Studies On Gasification Characteristics Of Brown Coal In Mild Conditions

The utilization of coal will dominate in our country's energy consumption for a long time in the future as there are relative abundance of coal resource, relative lack of oil and gas in our country. The effectively clean utilization of brown coal is the method meeting the needs of increased energy and solving the environmental problem. Compared with the high rank coal, brown coal with high oxygen content, high volatile, abundant alkali and alkaline earth metals is a good gasification material. It can be gasified at relatively lower temperature to obtain higher gasification efficiency. The reaction characteristics of brown coal are different from high rank coal during pyrolyzation and gasification in which it has higher reactivity. Therefore, it cannot be directly employed commercial gasification technology, the novel gasification process need to be developed. The new type gasification technology is that volatiles and char was reformed and gasified respectively with its being decoupled at relative lower temperature employed simulated entrained-flow reactor with high heating rate and the syngas was improved by catalytic conversion. It is necessary that studies on gasification performance of brown coal employed laboratory entrained-flow reactor to offer theoretical guidance and fundamental parameter for design and process development of gasifier.Aimed at features of brown coal, the small entrained-flow reactor was constructed for studying the gasification behavior of brown coal in various atmospheres at relative lower temperatures. The changes rule of gasification products was summarized. The microstructures of gasified char were measured, illustrating variation of aromatic rings system and O-containing functional groups, effects of char structure on volatilization of AAEM and relevance of char reactivity in the process of brown coal gasification. Effects of residence time and particle size on reaction behavior of brown coal in H2 O atmosphere were also investigated, clarifying major factors impacted gasification performance for different diameter of brown coal. The reactivity of subsequent char-O2 and char-CO2 were analyzed, and reaction kinetics function with CO2 was constructed with pre-exponential factor and activation energy being calculated. The gas included a small quantity of tar due to gasification in the low temperature need to be catalytic decomposed for improving gas quality and decreasing the adverse impact of tar. The activated char was prepared from Shengli brown coal, which feasibility for catalytic conversion to improve syngas quality was measured using it as catalyst or catalyst supporter. The experimental results offer fundamental data and theoretical guidance for the development of novel gasification technology. The main conclusions are as follows:(1) The experimental results from studies on gasification characteristics of SL brown coal in various atmospheres showed that carbon conversion in H2 O was gradually increased with the increased concentration of H2 O, and which increased significantly with addition of oxygen in the experimental conditions. When the oxygen was added to steam, H2 content was increased and the contents of CO2 and CO were dependent on oxygen. The O-containing functional groups like carbonyl and carboxyl decreased significantly, and aliphatic group as –CH3-,-CH2- decreased sharply after gasification. Aliphatic structure and O-containing group broke down to increase the order of structure with addition of oxygen. The steam played important role on increasing the aromatic ring systems, enhancing the order of structure. More small rings and amorphous carbon structure were reacted and consumed which promoted the increased order of char structure with addition of oxygen. These changes of char structure impacted subsequent char reactivity. The reaction index of subsequent char could be decreased by steam, further decreased with addition of oxygen. In the oxygen atmosphere alone, the lower concentration of oxygen played fewer roles on char reactivity.(2) The results by studying on physical and chemical structure of char indicated that steam could decrease the ratio of small fused rings and big fused rings and enhance the order of structure during gasification in H2 O and H2O+1%O2 atmosphere. The addition of oxygen play less role on aromatic structure at 800? and show important impact on structure at 900?. The effects of 1%-3% oxygen on changes of structure were less. The variation of O-containing structure was caused by oxidation and deoxidation reaction. The O-containing groups were decreased due to the major deoxidation reaction in H2 O atmosphere. The oxidation reaction was strengthened gradually with the increase of steam after oxygen being added and enhanced the O-containing groups as the temperature increasing to 900?. In the oxygen atmosphere, O-containing structure in char was increased gradually with increased oxygen. Based on the XPS analysis, the oxygen content of char was mainly decided by C=O group at 800?, COO- group played larger role at 900?. The volatilization of AAEM was influenced by the evolution of char structure. The impact of addition of oxygen on volatilization of Na followed different pattern at 800? and 900? in H2 O and 1%O2 atmosphere. At 800 ?, Oxygen promoted the volatilization of Na but inhibited volatilization at 900?. The interaction of char structure with AAEM would influence char reactivity. The char reactivity enhanced with increased ratio of small rings and big rings, and which increased with the increases of O-containing functional groups. The reaction index correlated well with Band area ratio, I(GR+VL+VR)/ID. The char structure was dominant factor influencing the transfer of AAEM and variation of char reactivity.(3) The research results for pore structure of char showed that shapes of adsorption isothermal curves obtained from the chars in various atmospheres were Type II and the shapes of the adsorption loop curves were mainly same. These indicated that there was a relatively continuous and integral pore system of the char with similar pore structures. The pore diameter distribution, pore quantity and pore volume were influenced by reaction atmosphere. According to pore volume and pore size distribution, the total pore volume of char from gasification in H2 O and 1%O2 atmosphere was more than the sum of pore volume for chars which from H2 O and which from 1%O2. The quantity of macropores of char was less in three atmospheres; the order from more to less of mesopores were H2O>O2>H2O+1%O2. The specific surface area of char presented the trend of increasing at first, then being steady with the increase of H2 O. The specific surface area of char during gasification in H2 O, 1%O2 mixture atmosphere increased linearly, and which also increased linearly with the increase of O2 in the low concentration of oxygen. The specific surface area of char from gasification in H2 O, 1%O2 mixture atmosphere was far more than which in H2 O or 1%O2 atmosphere alone. The relational expression for conversion versus ratio of micropore volume and total pore volume, and conversion versus specific surface area was X=0.196(Vm/V) +45.651, X=0.037S+48.066.(4) The temperature and particle size were major factors influencing carbon conversion but residence time play less role on conversion during steam gasification of brown coal with different particle size. At experimental temperature, in the steam gasification process of brown coal, internal diffusion do impact on gasification reaction in a certain degree for brown coal with big particle size, and played minor roles in small particle size. The gas composition was enhanced in a certain extent by secondary reaction of pulverized coal with big particle size, which was chiefly influenced by residence time for small particle size, decreasing with decreased residence time. The gas residence time did important impact on volatilization of Na species at the same gasification temperature. The concentration of Mg and Ca in char increased with the decreased residence time during brown coal gasification with small particle size, and which variation was less during gasification for big particle size. The concentrations of Mg and Ca in char from small particle size were more than which from big particle size. The retention of Mg and Ca in char from small particle size was mainly influenced by residence time and which from big particle size was impacted by internal diffusion and residence time.(5) The char-O2 reactivity and char-CO2 reactivity were measured used TGA which indicated that for the char from big particle size brown coal, there was minimum value for char-air reactivity in various gas-flows. The effects of gasifying agent flow on reactivity increased with increased temperature. The char reactivity with oxygen from brown coal with small particle size decreased with increases of residence time. Temperature played important role on char reactivity with CO2, higher temperature could activate inert components in char, decreasing the impacts of other factors.(6) The random nuclear model could well explain relation of carbon conversion versus time during char-CO2 reaction by studying the kinetic of char from gasification of brown coal. The difference of activation energy calculated using [-ln(1-x)] 1/2 and [-ln(1-x)] 1/3 was less, which were around 75-110 k J· mol-1. The activation energy of char during brown coal CO2 gasification with small particle size was slightly lower than which for char from brown coal with large particle size, indicating gasification reaction for char from brown coal with small particle size was conducted easily.(7) The char from brown coal could in-situ catalytic reform the volatiles as catalysts or catalyst support. Experimental results showed the char, Ca/char, and Ni/char catalysts had good activity for the catalytic conversion of volatiles containing tar, gas, etc. derived from brown coal steam gasification to produce hydrogen-rich syngas. All of the catalysts could promote the water-gas shift reaction, which enhanced H2 content and decreased CO content. The Ca/char and Ni/char catalysts showed high catalytic activity for promoting H2 production during brown coal steam gasification, whereas the catalytic effect of char alone was relatively small. Experiment results verified activated char from brown coal was an excellent catalyst support. The O-containing groups of catalysts played significant roles in the catalytic reforming of gas products derived from brown coal gasification. The disordered/amorphous carbon structure of the catalyst determined the activity during the catalytic steam gasification of brown coal. The pore structures of catalysts may not be dominant factors influencing the activity of catalysts in catalytic conversion and catalytic gasification processes, implying instead the importance of carbon structure and reactivity of the catalyst for these purposes.