Comparison of the Heating Values of Various Types of Fuel from Non-Wood Raw Materials

The combustion of waste fractions from the use of pruning of olive and orange trimmings used in the production of pulp for paper have been studied and the results were compared with those of other non-wood materials (Hesperaloe funifera, empty fru it bunches from o il palm, and banana trees), also used in the paper industry. Heating values were compared with experimental values estimated with elemental analysis, contents of the main components (cellulose, lignin, ext ractives with ethanol-benzene) and p roximate analysis (volat ile , ash and fixed carbon). The best estimate of values were obtained with the equation that correlates the heating values and the amount of carbon fixed and volatile, which reproduces the experimental values with less than 4% errors The values of flame temperature (between 1100 and 2400 °C depending on the excess air used and heat loss), dew point temperature (between 44 and 54 oC) and the air/fuel ratio (between 4.5 and 9.5 kg air / kg fuel) were calculated. Finally, the kJ prices obtained by combustion of the materials considered in this art icle were compared with those from fossil fuels and proved to be more expensive than the materials studied here (less than 3.4 €/MkJ) .


Introduction
Fro m little mo re than one century ago, the use of wood for the production of cellulose pulp for papermaking has increased progressively, reaching levels of consumption of wood similar to the oil [1]. Th is has led to a wood supply problem, which worsens over time. Fo r this reason, many of the investigations carried out in recent years have focused on finding new raw materials to avoid uncontrolled deforestation with serious ecological problems that occur in ecosystems. In this way the study of various materials has appeared such as agricultural, agro-industry and forest residues and alternative plants to those achieved in agri-food crops.
The use of agricultural and agro-industry residues and alternative plants to food crops seems to be a good alternative to wood raw material, which can lead to excellent papers with special propert ies and can serve as the sole source of raw materials in so me geographical areas.
Moreover, it is kno wn that consumers are increasingly interested in having papers obtained by using clean technol-ogies, or the fibers fro m recycled or non-wood plants; the use of agricultural and agro-industrial residues and non-wood plants could contribute to the preservation and maintenance of the environment, being able to reduce the large amount of wood used as feedstock in the production of paper pulp [1].
On the other hand, since paper consumption is parallel to a county´s standard of living, which is rising throughout the world, it is expected that this increase, in a greater or lesser degree, will continue in the future.
The non-wood raw materials, compared to wood raw materials, have the particularity that contain fractions with litt le use for the production of pulp, such as those formed by leaves, bark, pith and young stems, which have relat ively litt le cellulose content. However, these fractions, which can be called waste, should be processed along with the main fractions, rich in cellulose, to improve the economy of the pulping process. This is what is called biomass biorefinery, which uses all the co mponents of the raw materials [6,7].
The virtually unexp loited potential of this residual biomass invites the development of p rocedures for its use as an energy source. The potential energy of lignocellulosic materials can be explo ited by applying physical-chemical or biochemical procedures for conversion of their chemical energy into other simpler and more readily accessible types of energy [8,9]. The simplest physical-chemical procedure for exp loitation of lignocellu losic materials is combustion. The biomass of forest and agricultural residues (felling, straw, canes, stalks, etc.) has been widely used as fuel that in turn is employed for the production of water vapor or elec-trical energy at small factories. Nowadays these waste materials are interesting as energy sources, using them in combustion processes [10][11][12][13][14].
Co mbustion is the oldest alternative to the energy exploitation of bio mass. Its efficiency, like that of other thermal and physic-chemical methods, is marked ly conditioned by the moisture content of the biomass, which can amount to 15-20% even in dry materials, The moisture content of biomass used for energy production should never exceed 60%; otherwise, its heating value would be lower than that required to evaporate the moisture. The thermal y ield of combustion is typically 20-22% fo r a 25% excess of air and biomass with a mo isture content of 50%. Exceptional yields of up to 30% have also occasionally been reported [15,16].
In this paper the heating values and cost of the heat unit obtained by combustion of various lignocellu losic residues with those of fossil fuels have been compared. Have also been proposed equations for the prediction of heating values of such residues from their co mposition. Finally, with the help of elemental analysis, the flame temperatures of the lignocellulosic residues have been calculated as well as the dew point temperature of the combustion gases and air/fuel ratio required.

Heating Value
The gross calorific values (constant volume) were determined according to "CEN/TS 14918:2005 (E) So lid biofuels-Method for the determination of the calorific value" and UNE 164001 EX standards by using a Parr 6200 Isoperibol Calorimeter

Materials
The following lignocellulosic materials have been investigated: residual fractions (mainly leaves and young stems) of orange and olive pruning (which have been called residual orange and residual olive), and three non-wood materials for the pulp industries: EFB (empty fru it bunches), Hesperaloe funifera and banana stalks

Results and Discussion
The results of the elemental analysis and the contents of cellu lose, lignin, ethanol-benzene ext ractable, ash, volatile and fixed carbon are presented in Tables 1 and 2. The carbon content is relatively close to each other and is very similar to those found in the literature [15,16] fo r other lignocellulosic materials; wheat straw, sunflower stalks, vine shoots, cotton stalks and corn stalk stems. The lowest value for the case of banana trees and the highest for the olive pruning, whose wood proportion is higher, should be noted.
In relation to the hydrogen content can be indicated the same that in the case of the carbon content, also being the lowest value for the case of the banana and the highest in olive pruning The nitrogen content is low and differs more fro m one material to another, as observed in the residual fractions which have h igher values such as in the banana tree and Hesperaloe funifera, wh ich are less woody.
The sulphur percentages are very low, so the fuel gases of these lignocellu losic materials were poor in SO 2 , co mpared to gases from the co mbustion of fossil fuels. The cellulose content varies from 55% to the banana tree to 78% for H. funifera, and the lignin content varies from 10%H. funifera to 22% for the banana tree. Similar results were found for other lignocellu losic materials: wheat straw, sunflower stalks, vine shoots and cotton stalks [16].
The extracts values are very high for olive tree p runings and their residual fraction co mpared to other tested materials, which are in the order of other lignocellulosic materials such as wheat straw, sunflower stalks, vine shoots and cotton stalks [16].
The ashes values are abundant in the residual fraction of orange pruning and banana tree, and plenty for other materials tested, with respect to the pruning of olive trees, which are in the order o f hardwoods and softwoods [15,16].
Vo latile chemicals are high (78-80%), except fo r the EFB and banana tree, wh ich are relat ively lo w co mpared with other lignocellulosic materials: wheat straw, sunflower stalks, vine shoots and cotton stalks [16].
Finally, the fixed carbon is very high for EFB and very low for residual fraction fro m orange pruning, being intermed iate for other materials and those found in the literature (16). Table 3 presents the experiment results of the heating values of lignocellulosic materials tested.
In the literature [17,18] (5) where C is the total carbon content (%), Ce, L, E and A the contents of cellulose, lignin, extract ives and ash (all in%), and Ce' the cellulose content on an extractable-free basis (%).
By apply ing of the experimental data showed in Tab les 1 and 2 in the equations (1) to (5), the heating values presented in Table 3 were obtained, which also shows the values of the errors in the estimates for the experimental heating values.
As it can be seen, equation (3) is the best at reproducing the heating values of the test materials (errors less than 8% in the worst case for the residual olive pruning). Equation (1) reproduces the heating values with errors less than 7%, with the exception of the banana tree that reproduces an error close to 11%. Equation (2) reproduces the heating values with errors less than 7%, with the exception of olive pruning which reproduces with an error of almost 13%. Equations (4) and (5) reproduce good heating values of lignocellulosic materials, except those containing high values of ashes: residual orange tree and banana tree.
By correlating the experimental data (heating values against the sum of volat iles (V) and fixed carbon (Cf) contents) it is possible to obtain: HV = 123. 61 T + 6746.05 (6) where T is the sum of the volat iles and fixed carbon contents. This equation reproduces the heating values of all materials tested with errors less than 4%.
In the literature there is a similar equation for agricu ltural residues (16): HV = 339.82 T -14814.93 (7) The heating values of the materials tested here, are re-produced with small errors, except for materials with a high ash content, having the equation (7) the same defects as equations (4) and (5).
Jiméne z et al [15] also proposed a similar equation for the case of agricultural residues, food industry waste and forest residues from eucalyptus and holm: HV = 313.30 T -10814.08 (8) The above equation reproduced properly the heating values of the materials considered, apart fro m the banana tree which has a 12% error.
Using the elemental analysis of materials considered ( Table 1) and following the estimat ion techniques described in the literature [19,20] determines the values of flame temperature, dew point and air/fuel ratio, presented in Figures 1 to 9.
The high values of flame temperature demonstrate the possibility of using these materials in the production of steam.
The dew po int of all materials tested is low for co mbustion gases, thus avoiding condensation in chimneys and flue pipes, preventing corrosion that could cause the condensation; anyway, in the event of such condensation, the phenomenon would not be very serious given the small sulphur content of the material considered. This is an addit ional advantage that makes these fuels clean.         The cost of the various ligmocellu losic residues considered here are d ifferent. Thus, residual fractions fro m orange tree pruning and olive tree pruning, and empty fru it bunches fro m o il palm are the cheapest, since they only have to consider the costs of upgrading the energy plant (they are paper industry waste (residual fractions fro m pruning)) or agrofood (EFB), and other costs not required for other lignocellulosic residues, such as data collection in the fields and their transport to the combustion plant. As can be seen in table 4, the MkJ of energy obtained by combustion of industrial waste is cheaper than that obtained fro m the agricu ltural residues (olive tree and orange tree and banana tree pruning), which in turn is cheaper than the one obtained fro m mineral coal and even much cheaper than the one obtained from fossil fuel fluids. Moreover, we should emphasize some o f the advantages of the lignocellulosic residues studies: they are renewable and release very small amounts of sulfur dio xide in co mbustion gases and smaller amounts of ash than the solid fossil fuel, so, at the wo rst, they are good competitors with fossil fuels.

Conclusions
The values of residual heating fractions fro m o live tree and orange tree pruning have been determined and compared with those of other non-wood materials (Hesperaloe funifera, empty fruit bunches fro m o il-palm and banana tree), also used in the paper industry.
Heating values are compared with experimental values estimated with elemental analysis, contents of the main components (cellu lose, lignin, extractives with ethanol-benzene) and pro ximate analysis (volatile, ash and fixed carbon). The best estimates are obtained with the resulting equation to correlate the heating values and the amount of volatile and fixed carbon, which reproduces the experimental values with errors less than 4%.
The values of flame temperature (between 1100 and 2400 °C depending on the excess air used and heat loss) have been calculated, indicating that these lignocellulosic materials are suitable for the production of steam. The dew point temperature o f flue gas (44 to 54 ºC) has also been given, which when low prevents condensation of water vapor in chimneys and other potentially corrosive systems. It has been found that the air/fuel rat io is between 4.5 and 9.5 kg air/kg fuel, depending on the excess air used.
After comparing prices, the MkJ obtained by combustion of the materials considered here compared with those from fossil fuels, the cost of the materials studied here were much cheaper (less than 3.4 €/M kJ) .