Comparación de las percepciones según experiencias

The operating conditions play a very important role in biomass gasification in all respects, including carbon conversion, product gas composition, and tar reduction. The most important influencing parameters include gasification temperature, steam/biomass ratio, and residence time. The selection of these parameters also depends on the type of gasifier used. The influence of the operating parameters are summarized in Table 2.6.

2.4.1 Gasification Temperature

Researchers have conducted extensive studies reviewing the influence of temperature on tar production during biomass gasification. To achieve a high carbon conversion of the biomass and a low tar content, a high operating temperature (above 800 °C) in the gasifier is recommended. With the increase in temperature, combustible gas content, gas yield, hydrogen, and heating value all increased significantly, while the tar content decreased sharply.

Fagbemi et al [102] showed that tar yields augmented first while temperature rose up to 600oC, and then dropped after this temperature was surpassed. At higher temperatures, primary reactions were less significant and secondary reactions (i.e. tar cracking) prevailed. This led to considerable tar decomposition. Temperature not only affects the amount of tar formed, but also the composition of tar by influencing the chemical reactions involved in the gasification network [9]. Yu et al. [103] reported that tar yield was reduced by more than 40% when the temperature was raised from 700 to 900 °C.

With an increase in temperature, the amount of total oxygen-containing components drastically went down, the amount of substituted 1-ring and 2-ring aromatics also decreased, but the formation of 3- and 4-ring aromatics increased rapidly. An almost 40% increase in naphthalene content was reported at 900 °C. Furthermore, in the combustion zone of the gasifier, temperature plays a dominant role in the reactions between char and oxygen.

While this showed that higher temperatures are favorable for biomass gasification [104– 106], from an overall process perspective, the reduction of ash agglomeration requires lower temperatures [1,6,91]. This may limit, in practice, gasification temperatures up to 750 °C [1,18]. Moreover, Mahishi and Goswami [107] reported that the hydrogen at chemical equilibrium initially increased with temperature, reached a maximum and then gradually decreased at the highest temperatures. Therefore, several factors including tar content, gas composition determining gas heating value and char conversion should all be taken into consideration and weighted carefully in the selection of the gasifier operating temperature.

Table 2.6: Influence of different operating parameters of a gasifier [6,9,108,109]

Operating

parameters Advantages Technical Challenges

Temperature Increase

1.Yields reduced char and tar content

2.Yields higher carbon conversion and reduced methane in syngas 3.Yields increases syngas heating

value

1.Yields a decreased energy efficiency

2.Increases ash-related problems

Increase of pressure

1.Yields low char and tar content 2.Yields a compressed syngas

required for downstream utilization

1.Creates an increased uncertainty given the limited design and operational experience 2.Yields more expensive small

scale gasifier Increase of

S/B ratio

1.Yields low char and tar contents 1.Decreases heating value of syngas

2.4.2 Steam/Biomass Ratio

The steam/biomass ratio (S/B) or equivalence ratio (ER) strongly influences the type of gasification products. In case of air gasification, a high equivalence ratio (ER) results in a lower concentration of H2 and CO as well as in a higher CO2 content in the product gas. Thus, a higher ER decreases the heating value of the syngas. Increasing the ER also has a beneficial effect on reducing tar formation given the greater availability of oxygen to react with volatiles. This phenomenon is more significant at higher temperatures.

On the other hand, an increase in the steam/biomass ratio is expected to produce higher hydrogen and lower CO fractions as a result of the water-gas shift reaction. In addition, excess steam often drives the cracking of higher hydrocarbons and reforming reactions [109]. Nevertheless, the upper limit of steam/biomass ratio is set by gasification stoichiometry. Exceeding this limit yields excess steam in the product gas. The energy associated with excess steam and the enthalpy losses resulting from the unnecessary production of this steam need to be considered in the system energy balances. Such issues demonstrate the importance of selecting an optimal steam/biomass ratio in biomass steam gasification for achieving high process efficiency.

Herguido et al [110] reported the effect of steam/biomass ratio on the products from biomass steam gasification. They observed an increase in H2 (as high as 60%) and CO2 (from 10 to 30%) contents, a sharp decrease in CO (from 35 to 10%) content and a slight decrease in CH4 content when the S/B ratio was increased from 0.5 to 2.5. It also reduced the tars yield from 8% at S/B = 0.5 to almost nil at S/B = 2.5. However, there was a sharp decrease in the lower heating value which was attributed to the decrease in CO.

Steam gasification requires external energy input as it is a endothermic process. The use of some small amounts of oxygen along with steam can provide the necessary heat for gasification. The process is known as auto-thermal gasification. In view of this, many researchers used steam–oxygen mixtures for biomass gasification. Aznar et al [111] reported more than 85% reduction in the total tar when they increased the (steam+O2)/biomass ratio termed as gasifying ratio (GR) from 0.7 to 1.2. They also reported that low GR values produced light tars which could be easily converted using a

catalyst. Gil et al [73] recommended H2O/O2 ratio of around 3.0 (mol/mol) for auto- thermal gasification. They observed a decrease in H2 content from 29% to 13%, a decrease in CO content from 50% to 30%, an increase in CO2 content from 14% to 37%, a slight decrease in CH4 content from 7% to 5% and a change in C2 hydrocarbons from 3.5% to 2.3%, when the GR was increased from 0.6 to 1.7. Tar content of the raw gas was also sharply decreased with GR; with less than 5 g/m3 at a GR of 1.2.

2.4.3 Operating Pressure

Several researchers have investigated pressurized biomass gasification [112]. When the pressure was increased, a reduction in the amount of light hydrocarbons and tars were observed at higher ERs. This occurred with 100% carbon conversion. Although the total amount of tar decreased with greater pressures, the fraction of polycyclic aromatic hydrocarbons increased.

2.4.4 Residence time

Residence time has a significant influence on the amount and composition of the produced tars. According to Kinoshita et al [108], the fraction of oxygen-containing compounds tends to decrease by increasing residence time. Furthermore, yields of one and two aromatic ring compounds (except benzene and naphthalene) decrease with residence time whereas that of three and four ring species increases. Corella et al [113], observed a decrease in the total tar content when the space time was augmented in biomass gasification with a bed of dolomite.