known, the calculation algorithms are used to obtain four different types of equation to correlate the LHV of the gas with the temperature of the process. The experimental values are compared with the calculated values using the proposed equations. Figures compare the LHV curve based on the temperatures with the curves by the correlations of exponential, power, logarithmic and polynomial. For each curve the mathematical expression that represents it is specified.

All the trends (within the range considered) were found to rise in temperatures despite of the diversity. A higher temperature corresponds to a higher energy content of the gas in terms of LHV

Table 3 illustrates the correlation coefficients (CC) between the experimental data and those obtained with the equations.

The relative errors(R) committed using the equations for predictive purpose together with their mean of squares are shown in Table.4

By observing the correlation coefficients and the mean percentage of [R.sup.2], the best matches are obtained with the polynomial, exponential and power equations.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Summary charts

All the results recorded are summarised in this section. In addition to the parameterisation of the polynomial and power correlations, the tables include the temperature range in which they are validated with the relative error that is committed by using the equation to calculate the LHV of the gas at the initial temperature in said range of validity.

Polynomial correlation

The polynomial correlation is of the type:

LHV = A [X.sup.2] + B X + C (kJ/[Nm.sup.3])

A,B,C are the parameters to use in the correlations according to the type of biomass, [DELTA]X is the correlations range of validity expressed as the initial to the final temperature in [degrees]C and [[DELTA].sub.i] % the percentage relative error committed in the temperature range of validity. Table 5 and 6 illustrates the correlation parameters used in the literatures for different types of biomass. [3]

Power correlation

The power correlation is of the type

LHV = A XY (kJ/ [Nm.sup.3])

All the parameters are contained in the Table 6all the symbols have the same meaning of Table 5 collected from the literature of various biomass. [3]

Exponential correlation

The exponential correlation is of the type

LHV = [Ae.sup.BX] (kJ/[Nm.sup.3])

All the parameters are contained in the Table 6all the symbols have the same meaning of Table 5 collected from the literature of various biomass. [3]

Conclusions

This paper contributes to estimate alternative method for calculating the LHV of the gas. The analysis of Eucalyptus wood gas was performed and the assessment of the relative error s and empirical calculations considered, the Exponential equation is the one that most effectively correlates the LHV and gasification temperature variables. . This approach has made it possible to disregard the conceptual difficulties that a strict study of the reactions concerned in the gasification process would involve, given their complexity, which is influenced by an enormous number of parameters that are difficult to monitor and would have demanded on excessive burden of experimental work. Combining a mathematical formula with the representation of the trend of the LHV = f (T) for each of the materials analysed would even so to facilitate the optimisation of the gasification processes with a view, to obtain the best possible quality in terms of energy content of the gas, or comparing different types of biomass. And the parameters A,B,C values are formulated for the Eucalyptus wood.

References

[1] P.V. Iyer. Thermochemical characterisation of biomass book published by IIT Delhi

[2] Reed T.B. Biomass gasification Principles and Technology,1981(Noyes data Corporation, Parkridge, New Jersy, U.S.A),pp.264,291

[3] 3 L. Bomprezzi, P. Pierpaoli and R. Raffaelli. The heating value of gas obtained from biomass gasification : a new method for its calculation or prediction. Proc Instn Mech Engrs Vol 216 Part A: J Power and energy pp. 447-452.

[4] Hoque,M.M. and Bhattacharya, S. C. Fuel characteristics of gasified coconut shell in a fluidized and a spouted bed reactor.Energy,2001,26,101-110.

[5] P.M. Lva,, Z.H. Xiong, J. Chang, C.Z. Wu, Y. Chen, J.X. Zhu. An experimental study on biomass air-steam gasification in a fluidized bed. Bio resource Technology 95 (2004) 95-101.

[6] Elliott, D.C., Butner,R.S. and Sealock, L.J. Low temperature gasification of high moisture biomass, International Conference on Research in Thermochemical biomass conversion, Phoenix, Arizona,U.S.A., research in thermochemical biomass conversion,1988 (Elsevier Science Publishers LTD, London and Newyork), pp696-710.

[7] Williams, P.T. and Nugranad, N. Comparision of products from the pyrolysis and catalytic pyrolysis of rice Husks. Energy 2000,25,493-513.

[8] Reed T.B. Biomass gasification Principles and Technology,1981(Noyes data Corporation, Parkridge, New Jersy, U.S.A),pp.134,135.

[9] Reed T.B. Biomass gasification Principles and Technology,1981(Noyes data Corporation, Parkridge, New Jersy, U.S.A),pp.266-269.

[10] A.V. Bridgwater, G.V.C. Peacocke. Fast pyrolysis processes for biomass Renewable and Sustainable Energy Reviews 4 (2000) 1-73.

[11] J. M. Encinar, F. J. Beltran, A. Bernalte, A. Biro and J. F. Gonzalez. Pyrolysis of two agricultural residues: Olive and Grape bagasse. Influence of particle size and temperature. Biomass and Bio energy Vol. 11, NO. 5, pp. 397409, 1996

P.K. Srividhya*, S. Jayaraj and C. Muraleedharan

Department of Mechanical Engineering

National Institute of Technology Calicut

NIT Campus (P.O), Calicut, Kerala, INDIA. PIN-673601

*email- [e-mail]

Table 1: Volumetric composition of the gas (kg/Nm3/ of Eucalyptus wood)

Temperature [degrees]C   CO%    C[O.sub.2]%   C[H.sub.4]%   [H.sub.2]%

650                      20.2    9.6          0.78          16.3
700                      20.9    9.7          0.80          16.5
750                      21.1    9.9          0.91          16.8
800                      21.3   10.0          0.94          17.1
850                      21.5   10.2          0.98          17.3
900                      21.8   10.4          0.99          17.4
950                      22.1   10.7           1.0          17.5
1000                     22.3   10.8           1.1          17.5
1050                     22.5   10.8           1.2          17.6

Table 2: Ultimate analysis of wood on dry basis

Ultimate analysis   % by wt. dry basis

Carbon                    46.3
Hydrogen                   5.82
Oxygen                    44.49
Nitrogen                   0.3
Sulphur                    0.01

Table 3: Correlation coefficients between calculated LHV and
experiment of Eucalyptus

Temperature                  LHV(kJ/N[m.sup.3])
[degrees]C
              Experimental   Polynomial   Exponential   Power

650           4584.39        4636.623     4615.754      4616.525
700           4701.35        4731.973     4682.655      4697.273
750           4809.15        4821.323     4750.525      4773.716
800           4855.97        4904.673     4819.379      4846.349
850           4927.86        4982.023     4889.232      4915.585
900           4980.04        5053.373     4960.096      4981.767
950           5032.23        5118.723     5031.988      5045.190
1000          5093.298       5178.073     5104.922      5106.105
1050          5165.16        5231.423     5178.913      5164.730
[R.sup.2]     1              0.995847     0.999867      0.998655

Table 4: Percentage of relative error squared at various
temperatures for the correlation of Eucalyptus and their means

Temperature                 [R.sup.2]
[degrees]C    Polynomial    Exponential   Power

650           1.2982        0.4680        0.4914
700           0.4243        0.1581        0.0075
750           0.0641        1.4860        0.5429
800           1.0059        0.5678        0.0393
850           1.2081        0.6145        0.0621
900           2.1684        0.1604        0.0012
950           2.9542        0.0000        0.0663
1000          2.7704        0.0521        0.0632
1050          1.6458        0.0709        0.0001
Mean          1.5044        0.3975        0.1415

Table 5: Polynomial correlation Parameters

Biomass           A (MJ/N[m.sup.3])       B(MJ/N[m.sup.3])

Rubber           2.821 x [10.sup.-4]           -0.57
Plastic                 -83.7                  0.106
Manure           3.679 x [10.sup.-4]          - 0.517
Rice husks              -23.42                 0.038
Bagasse                 -15.8                  0.151
Paper                  -102.53                 0.158
Compost           3.077 x[10.sup.-4]          - 0.123
Sawdust           1.758 x[10.sup.-4]           -0.248
Dry wood                -29.7                  0.048
Coconut shells          -27.19                  0.04

Biomass            C(MJ/N[m.sup.3])     [DELTA]X [degrees]C

Rubber                 292.271                743-1015
Plastic                -17.392                500-850
Manure                 192.857                617-711
Rice husks              -1.859                400-600
Bagasse                -26.676                400-800
Paper                  -45.602                500-900
Compost                 16.687                250-400
Sawdust                 98.492                650-800
Dry wood               -11.697                627-1127
Coconut shells          -13.51                627-842

Biomass           [[DELTA].sub.i] %

Rubber                   0.18
Plastic                 -3.38
Manure                   6.78
Rice husks              -2
Bagasse                 -3.3
Paper                    0.67
Compost                 -0.1
Sawdust                 -0.52
Dry wood                 2.7
Coconut shells          -2.7

Table 6: Power correlation parameters

Biomass                 A (MJ/N[m.sup.3])     B (non Dimensional)

Rubber                  1.256x [10.sup.18]          -5.8408
Plastic                 1.440 x [10.sup.1]           0.0308
Manure                  2.093 x [10.sup.5]          -1.4999
Rice husks             9.136 x [10.sup.-2]           0.7925
Bagasse                       2.138                  0.3412
Paper                  6.424 x [10.sup.-3]           1.1893
Compost                4.186 x [10.sup.-6]           2.5367
Sawdust                8.153 x [10.sup.-1]           0.4027
Dry wood               6.024 x [10.sup.-1]           0.4259
Coconut shells         1.599 x [10.sup.-4]           1.5228

Biomass                [DELTA]T [degrees]C     [[DELTA].sub.i] %

Rubber                       743-1015                 -12
Plastic                      500-850                  4.3
Manure                       617-711                  4.5
Rice husks                   400-600                 -1.3
Bagasse                      400-800                  8
Paper                        500-900                  14
Compost                      250-400                  0.35
Sawdust                      650-800                 -3.8
Dry wood                     627-1127                 9
Coconut shells               627-842                 -2.7