Abstract: The steam gasification characteristics of biomass char, coal char and the mixture of biomass char and coal char were studied in a thermobalance device. The steam gasification experiments of biomass char (rice stalk char, sorghum stalk char and corn stalk char), Shenmu coal char and the mixture of biomass char and coal char were carried out by temperature programmed thermogravimetry. The results show that the gasification rate of biomass char and coal char at a certain temperature is: sorghum char> rice stalk char> corn char> Shenmu coal char. The kinetic analysis of the steam gasification reaction of three kinds of biomass coke, coal coke, bio-coke and coal coke mixture is carried out, and the analysis shows that the continuous reaction model can reflect the steam gasification reaction dynamics of the coke sample to a certain extent.

0 Introduction

As a kind of renewable energy, the efficient and clean utilization of biomass has attracted more and more attention. Pyrolysis gasification of biomass is an efficient way of energy utilization, which can produce fuel gas or chemical synthesis gas. China's crop straw yield is large, efficient and clean large-scale utilization of backward technology, resulting in serious waste of biomass resources. The energy density of biomass is low, the temperature of separate gasification is low, and more tar is generated during gasification, which not only reduces the utilization efficiency of biomass, but also adversely affects the stable operation of the gasification process; in addition, the supply of biomass is affected by the season, so that the scale of separate gasification of biomass is limited.

The gasification temperature of coal is high, and the co-gasification of biomass and coal can not only improve the gasification efficiency of biomass and reduce the generation of tar, but also solve the seasonal problem of biomass supply and provide a new technical way for the efficient utilization of biomass. Studies have shown that the co-gasification process of biomass and coal may have a synergistic effect [3,4], mainly because of the high volatile content of biomass, the high reactivity of biomass char and the good catalytic effect of alkali metals in biomass ash on the gasification process of coal char. The research on co-gasification of biomass and coal at home and abroad is still in the laboratory stage of small pilot scale. In this paper, the steam gasification characteristics of three crop straw chars and one coal char and their mixtures were investigated using a thermobalance, aiming to provide some basic data for the fluidized bed gasification of biomass and coal mixtures.

1 Experimental part

In order to investigate the gasification characteristics of different biomass and biomass and coal mixtures, the steam gasification experiments of biomass char (rice stalk char, sorghum stalk char and corn stalk char), coal char and their mixtures were carried out on a thermobalance. The experiment was carried out on the ZRP-2P thermal analyzer produced by Shanghai Balance Instrument Factory, and a water vapor generator was added to the original thermal balance. The device is shown in Figure 1. Rice stalk (D), sorghum stalk (G), corn stalk (Y) and Shenmu coal (SM) were respectively placed in vessels with lids, put into muffle furnace, and kept at 900 ℃ for about 7min to prepare rice stalk coke (DJ), sorghum stalk coke (GJ), corn stalk coke (YJ) and Shenmu coal coke (SMJ). The coke sample is ground and the sample below 80 mesh is taken as the material for the thermogravimetric experiment. Rice stalk coke (DJ), sorghum stalk coke (GJ) and corn stalk coke (YJ) were uniformly mixed with Shenmu coal coke (SMJ) at a mass ratio of 1:4, and the samples were recorded as DSMJ,GSMJ and YSMJ, respectively, as raw materials for co-gasification of the mixture. The industrial analysis and elemental analysis of the samples are shown in Table 1 on page 45.

2 Results and discussion

Separate gasification of 2.1 biomass char and coal char

Different materials were carried out under the same experimental conditions, each time about 10mg, water vapor flow rate was 25 mL/min, nitrogen flow rate was 75 mL/min. The experiments were conducted between room temperature and 1200°C with a heating rate of 20°C/min. According to the relevant raw data obtained by the thermal weight meter, the carbon conversion rate X of the steam gasification of the coke sample 2 is calculated by the formula (1), and the temperature relationship diagram of the carbon conversion rate 2 is plotted (see Figure 2). The reaction rate k of the coke sample is calculated from the continuous reaction model equation (2), and the reaction rate 2 temperature relationship is plotted (see Figure 3).

Figure 2 shows the relationship between the thermal weightlessness process of gasification of three biomass chars and Shenmu coal chars in a water vapor atmosphere and the change of carbon conversion rate with temperature. As can be seen from Figure 2, the gasification temperature of biomass char is low,

The gasification reaction of the three kinds of biomass char starts at about 700 ℃, while the coal char starts to react with water vapor at 850 ℃. Figure 3 shows the reaction rate 2 temperature relationship for biomass char and coal char. It can be seen from Figure 3 that the reaction rate increases with the increase of temperature, and the gasification reaction rate of the three types of biomass is greater than that of coal char at the same temperature. From the industrial analysis, it can be seen that the volatile content of the three kinds of biomass is not much different, but it is much larger than that of Shenmu coal. In the process of coking, the removal of a large number of volatile matter increases the pore specific surface area of biomass coke, and the gasification agent is easy to contact with the solid surface, so the reaction rate is faster.

The gasification rate of the three biomass chars at a certain temperature increases in the order of GJ>DJ>YJ. Due to the different types of biomass, the amount and structure of cellulose, hemicellulose and lignin are also different, and the type and amount of minerals are also different, which will lead to different gasification reaction rates.

Co-gasification of 2.2 biomass char and coal char

In the co-gasification of biomass char and coal char, the mass ratio of biomass char to coal char is 1 ∶ 4, and the experimental conditions are the same as those of separate gasification. The steam gasification carbon conversion rate X of coke sample 2 is calculated by formula (1), and the results are shown in Figure 4. As can be seen from Figure 4, the weight loss process of the three biomass char and coal char mixtures is basically the same, mainly because the weight of coal char in the mixture is relatively large, accounting for 80%. It can also be seen from Figure 4 that the order of the increase of the conversion rate of the mixed char at the same temperature is GSMJ>DSMJ>YSMJ, which is the same as the order of the reaction rate of the biomass char. Robert et al. [5] have shown that potassium in biomass catalyzes the gasification of coal char when potassium-rich biomass is co-gasified with coal. Among the biomass used in this experiment, GJ has the largest potassium content (3.93%), while YJ has the smallest potassium content (1.18%). The experimental results may be related to the catalysis of alkali metals.

Kinetic Analysis of Steam Gasification of 2.3 Biomass Char and Coal Char and Their Mixtures

The relationship between the conversion rate of coke-like carbon and time is obtained from the experimental results at 800 ° C. to 1050 ° C., the reaction rate k is obtained from the continuous reaction model equation (2), and the relationship (4) between lnk and 1/T is obtained from the Arrhenius equation (3). The relationship between lnk and 1/T is drawn, and the results are shown in Figure 5. The activation energy E and pre-index factor A of coke sample steam gasification reaction can be obtained. The results are shown in Table 2.

As can be seen from Figure 5, the continuous reaction model reflects the steam gasification reaction dynamics of the coke sample to a certain extent, and can be used as a model to solve the dynamic parameters. The order of increasing activation energy of steam gasification reaction of three kinds of biomass char and coal char is DJ

The order of increasing activation energy of steam gasification reaction of three kinds of mixed coke is DSMJ

3 Conclusions

The steam gasification characteristics of biomass char, coal char and the mixture of biomass char and coal char were studied in a thermobalance device, and the results showed that the reaction activity of biomass char selected in this experiment was greater than that of coal char, and the steam gasification reaction rates of biomass char and coal char increased in the order of sorghum stalk char> rice stalk char> corn stalk char> Shenmu coal char. The kinetics of steam gasification reaction of three kinds of biomass char, coal char and their mixture was analyzed, and the results showed that the continuous reaction model could reflect the steam gasification reaction kinetics of the coke to a certain extent.