Effect of Temperature on Hydrothermal Gasification of Paper Mill Waste, Case Study: The Paper mill in North of Iran

Hydrothermal process can be divided into three parts. The first part is hydrothermal carbonization aimed at production of hydrochar and is performed in operating temperature range between 180-250 oC. The other subject is Hydrothermal liquefaction which is performed in operating temperature range of 200-370 oC and aimed at production of heavy oil and in the end the last part, which contains super-critical conditions, is named hydrothermal gasification with the purpose of production of flammable gases and this is the matter of the article.
Overall, about 35% of the input materials to paper mills are in the form of different wastes such as wastes of processing recycled fibers, sludge, waste of water and wastewater purification system and also de-inking sludge part. In the northern regions of Iran, as the necessary conditions for producing paper are available, several paper mills are existed. Now, in order to manage waste of paper mills in northern Iran, the landfilling and burning method is being used, each with their problems. For instance, on one hand landfilling requires an extensive land area and in addition the land value is so high in north of Iran and on the other hand the level of underground waters are high in north of the country and it may get polluted. Incinerators also pollute the air and the burning area and as a result contribute to increase greenhouse gases.
Materials and Methods
Samples are taken during a year in 2014 at the beginning of each season, in four times and from five paper mills in order to provide a model of paper mill waste which would be the closest to the real model. The sampled waste was gray, with the smell of wet newspaper and containing particles and fibers with dimensions of 1 to 5 mm. pH of the waste sample was close to the neutral pH=7. The moisture content of the paper mill waste was 39% and needed a pre-treatment to prepare for the hydrothermal gasification process.
Feedstock has been analyzed approximately (proximate analysis) before entering the reactor in order to measure the moisture, volatile substances, fixed carbon and ash of input material entering the reactor. Then, after determining the area under the peak of each of the components, the produced gas is calibrated and converted to mol/lit.
Results and Discussion
The amount of output gas is directly related to the temperature and gas production increases when temperature rises. This increased efficiency is probably due to the cracking of heavy hydrocarbons. Another reason could be due to Char endothermic reactions, when conditions become suitable for the development of the reactions with increasing temperature. With enhancing temperature from 500 oC to 750 oC, gas output increases from 27.09 to 45.22 mol/kg. It should be noted that according to the papers in the field of gasification, efficiency improvements in gas production are also being observed in hydrothermal gasification technique. This increase in the volume of the gas begins from 3.034 lit and continues up to 5.064 lit. It means that the amount of the volume has increased of 66.9 % at this temperature range and only under the effect of temperature changes.
In summary the changes of gas amounts are as below: the amount of CO2 and H2 is enhanced and the amount of CO and CH4 is reduced in whole and it illustrates a more optimized mode compared to the gasification mode without presence of moisture. Because in normal mode (the typical gasification), less hydrogen gas is generated and we can fetch up the result that water enters into this process as a catalyst and reactant helper to produce H2 and facilitates reactions to produce hydrogen as more as possible. In fact water has a role in which more water-gas transfer reactions occur in the presence of water and the conversion rate of water-gas increases with increasing temperature as a result of increasingly breaking carbon bonds.
Efficiency level of generated gas has been increased like typical gasification. The result was not unexpected because water evaporates due to the temperature enhancement and water presence and it generates more pressure in accordance with higher temperature, then in a high pressure cracking reactions occur in better conditions and materials break more easily. The changes are located in the range of 0.61 to 1.01 (m3/kg).
In general we can conclude that the amount of generated gas enhances by increasing temperature in hydrothermal gasification and in contrary its heating value rate decreases while carbon conversion rate is increased. As a result in order to achieve the aim of optimizing we need to consider a temperature in which the generated gas amount would be much and in addition its heating value would be high. Here, the acceptable heating value is considered more than 10 and its heating period is considered between 500 °C to 600 °C. In this period, the gas yield rate is placed in the range of 0.61 to 0.77. As we get closer to the temperature of 500 °C, the heating value of the gas coming from the reactor increases.
Conclusion
In this study, north of Iran’s paper mill wastes was examined and the testing process was hydrothermal gasification process. Wastes from five factories were sampled at the beginning of each season and after pre-treatment phase they were imported into the hydrothermal gasification reactor in the waste to energy laboratory of Environment Faculty of Tehran University. Results from studying waste characteristics before entering hydrothermal gasification process and its output products showed that changes are as below: The amount of CO2 and H2 is enhanced and the amount of CO and CH4 production is reduced in whole and it illustrates a more optimized mode compared to gasification mode without the presence of water.