Pyrolysis, biomass temperature

Thermochemical Liquefaction. Most of the research done since 1970 on the direct thermochemical Hquefaction of biomass has been concentrated on the use of various pyrolytic techniques for the production of Hquid fuels and fuel components (96,112,125,166,167). Some of the techniques investigated are entrained-flow pyrolysis, vacuum pyrolysis, rapid and flash pyrolysis, ultrafast pyrolysis in vortex reactors, fluid-bed pyrolysis, low temperature pyrolysis at long reaction times, and updraft fixed-bed pyrolysis. Other research has been done to develop low cost, upgrading methods to convert the complex mixtures formed on pyrolysis of biomass to high quaHty transportation fuels, and to study Hquefaction at high pressures via solvolysis, steam—water treatment, catalytic hydrotreatment, and noncatalytic and catalytic treatment in aqueous systems. 

Slow pyrolysis of biomass operates at relatively low heating rates (0. l-2°C/s) and longer solid and vapor residence time (2-30 min) to favor biochar yield (Nanda et al., 2014b). Slow pyrolysis operates at temperature lower than that of fast pyrolysis, t q)ically 400 10°C and has a gas residence time usually > 5 s. Slow pyrolysis is similar to carbonization (for low temperatures and long residence times). During conventional pyrolysis, biomass is slowly devolatilized facilitating the formation of chars and some tars as the main products. This process yields different range of products with their properties dependent on temperature, inert gas flow rate and residence time. 

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. 

Another emerging area m biofuels is pyrolysis, which is the decomposition of biomass into other more usable fuels using a high-temperature anaerobic process. Pyrolysis converts biomass into charcoal and a liquid called biocrude. This liquid has a high energy density and is cheaper to transport and store than the unconverted biomass. Biocrude can be burned in boilers or used in a gas turbine. Biocrude also can be chemical by altered into other fuels or chemicals. Use of pyrolysis may make bioenergy more feasible in regions not near biomass sources. Biocrude is about two to four times more expensive than petroleum crude. 

A pilot plant for the high temperature pyrolysis of polymers to recycle plastic waste to valuable products based on rotating cone reactor (RCR) technology. The RCR used in this pilot plant, the continuous RCR was an improved version of the bench-scale RCR previously used for the pyrolysis of biomass, PE and PP. 9 refs. 

These both involve heating biomass (mainly wood), largely in the absence of oxygen, at temperatures from a few hundred degrees centigrade (thermolysis) up to 1500 °C (pyrolysis). At the lower temperature char or... 

The highly oxygenated bio oil can be de-oxygenated, and thereby upgraded, over acidic zeolite catalysts through the formation of mainly water at low temperatures and C02 and CO at higher temperatures [1-3], Successful catalytic pyrolysis of woody biomass over Beta zeolites has been performed in a fluidized bed reactor in [4]. A drawback in the use of pure zeolitic materials has been the mechanical strength of the pelletized zeolite particles in the fluidized bed. 

Pyrolysis was performed at 400°C and catalytic de-oxygenation at 450°C. The gas flow through the bottom of the reactor was 1.0 1/min and through the feeder 2.0 1/min. The furnace was heated with a heating rate of 5°C/min to 490°C with an isothermal temperature plateau at 300°C for 30 min. The set-point for the LAUDA cooler was -20°C. When the set-points of the furnace and cooler were reached the feeding of the biomass started. 

Somewhat related is a process proposed and demonstrated on labscale by the University of Siegen (Germany). The process is called the (Herhof)-Integrierte Pyrolyse und Verbren-nung (IPV) process and is decribed in detail by Hamel et al.60 In this process, biomass is converted with high-temperature steam to pyrolysis gas in a fixed-bed reactor. The generated carbon from this reactor is led to a stationary FB combustor from which the hot ash is returned to the first-mentioned reactor. The ash works catalytically to reduce the tar content of the gas produced. The gas is further cleaned and conditioned using a scrubber and electrostatic filter from which the catch is returned to the FB combustor. 

In the first reaction, pyrolysis, the dissociated and volatile components of the fuel are vaporized at temperatures as low as 600°C (1100°F). Included in the volatile vapors are hydrocarbon gases, hydrogen, carbon monoxide, carbon dioxide, tar, and water vapor. Because biomass fuels tend to have more volatile components (70 to 86% on a dry basis) than coal (30%), pyrolysis plays a larger role in biomass gasification than in coal gasification. 

Table 4.1 Biomass Pyrolysis Product Slate As A Function of Heating Rate, Residence Time, and Temperature...

Pyrolysis processes can be divided into two groups low temperature and high temperature. The products of pyrolysis processes differ and can be controlled by temperature and the rate of material heating. Table 4.1 provides a summary of variations of the product slate for biomass and coal feedstocks. 

Zanzi, R., Sjostrom, K. and Bjornbom, E., Rapid Pyrolysis of Agricultural Residues at High Temperatures, submitted to Biomass Bioenergy, 2001... 

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... 

Pyrolysis has a long history in the upgrading of biomass. The dry distillation of hardwood was applied in the early 1990s to produce organic intermediates (methanol and acetic acid), charcoal and fuel gas [3]. Today s processes can be tuned to form char, oil and/or gas, all depending on the temperature and reaction time, from 300 °C and hours, to 400-500 °C and seconds-minutes, to >700 °C and a fraction of a second [3, 19, 23, 24], The process is typically carried out under inert atmosphere. We illustrate the basic chemistry of pyrolysis by focusing on the conversion of the carbohydrate components (Fig. 2.4). The reaction of the lignin will not be covered here but should obviously be considered in a real process. Interested readers could consult the literature, e.g., [25]. Pyrolysis is discussed in more details elsewhere in this book [26],... 

As discussed above, the pyrolysis of biomass at high temperature (>1000 °C) results in the formation of synthesis gas, a valuable mixture of CO and H2. The decomposition of carbohydrate to synthesis gas is an endothermic reaction since the heating value of product is —125% of that of the feedstock (Reaction 1). The reaction becomes nearly thermo-neutral upon burning about 1/4 of the products. Since the thermodynamics favors the combustion of H2 over CO, the gasification reaction resemble the theoretical Reaction (2). Indeed numerous gasification processes feed 02 or air to drive the gasification reaction. 

Another approach to produce chemicals via degraded molecules is the fast pyrolysis of biomass at high temperatures in the absence of oxygen. This gives gas, tar and up to 80 wt.% of a so-called bio-oil liquid phase, which is a mixture of hundreds molecules. Some of compounds produced by pyrolysis have been identified as fragments of the basic components of biomass, viz. lignin, cellulose and hemicellulose. The bio-oil composition depends upon the nature of starting... 

The literature on biomass fast pyrolysis is quite extensive and excellent research and technology reviews are available [51-55]. For an optimal fast pyrolysis process in terms of organic liquid yield the temperature is around 500 °C the biomass particle size should be small (<5 mm). 

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