Pyrolysis, slow gases

In slow pyrolysis, the gas phase contains less methane and ethylene and more ethane and propane than hy flash pyrolysis (see Tables 10.4 and 10.7). The product yields obtained in the literature by different authors for the PE for slow pyrolysis (Pinto, Madorsky, Bockhom, Tsuji and Williams) and fast pyrolysis (Kaminsky, Williams, Scott and Conesa) are respectively presented in Eigures 10.2 and 10.3. 

At higher temperature (700°C), in slow pyrolysis, the gas phase contains less methane and more propane, propene and butene than by flash pyrolysis (see Tables 10.9 and 10.12). 

Pyrolysis is a type of gasification that breaks down the biomass in oxygen deficient environments, at temperatures of up to 400°F. This process is used to produce charcoal. Since the temperature is lower than other gasification methods, the end products are different. The slow heating produces almost equal proportions of gas, liquid and charcoal, but the output mix can be adjusted by changing the input, the temperature, and the time in the reactor. The main gases produced are hydrogen and carbon... 

Polak and Molenaar described a method for the determination of acetylcholine from brain tissue by pyrolysis-gas chromatography-mass spectrometry [200]. The deuterium-labeled acetyl-choline is pyrolytically demethylated with sodium benzenethiolate, followed by quantitative GC-MS analysis. In this method, care must be taken so that the samples do not contain appreciable amounts of choline since exchange of deuterium-labeled groups between acetylcholine and choline during pyrolysis may yield erroneous results. The same authors have also reported a method for the determination of acetylcholine by slow pyrolysis combined with mass fragment analysis on a packed capillary column [201]. 

The initial temperature for the decomposition of LDPE is lower than that of HDPE [17], Eor the two types of PE, total degradation occurs at 490°C. The gas phase composition in slow pyrolysis is presented in Table 10.7. 

During the slow pyrolysis of polyethylene, for a temperature increase from 400 to 700° C, the yield in liquid phase remains higher than 80% with a very small increase in the yield of gas phase (less than 20%). On the other hand, in flash pyrolysis of polyethylene, an increase of temperature from 550 to 700°C leads to a decrease of the yield in the liquid phase to less than 40% with an increase in the yield of the gas phase up to 60%. 

For an experimental determination of the pyrolysis time, a fluidised bed of sand and a large volume hot vessel have been used [20]. The method for reaction rate measurements in the fluidised bed is partly explained in Fig. 7. The pyrolysis gases in the fluidising nitrogen gas stream are combusted after 02 addition downstream from the vessel and the CO2 plus some CO are monitored with NDIR-analysers. Thus, the course of a slow pyrolysis has been followed with a time resolution of several seconds. After pyrolysis, the O2 is added upstream from the fluidised bed and the combustion of the pyrolysis char can be followed in the same way. The equipment is calibrated by injection of a known C02-volume to the bed. Area and time response of the resulting calibration signal are needed for data analysis. 

In pyrolysis the wood material is heated rapidly to about 500 °C at which temperature the wood decomposes to a maximum amount of liquid product. At lower temperatures more char is formed and less liquid and gas, and at higher tenperatures the energy requirements are higher without producing noticeably more liquid. The pyrolysis process is carried out in a fluidised bed where milled material is fed into the bed and the product stream is condensed at temperatures between 30 and 60 °C. The char is usually separated before the condenser and used as fuel - along with the gas -to provide heat to the fluidised bed. The fluidised bed may be bubbling or circulating. In both cases a fast pyrolysis is obtained in contrast to slow pyrolysis which usually yields lower amounts of liquids. 

These orientation tests enabled us to identify the problems encountered in waste wood slow pyrolysis and the modifications to make. In order to gain time and improve the analysis, it is recommended to inject hot gas in the kiln and to inject the cold gas at the completion of the test to decrease the temperature inside the kiln. The pyrolysis experiment required in fact two days to be carried out one day for the experiment and another to empty and refill the kiln. 

These reactions increase the heating value of the gas product, since methane has a high heat of combustion. However, these reactions are very slow except under high pressure and in the presence of a catalyst. Another source of the methane in the syngas is the pyrolysis process. Reaction R-4.11 is the reverse steam methane reforming reaction. All reactions that produce methane are exothermic reactions. 

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