Physical Aspect On Biomass Pyrolysis

From a thermal standpoint, we may divide the pyrolysis process into four stages. Although divided by temperature, the boundaries between them are not sharp; there is always some overlap :

-Drying (~100 oC). During the initial phase of biomass heating at low temperature, the free moisture and some loosely bound water is released. The free moisture evaporates, and the heat then conducted into the biomass interior.
-Initial Stage (100-300 oC). In this stage, exothermic dehydration of the biomass take place with the release of water and low-molecular-weight like CO and CO2.
-Intermediate Stage (>200 oC). This is primary pyrolysis, and it takes place in the temperature range of 200 to 600 oC. Most of the vapor or precursor to bio-oil is produced at this stage. Large molecules of biomass particles decompose into char (primary char),  condensable gases (vapors and precursors of the liquid yield), and noncondensable gases.
-Final Stage (~300-900 oC). The final stage of pyrolysis involves secondary cracking of volatiles into char and noncondensable gases. If they reside in the biomass long enough, relatively large-molecular-weight condensable gases crack, yielding additional char (called secondary char) and gases. This stage typically occurs above 300 oC (Reed, 2002, P.III-6). The condensable gases, if removed quickly from reaction site, condense outside in the downstream reactor as tar or bio-oil. It is apparent from figure below that a higher pyrolysis temperature favor production of hydrogen, which increase quickly above 600 oC. An additional contribution of shift reaction further increase the hydrogen yield above 900 oC, in which typically used in gasification process since biomass pyrolysis use in lower temperature than gasification (range 400-600 oC).

Shift Reaction

Releses of Gases During Pyrolysis of Wood

Temperature has a major influence on the product of pyrolysis. The carbondioxide yield is high at lower temperature and decrease at higher temperature. The release of hydrocarbon gases peaks at around 450 oC and then starts decreasing above 500 C, boosting the generation of hydrogen.

Hot char particles can catalyze the primary cracking of the vapor released within biomass particle and the secondary cracking occurring outside the particle but inside the reactor. To avoid cracking of condensable gases and thereby increase the liquid yield, rapid removal of the condensable vapor is very important. The shorter the residence time of the condensable gas in the reactor, the less the secondary cracking and hence the higher the liquid yield. There’s always see the market demand of biomass pyrolysis products to meet that need. To see how our continous pyrolysis plant can do this, please click here and to find what real application on biomass pyrolysis for South East region please read the all articles in this blog.

Effect of Heating Rate in Pyrolysis Process

The rate of heating of the biomass particle has an important influence on yield and composition of the product. Rapid heating to a moderate temperature (400-600 oC) yields higher volatiles and hence more liquid, while slower heating to that temperature produces more char. The operating parameters of a pyrolyzer are adjusted to meet the requirement of the final product of interest. Tentative design norms for heating in a pyrolyzer include the following :

-To maximize char production, use a slow heating rate (<0.01-2.0 oC/s), a low final temperature, and a long gas residence time.
-To maximize liquid yield, use a high heating rate, a moderate final temperature (450-600 oC), and a short gas resiedence time.
-To maximize gas production, use a slow heating rate, a high final temperature (700-900 oC), and a long gas residence time.

Production of charcoal through carbonization uses the first norm, more detail about our pyrolyzer please click here or if you want more considerations about charcoal production please click here.

Waste Heat Recovery From Pyrolysis Plant

Urgency of availability of cheap energy is the solution of energy problems today. Waste heat recovery is the best option for this.With a pyrolysis unit with waste as feedstock then produce heat  and fuel as among of the products, of course this is a powerful solution in the current era of energy crisis. Coupled with the application of effective waste heat recovery that makes almost all the energy produced can be utilized optimally. That way the integration of the pyrolysis unit will be needed by various industries, such as the scheme below.



The high volume of biomass wastes generated in various agro-industry and on the other hand the large energy requirements for processing these products, so it's time toconsider the application of pyrolysis, the reasons include:


a. energy efficiency
b. Reduce environmental problems caused by waste biomass and emissions
c. The added value generated
d. Sustainable business


Waste heat recovery is the second stage of process of energy efficiency after you apply the pyrolysis unit in your industry, so that almost all the energy produced can be used optimally as possible by reducing energy losses in the pyrolysis process. Application ofpyrolysis unit and the waste heat recovery in a specific industry will be carefully analyzed so that the application system according to the relevant industry. Studies conducted by the Eastern Asia University, Pathumthani, Thailand showed that the waste heat recoveryfrom the pyrolysis unit to contribute significantly to energy efficiency.

Which one is better, pelletization of torrefied biomass or torrefaction of pelletized wood?

Torrefaction, a process different from carbonization, is a mild pyrolysis process carried out in a temperature range of 230 to 300 °C in the absence of oxygen.  This thermal pretreatment of biomass improves its energy density, reduces its  oxygen-to-carbon (O/C) ratio, and reduces its hygroscopic nature. During this process the biomass dries and partially devolatilizes, decreasing its mass while largely preserving its energy content. The torrefaction process removes H2O and CO2 from the biomass. As a result, both the O/C and the H/C ratios of the biomass decrease. But Torrefaction will increases the relative carbon content of the biomass. The properties of a torrefied wood depends on torrefaction temperature, time, and on the type of wood feed. Torrefaction also modifies the structure of the biomass, making it more friable or brittle. This is caused by the depolymerization of hemicellulose. This makes it easier to co-fire biomass in a pulverized-coal fired boiler or gasify it in an entrained-flow reactor. There is a 29 to 33% increase in energy density (energy per unit mass) of the biomass through torrefaction. This increases its higher heating value (HHV) to about 20 MJ/kg.  To know more advantages of the torrefaction, please click here.

In biomass, hemicellulose is like the cement in reinforced concrete, and cellulose is like the steel rods. The strands of microfibrils (cellulose) are supported by the hemicellulose. Decomposition of hemicellulose during torrefaction is like the melting away of the cement from the reinforced concrete. Thus, the size reduction of biomass consumes less energy after torrefaction.

During torrefaction the weight loss of biomass comes primarily from the decomposition of its hemicellulose constituents. Hemicellulose decomposes mostly within the temperature range 150 to 280 °C, which is the temperature window of torrefaction. As we can see from  Figure below, the hemicellulose component undergoes the greatest amount of degradation within the 200 to 300 °C temperature window. Lignin, the binder component of biomass, starts softening above its glass-softening temperature (~130 °C), which helps densification (pelletization) of torrefied biomass. Unlike hemicellulose, cellulose shows limited devolatilzation and carbonization and that too does not start below 250 °C.

Weight loss in wood cellulose, hemicellulose, and lignin during torrefaction

Thus, hemicellulose decomposition is the primary mechanism of torrefaction. At lower temperatures (< 160 °C), as biomass dries it releases H2O and CO2. Water and carbon dioxide, which make no contribution to the energy in the product gas, constitute a dominant portion of the weight loss during  torrefaction. Above 180 °C, the reaction becomes exothermic, releasing gas  with small heating values. The initial stage (< 250 °C) involves hemicellulose depolymerization, leading to an altered and rearranged polysugar structures (Bergman et al., 2005a). At higher temperatures (250–300 °C) these form chars, CO, CO2, and H2O. The hygroscopic property of biomass is partly lost in torrefaction because of the destruction of OH groups through dehydration, which prevents the formation of hydrogen bonds.

A typical reaction time is about 30 minutes. The properties of torrefied wood depend on (1) the type of wood, (2) the reaction temperature, and (3) the reaction time. Pelletization may not increase the energy density on a mass basis, but it can increase the energy content of the fuel on a volume basis. Pelletization of torrefied biomass is better than torrefaction  of pelletized wood from the standpoint of process energy consumption and  product stability.This is because :

a. Torrefied biomass (torrefied wood), for example using sawdust as feedstock, so the torrefaction process will consume less energy due to the smaller particle size than the pelletized wood (wood pellets).  Surface material can be in contact with the process of torrefaction is also larger in general when the particle size is smaller, so that better product quality (product stability). Normally before entering the torrefaction process feedstock will be diminished to the size of a certain size and drying up to a certain moisture content.
b. Physical form of pelletized wood (wood pellets) will be damaged due to torrefaction so irregular and will tend to shrink. While torrefied biomass has no problem with it because the physical form of the final product after pelletization.

Selection of Pyrolysis Technology to Produce Charcoal from Biomass

Pyrolysis is a thermochemical decomposition of biomass into a range of useful  products, either in the total absence of oxidizing agents or with a limited supply that does not permit gasification to an appreciable extent. It is one of several  reaction steps or zones observed in a gasifier if we use gasification application. During pyrolysis, large complex hydrocarbon molecules of biomass break down into relatively smaller and simpler molecules of gas, liquid, and char.

Pyrolysis has similarity to and some overlap with processes like cracking, devolatilization, carbonization, dry distillation, destructive distillation, and thermolysis, but it has no similarity with the gasification process, which involves chemical reactions with an external agent known as gasification medium. Pyrol-ysis of biomass is typically carried out in a relatively low temperature range of 300 to 650 °C compared to 800 to 1000 °C for gasification. Other review the difference between pyrolysis and gasification, please click here.

The product of pyrolysis depends on the design of the pyrolyzer, the physical and chemical characteristics of the biomass, and important operating parameters such as

-  Heating rate

-  Final temperature (pyrolysis temperature)

-  Residence time in the reaction zone

Besides these, the tar and the yields of other products depend on (1) pressure,  (2) ambient gas composition, and (3) presence of mineral catalysts (Shafizadeh, 1984).

By changing the final temperature and the heating rate, it is possible to change the relative yields of the solid, liquid, and gaseous products of pyrolysis.  Rapid heating yields higher volatiles and more reactive char than produced by  a slower heating process; slower heating rate and longer residence time result in secondary char produced from a reaction between the primary char and the volatiles.

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Type of Pyrolysis

Based on heating rate, pyrolysis may be broadly classified as slow and fast. It is considered slow if the time, theating, required to heat the fuel to the pyrolysis temperature is much longer than the characteristic pyrolysis reaction time,  tr, and vice versa. That is:

-Slow pyrolysis: theating is bigger than tr

-Fast pyrolysis: theating is smaller tr

These criteria may be expressed in terms of heating rate as well, assuming a simple linear heating rate (Tpyr/theating, K/s). The characteristic reaction time, tr, for a single reaction is taken as the reciprocal of the rate constant,  k, evaluated at the pyrolysis temperature (Probstein and Hicks, 2006, p. 63).

There are a few other variants depending on the medium in and pressure at which the pyrolysis is carried out. Given specific operating conditions, each process has its characteristic products and applications. In the following list, the first two types are based on the heating rate while the third is based on the environment or medium in which the pyrolysis is carried out: (1) slow pyrolysis, (2) fast pyrolysis, and (3) hydropyrolysis.

Slow and fast pyrolysis are carried out generally in the absence of a medium.  Two other types are conducted in a specific medium: (1) hydrous pyrolysis (in H2O) and (2) hydropyrolysis (in H2). These types are used mainly for the production of chemicals.

In slow pyrolysis, the residence time of vapor in the pyrolysis zone (vapor residence time) is on the order of minutes or longer. This process is used primarily for char production and is broken down into two types: (1) carbonization and (2) conventional.

In fast pyrolysis, the vapor residence time is on the order of seconds or milliseconds. This type of pyrolysis, used primarily for the production of bio-oil and gas, is of two main types: (1) flash and (2) ultra-rapid. Carbonization produces mainly charcoal; fast pyrolysis processes target production of liquid or  gas.

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Production of charcoal through Pyrolysis

Carbon is a preferred product of biomass pyrolysis at a moderate temperature.  Thermodynamic equilibrium calculation shows that the char yield of most biomass may not exceed 35%. See table below gives the theoretical equilibrium yield of biomass at different temperatures. Assuming that cellulose represents biomass, the stoichiometric equation for production of charcoal (Antal, 2003) may be written as :

 

Charcoal production from biomass requires slow heating for a long duration but at a relatively low temperature of around 400 °C. An extreme example of a pyrolysis or carbonization is in the coke oven in an iron and steel plant, which pyrolyzes (carbonizes) coking coal to produce hard coke used for iron extraction. This is an indirectly pyrolyzer that operates at a temperature exceeding 1000 °C and for a long period of time to maximize gas and solid coke production.

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The best biochar  for improving soil quality (agricultural application) can be produced with slow pyrolysis process, more review on this, please click here.  The best charcoal for activated carbon production also can be produced with this process, more explanation please click here. We can also produce high fixed carbon charcoal with this technology, read more click here. In simple words we will produce charcoal as you wish.

Biobased Economy through Biomass Torrefaction

Lately a number of places in Indonesia has begun the production of wood pellets and wood chips as a renewable fuel. Biomass waste treatment has reduced the waste pollution and provide economic benefits. Since its application to energy the higher the energy content, the better it will be in addition to other properties. Through torrefaction of biomass will experience a thermal process that makes the content of volatiles is reduced, leaving the higher energy content / energy density (or energy content / unit mass is usually presented in kcal / kg) in the biomass solids.

Torrefaction of biomass which is then followed by compaction of pellets or briquettes will make the energy content per volume (one of which is expressed in units GJ/M3) the greater. And it will save on transportation costs. Torrefaction becomes an important concern lately because of the benefits torrefaction properties of these products, compared to wood pellets or wood chips. Appropriate technology that can be relied greatly needed for the commercialization process. JF BioCarbon have an effective technology for torrefaction, the more details please click here.

Many people noticed that the biomass torrefaction will soon find its golden ages at some future time. Indonesia and Malaysia in particular as a country rich in the amount of biomass it will be great potential for applying this technology. The palm oil industry is one of the potential with huge potential for implementation. A large number of oil mills and the high solid waste generated indicating the potential magnitude of the abundant raw materials. In terms of market is a matter that can not be denied that the energy needs will continue to increase directly proportional to the increase in human population. Biomass torrefaction is one way the most efficient utilization of biomass for energy. For further details, please click here.

Only With Continuous Pyrolysis, Charcoal Briquette Industry Will Get a Supply Of High Quality Raw Materials

Charcoal briquette plant with a large capacity can only be supplied charcoal produced from continuous pyrolysis technology. The quality of products are standard and stable as well as the quantity of large quantities can only be met when using continuous pyrolysis technology in the process of charcoal production. Charcoal of satisfactory market quality can be made in kilns of any size or type when suitable coaling temperature and time conditions are present. It is perhaps more difficult to produce charcoal of consistently high quality in uninsulated metal kilns because of rapid and large heat loss.

The growing popularity of charcoal briquette has spurred great interest recently because its benefit on specific fuel application. Some information on plant equipment, manufacturing detail and the practicability of briquette production with contionous pyrolysis system to provide a few items of special interest.

Equipment : The equipment required for briquette manufacture is highly specialized. Powered units are required for grinding and mixing dry and wet charcoal, wet forming the briquettes, moving material in the process, and continous drying. Production rates are 1 to 3.5 tons of briquettes per hour. The equipment for both capacities is basically the same, but somewhat larger and heavier machines are needed for 3.5 ton output. Standard equipment for a 1-ton-per-hour briquetting plant includes the following :

-Briquette press with paddle feeder

-Hammer mill

-Charcoal feeder with surge hopper

-Paddle mixer

-Vertical fluxer

-Starch feeder or pump

-Briquette drier

-Boiler, 30 horsepower - - 15 pounds per square inch gage pressure

-Conveyors

-Bagging machine

-Building, 60 feet by 120 feet, with 20 feet clear height.

The labor requirements per shift are eight men, including a foreman, a machine operator, a night-shift maintenance man, a bagger and three men for warehouse and miscellaneous jobs.

Plant processing :-In general , charcoal lump and fines as received or from plant storage are fed by screw conveyor to hammer mill or crusher for feed material of 1/8-inch and smaller screen size. The ground charcoal is moved mechanically or by air to a surge bin for metered flows to the mixer, metered amounts of about 5 percent of binder (potato, corn or cassava starch) with water are added. After agiataion in a paddle mixer, the mixture is run through the fluxer for more throrough working of the mass before it is transferred to the press feeder for regulated flow to the forming press.

From the press, the wet or green briquettes are moved by belt conveyor to a special device for uniform loading and continous passage through the drier. The conditions for the drying are usually a 3-to 4-hour period at a temperature of about 275 F. The processing steps are carried out as shown in figure below.

Because of the large daily charcoal requirements and the investment necessary for even the smallest commercial briquette operation, it is not practical for the smaller kiln operator to undertake such manufacture. Operating the smallest commercial plant at a production rate of about 10 tons of briquette per day would require at least 250 tons of charcoal monthly.  Briquetting plants usually operate on two or three shifts per day for most economical production.

 

Only charcoal plant with level of production above 10 tons/day adequate for charcoal briquette plants need.  JFE project can provide charcoal plant (continous pyrolysis technology)  to meet that needs include high specification (quality) of charcoal requirement if it’s needed.