Straw pyrolysis

Heterogeneous kinetics of straw pyrolysis and straw gasification are essentia data for reactor design. Pyrolysis is a relatively fast process. In view of the poor heat conduction of straw and straw char, the pyrolysis time is the time which is required to heat the center of the particle to the decomposition temperature. This is a rather simplified model, but allows a reasonable time estimate in view of the order of magnitude. 

Thermal Analysis and Kinetic Modelling of Wheat Straw Pyrolysis... 

The model for straw pyrolysis by Lanzetta and Di Blasi [7] was applied to the data in this study even though the conditions of the TGA-e periments are very different from the conditions used to fit the parameters in the model. The heating rate in the present study (5-40 C/min) is much lower than in the study by Lanzetta and Di Blasi [7], who used heating rates of 1500-4200°C/min, and the maximum final temperature was higher in this study (700°C) compared to 375 C by Lanzetta and Di Blasi [7], The reaction scheme used by Lanzetta and Di Blasi [7] is given in Fig. 10. 

Fig. / / Modelling of wheat straw pyrolysis at IO C/min using the model by Lanzetta and Di Blasi [7],...

In the experiment, the straw was heated up in a lab-scale pyrolyzer with external heating element. The temperature in the pyrolyzer was increased gradually and kept at about 800 C for a period of time. The temperature of the fuel was registered as a function of time. The ten erature history of the straw pyrolysis is shown in Fig. 1. 

Basic investigations on the pyrolysis of different biofuels under different process and fuel parameters have been carried out in previous investigations. Especially gaseous and condensable pyrolysis products have been the focus of those investigations [13, 17], For the sake of con leteness main results of straw pyrolysis are presented here. 

Figure 4 shows the yield of the main conqranents of straw pyrolysis. The composition of the pyrolysis gas changes with increasing pyrolysis tenq>eratiire to the favour of... 

Figure 4 Main gaseous components of straw pyrolysis gas Tar Components...

Figure 5 Production of tars during straw pyrolysis in dependence on the pyrolysis... 

Char samples were taken om the pyrolysis experiments at the entrained flow reactor. Figure 6 shows the devolatilisation and the composition of the char from straw pyrolysis at different tenqreratures and air ratios and of the raw straw. The results of the thermogravimetric measurements are related to the ash content of the raw fuel. Straw shows a high devolatilisation already at low temperatures. The devolatilisation rate is almost not dependant on the air ratio in the entrained flow reactor. Deviations between the values are within the tolerance of the sampling and measurement system. 

Figure 11 Relation between the air ratio in the reduction zone and fuel mass flow rate of the straw pyrolysis process (X test runs)...

Figure 4. Gas composition from straw pyrolysis—Series II (Note heavy com ...

It is interesting to compare these experiments with previous experiments on peat pyrolysis using the same unit. Ground sod peat with a degree of decomposition of 27% (Baturin formula - Baturin, 1975 H4 on the Von Post scale) was air dried to 12% moisture content and fed in at rates up to 18 kg/h (Campion, 1978). Yields and calorific values at reaction temperatures of 1120 K are compared with stover and straw pyrolysis at 1170 K in Table III. 

A report on the continuous flash pyrolysis of biomass at atmospheric pressure to produce Hquids iadicates that pyrolysis temperatures must be optimized to maximize Hquid yields (36). It has been found that a sharp maximum ia the Hquid yields vs temperature curves exist and that the yields drop off sharply on both sides of this maximum. Pure ceUulose has been found to have an optimum temperature for Hquids at 500°C, while the wheat straw and wood species tested have optimum temperatures at 600°C and 500°C, respectively. Organic Hquid yields were of the order of 65 wt % of the dry biomass fed, but contained relatively large quantities of organic acids. 

The Biolig process of the research center Karlsruhe FZK, Germany. Here, flash pyrolysis, with emphasis on straw as feedstock, is tested to produce a bio-oil-char slurry. The pyrolysis reactor compares to the ER reactor (Lurgi-Ruhrgas) by which sand as heat carrier is mixed and transported together with biomass in a double (twin) screw feeder. A novel unit is constructed with a biomass processing capacity of 12 t/day. 

Putun, A.E. 2002. Biomass to bio-oil via fast pyrolysis of cotton straw and stalk. Energy Sources 24 275-285. 

Over the past two decades, considerable interest has been directed toward the conversion of cellulosic biomass (such materials as wood wastes, bagasse, and straw) into useful products, notably fuels. Several procedures, including fermentation, gasification, liquefaction, and pyrolysis, have been commercially applied to carbohydrates with various degrees of success. In order to use the polysaccharides present in lignocel-lulosic materials as a substrate in fermentation processes, pretreatments are necessary, such as with steam (under slightly acid conditions) or... 

Boon, J. J., 1989, An introduction to pyrolysis mass spectroscopy of lignocellulosic material case studis of barley straw, com stem and Agropyron, in Physico-chemical Characterisation of Plant Residues for Industrial and Feed Use, A. Chesson, and E. R. 0rskov, eds., Elsevier Applied Science London, pp. 25-49. 

In principle, most types of biomass can be used as a raw material in the pyrolysis process [14]. Most of the research has been carried out using different wood as feedstock, although more than 100 different types of biomass have been tested [14]. Besides wood, these materials include forest residues, such as bark black liquor and agricultural residues such as straw, olive pits, and nut shells [12, 14], Additionally, pure biological polymers cellulose (linear polymer of D-glucose units), hemicellulose (heteropolymers of different hexoses and pentoses), and lignin (heteropolymer of p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol [19]) - have been tested as raw materials in the pyrolysis. 

Flash pyrolysis in FFR is a usefnl means to remove snlphnr from coal [19, 21]. As shown by Li et al. [22], it can also be ntilized to remove heteroatom molecules from biomass. Both the yield and the composition of the resnltant gas depend on the biomass composition. The gas ontpnt is richer in hydrogen in the case of cellulose and hemicel-Inlose than in the case of lignin. Smaller biomass particle sizes and higher fast pyrolysis temperatnres also boost hydrogen content. The total of carbon monoxide and hydrogen content is reported to be 65.4% for legnme straw and 55.7% for apricot stone. 

The results of this study are summarised in Figure 2. The results were calculated so that the amount of potassium was normalised to one either in the char (immediately after pyrolysis, i.e. at 0% char conversion), or in the original straw. Hence, the water-soluble potassium and the total potassium were conqjarcd, as shown in Figure 2. 

Fig. 2 The amount of potassium in straw char as a function of char conversion. Symbols without line amount of water-soluble potassium (normalised values) sym-bols with the solid line amount of total potassium. In the upper graph, potassium is de-noted as 1 in char after pyrolysis in the tower graph in straw dry matter, respectively.

In addition, the total potassium level apparently was reduced during pyrolysis at the char conversion of 0%, the total potassium content was 10% and the soluble potassium content 20% lower than that in straw, respectively. This indicated that during pyrolysis, a part of potassium would have been reacted to insoluble potassium, and moreover a part was evaporated during pyrolysis. However, due to the relatively large scatter of the measuring points, a more detailed study is needed to verify this observation. 

The second step is an autothermal gasification of the pyrolysis gas and the solid pyrolysis char at temperatures above 800°C after partial combustion with air or oxygen. The pyrolysis char carries the whole ash and must be separated from the bed material prior to gasification. Straw char is a rather brittle material and is easily crushed to a fine powder, which gasifies faster. After a relatively fast pyrolysis step the pyrolysis gas contains > 50% of the energy. 

The following examples of pyrolysis equations are oversimplified, but show the essentials. The simplified formula C3(H 0)2 (MW 72) represents the dahf-composition of the organic part in wood, straw or any other lignocellulosic biomass. 

Pyrolysis kinetics for cylindrical 12 mm diameter straw pellets in a fluidised bed of sand at different temperatures are shown in Fig. 8. The long pyrolysis times around 100 s are a consequence of the large particle dimension. This demonstrates, that pelletisation is not an advantage it is expensive and destroys the high reactivity of untreated straw with thin ca. 0.5 mm thick walls. 

At comparable temperatures, the pyrolysis of straw chops proceeds more than 10 times faster than straw pellet pyrolysis. This is demonstrated in Fig. 9. Small cm-sized single walled straw chops have been added to a large preheated steel vessel with a sand layer at the bottom. The pyrolysis gases quickly displace a certain amount of inert gas in the thermostated vessel. The released inert gas is collected in a burette, whose level is observed with a TV-camera. A time resolution of < 0.1 s has been obtained in this way. Fig. 9 shows that the pyrolysis times at > 600 C are less than 10 s. Nodes in the straw stem halm must be squeezed to maintain short reaction times. 

Fig. 8 Pyrolysis kinetics for cylindrical 12 mm straw pellets in a fluidised bed [20]...

Most of the particular difficulties of straw gasification are caused by the high K and Cl-content. The behaviour of these impurities in the successive process steps is therefore of special importance. The selection of a method for their removal is a major process decision. Chlorine volatilisation starts at relatively low pyrolysis temperatures of about 200 C. About half of the chlorine can be removed into the pyrolysis gas up to about 500°C. The rest of the HCl is volatilised together with the potassium at higher temperatures. At lower pyrolysis temperature, K can be kept completely within the char particles together with the residual ash, but the chlorine distributes between char and gas. 

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