Study of the emissions causing respiratory diseases during indoor air gasification

Miranda García-Cuevas, Mª Teresa

Departamento de Ingeniería Mecánica Energética y de los Materiales/ Universidad de Extremadura/ Avda. de Elvas s/n / 06071 Badajoz, España+34924289600 – (Ext. 86772) Fax: 924289601 / tmiranda@unex.esRomán Suero, SilviaDepartamento de Ingeniería Mecánica Energética y de los Materiales/ Universidad de Extremadura/ Avda. de Elvas s/n / 06071 Badajoz, España+34924289600 – (Ext. 86639) Fax: 924289601 / sroman@unex.esNogales Delgado, SergioDepartamento de Ingeniería Mecánica Energética y de los Materiales/ Universidad de Extremadura/ Avda. de Elvas s/n / 06071 Badajoz, España+34924289600 Fax: 924289601 / senogales@alumnos.unex.esMontero Puertas, IreneDepartamento de Ingeniería Mecánica Energética y de los Materiales/ Universidad de Extremadura/ Avda. de Elvas s/n / 06071 Badajoz, España+34924289600 – (Ext. 86725) Fax: 924289601 / imontero@unex.esArranz Barriga, José IgnacioDepartamento de Ingeniería Mecánica Energética y de los Materiales/ Universidad de Extremadura/ Avda. de Elvas s/n / 06071 Badajoz, España+34924289623 Fax: 924289601 / jiarranz@unex.esRojas Moreno, CarmenDepartamento de Ingeniería Mecánica Energética y de los Materiales/ Universidad de Extremadura/ Avda. de Elvas s/n / 06071 Badajoz, España+34924289600 – (Ext. 86690) Fax: 924289601 cvrojas@unex.es

ABSTRACT

This work deals with the study of the emissions associated to the thermal degradation of some densified biomass samples (pellets) made up with mixtures of olive pomace and pirenean oak. The influence of the pellet composition on the emissions generated during air gasification processes was studied and related to the thermal decomposition.

It was proven that the release of the aromatic compounds studied (benzene and toluene) occurs in the temperature range of 275-455 ºC, and is related to the most significant weight loss observed in the TG curves. It was observed that the maximum of the evolved gases is towards lower values of temperature as the content of olive pomace is increased.

Key words

Thermal degradation, Toxic emissions, biomass

INTRODUCTION

The use of biomass as energy source is a promising technique that has been used by human beings thousands of years ago. Nowadays, processes related to thermal conversion of biomass to bio-fuels and other products is gaining prominence due to many economical and environmental reasons (such as global warming associated with fossil fuel exploitation, energetic dependence etc.). Thus, a great effort is being done worldwide to improve the design of biomass conversion processes and to control the variables in order to optimize the products. Moreover, biomass exploitation plants can be implemented in rural areas, which offers an interesting alternative to isolated places in developing countries.

In this sense, biomass (as forest and industrial residues) is commonly used for cooking and heating in developing countries. In these uses, biomass is often burnt inside the houses, under deficient conditions which usually do not favor the complete combustion. Moreover, no analyses on the potential toxicity of the gases generated in these processes are made.

The gas generated during biomass thermal degradation is composed of many chemical compounds, some of which are of concern for health [1]. Among these compounds, particles, polycyclic aromatic hydrocarbons have been identified [2]. Reported respiratory symptoms in adults with chronic exposure to indoor biomass burning processes include bronchitis, respiratory failure, and cor pulmonale [3].

With these premises, the target of this work was to study the aromatic emissions (benzene and toluene) associated to the combustion of some densified biomass samples (pellets) made up with mixtures of olive pomace and pirenean oak, whose potential as energy fuels has already been proven [4]. The influence of the pellet composition on the emissions of these compounds during air gasification processes was studied and related to the thermal decomposition rate.

EXPERIMENTAL

Raw materials

Olive waste (orujillo) and wood residues (pirenean oak) were provided by local manufacturers. These residues were characterized in terms of their proximate analysis following the technical specifications CEN/TS 1474-2, CEN/TS 15148 and CEN/TS 14775, for moisture, volatile matter and ash, respectively [4]. The ultimate analysis was made with an elemental analyzer (Eurovector EA 3000), according to the norm CEN/TS 15104 (for determining the content of C, H and N) and CEN/TS 15289 (for S) [5].

Table 1 shows the elemental analyses of the five samples prepared (content of olive pomace ranging 0-100%). The samples used were named according to Oru-X, where X stands for the percentage of olive pomace corresponding to the densified pellets.

                             Table 1. Ultimate analyses of the densified pellets.              

As it can be inferred from Table 1, the five materials show similar percentages of carbon, close to 50%, as it is typical in lignocellulosic materials. Also, the low values of nitrogen and sulphur are advantageous from the environmental point of view.

The densified pellets were prepared using a GR150E2 pelletizer, following the method described elsewhere [6]. The pellets were made with different proportions of wood and olive pomace (0, 25, 50, 75 and 100%).

Thermogravimetric study and control of emissions

This study was carried out using a coupled TG-MS equipment (Thermogravimetric system, TA instruments; Mass Spectrometer, Pfeiffer Tecnovac Thermostar GDS301 T3). The gas line between the TG and MS was heated to 200 ºC in order to avoid cold points and thus prevent the condensation of some of the gaseous products. The mass spectrometer signals were assigned to the following gaseous species: benzene: m/z = 78 and toluene: m/z = 91.

In order to guarantee reproducibility of results each run was made three times and the signals were normalized to the initial mass of the sample. Thus, it was possible to compare the peak height of the same compound evolved from different samples.

For the TG-MS analyses, the pellets were crushed and an initial mass of 15.0±0.1 mg was used, employing air (100 cm3 min-1) and a heating rate of 20 ºC min-1. The analysis was made in the temperature range 25-750 ºC.

DISCUSSION OF RESULTS

Figures 1 and 2 show the ion intensity profiles of signals m/z of 78 and 91, which can be associated to benzene and toluene emissions, respectively.

Figure 1. Ion intensity of m/z=78 (benzene) for the samples studied.

Figure 2. Ion intensity of m/z=91 (toluene) for the samples studied.

As it can be observed, there is a defined peak approximately centered in 350 ºC in both cases, whose shape seems to be modified for samples Oru-75 and Oru-100. In these samples, the peak is divided into two smaller peaks, being this bifurcation more marked as the content of olive pomace is increased. Also, there is a clear increase in the intensity of the signal of both compounds as the olive pomace content is greater, especially for olive contents higher than 75%.

In order to justify the different pattern found in the profiles of the emissions, we have studied the weight losses associated to the cases Oru-0 and Oru-100, which have been plotted in Figure 3.

In this Figure, several factors have to be pointed out. First, it is noticeable that the TG curves of the two samples exhibit some differences: Oru-0 starts its weight loss at a higher temperature (245 ºC, while Oru-100 starts its weight loss at about 215ºC), which in turn could indicate a lower reactivity in the case of Oru-100. Moreover the weight loss follows a continuous decrease in the case of Oru-0, whereas Oru-100 shows two different slopes; the first weight loss of Oru-100 is more marked than that of Oru-0, probably due to its higher reactivity. On the other hand, the second weight loss of Oru-100 shows a softer slope.

These thermogravimetric characteristics can be related to the profiles found in the emissions of benzene and toluene. According to the results obtained, in the samples with a higher content of olive pomace (Oru-75 and Oru-100), the gases are released in two stages; this could be associated with the fact that during combustion these samples are decomposed in two processes. It was observed that this trend decreased with pirenean oak content, finding that for Oru-0 just one peak is seen, in accordance with the one-stage thermal degradation in the temperature range described was found. At a first glance, this could indicate that olive pomace is composed by a higher proportion of any component that is harder to decompose.

Figure 3. Weight loss curves and ion intensities of m/z=78 (benzene) and m/z=91 (toluene) for samples Oru-0 and Oru-100.

Finally, in order to make an evaluation of the energetic potential of the biomass samples studied here, their high heating value (HHV) was determined. The results obtained have been collected in Table 2.

Table 2. HHV (kcal kg-1 db) of the densified pellets.

As it can be inferred, the HHV show greater values as the olive pomace content increases. Having into account the previous discussion about the intensity of the emissions studied, it could be concluded that the optimal compromise situation involving both interesting energetic characteristics and low emissions of benzene and toluene, could be those corresponding to a content of olive waste below 50%.

CONCLUSIONS

The use of biomass as an energy source olive by means of combustion processes in many applications is often used in developing countries. This offers many environmental and economical advantages such as zero CO2 emissions and independence from fossil fuels. However, indoor combustion processes have to be controlled with respect to their emissions, which can contain several toxic compounds. In this work, gas species evolved during combustion processes of biomass samplesmade up from mixtures of olive pomace and pirenean oak at different proportions, were studied by thermogravimetric analysis coupled with mass spectroscopy (TG-MS). The results indicate that under the experimental conditions studied, the quantities of benzene and toluene increase as the olive pomace content is greater, especially for olive contents higher than 75%. Considering the high heating power of the samples prepared, it is suggested that the optimal conditions correspond to olive pomace contents below 50%.

REFERENCIAS

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5. CEN/TS 335 Biomass standards. Technical specifications CEN/TS-Solid Biofuels (2004).

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