Basu P. Biomass Gasification and Pyrolysis: Practical Design and Theory
Подождите немного. Документ загружается.
267
7.10 Major Challenges
X
c
= carbon conversion fraction
Y = gasification yield
µ = viscosity (N.s/m
2
)
µ
i
= number of carbon atoms of component i in the gas product
η = heat exchange efficiency
τ = residence time (s)
Biomass Handling
Chapter 8
Biomass Gasification and Pyrolysis. DOI: 10.1016/B978-0-12-374988-8.00008-8
Copyright © 2010 Prabir Basu. Published by Elsevier Inc. All rights reserved.
269
8.1 IntroductIon
Liquids and gases are relatively easy to handle because they continuously
deform under shear stress—they easily take the shape of any vessel they are
kept in and flow easily under gravity if they are heavier than air. For these
reasons, storage, handling, and feeding of gases or liquids do not generally pose
a major problem. On the other hand, solids can support shear stress and do not
flow freely. This problem is most evident when they are stored in conical bins
and are withdrawn from the bottom. Because they do not deform under shear
stress, solids can form a bridge over the cone and cease to flow.
Biomass is particularly notorious in this respect, because of its fibrous
nature and nonspherical shape. The exceptionally poor flow characteristic of
biomass poses a formidable challenge to both designers and operators of
biomass plants. The cause of many shutdowns in these plants incidents can be
traced to the failure of some parts of the biomass-handling system.
This chapter describes the design and operating issues involved in the flow
of biomass through the system. It discusses options for the handling and feeding
of biomass in a gasification plant.
8.2 desIgn of a BIomass energy system
A typical biomass energy system comprises farming, collection, transportation,
preparation, storage, feeding, and conversion. This is followed by transmission
of the energy produced to the point of use. The concern here is with the handling
of biomass upstream of the conversion system—that is, a gasifier in the present
context. Biomass farming is a subject by itself and is beyond the scope of this
chapter.
Biomass fuel can be procured from the following sources:
Energy crop or forestry
Ligno-cellulose wastes that are from forestry, agriculture, wood, or other
industries
Carbohydrates such as fat, oil, and other wastes
270
chapter
|
8 Biomass Handling
Biomass has two major (Figure 8.1) applications: (1) energy production through
gasification or combustion, and (2) production of chemicals and fiber-based
items (e.g., paper).
The collection methods for biomass vary depending on its type and source.
Forest residues are a typical ligno-cellulose biomass used in gasification plants.
They are collected by various pieces of equipment and transported to the gas-
ification plant by special trucks (or rail cars in some cases). There, the biomass
is received, temporarily stored, and pretreated as needed. Sometimes the plant
owner purchases prepared biomass to avoid the cost of onsite pretreatment. The
treated biomass is placed in storage bins located in line with the feeder, which
feeds it into the gasifier at the required rate.
Biomass typically contains only a small amount of ash, but it is often
mixed with undesirable foreign materials. These materials require an elaborate
system for separation. If the plant uses oxygen for gasification, it needs an air-
separation unit for oxygen production. If it uses steam, a steam generator is
necessary. Thus, a biomass plant could involve several auxiliary units. The
capacity of each of these units and the selection of equipment depend on a large
number of factors. These are beyond the scope of this chapter.
Forestry and agriculture are two major sources of biomass. In forestry, large
trees are cut, logged, and transported to the market. The logging process
involves delimbing, and taking out the large-diameter tree trunks as logs.
The processes involved in biomass harvesting, such as delimbing, deburking,
and chipping, produce a large amount of woody residue, all of which constitutes
a major part of the forest residue. The entire operation involves chopping the
tree into chips and then using those chips to make fuels or feedstock for pulp
industries.
Forest
Chemicals
Energy
Pulp mill/chemical plant Power plant
f
Igure
8.1
Biomass is used for the production of energy or for commercial products such as
paper or chemicals.
271
8.3 Biomass-Handling System
8.3 BIomass-HandlIng system
A typical biomass gasification plant comprises a large number of process units,
of which the biomass-handling unit is the most important. Unlike coal-fired
boiler plants, an ash-handling system is not a major component of a biomass
gasification plant because biomass contains a relatively small amount of ash.
Normally it does not produce a large volume of spent catalysts or sorbents.
Transportation, feed preparation, and feeding are more important for biomass
than they are for coal- or oil–gas-fired units.
The biomass-handling system can be broadly classified into the following
components:
Receiving
Storage and screening
Feed preparation
Conveying
Feeding
The design of the handling system is very similar to that of a biomass-fired
steam plant. Figure 8.2 illustrates the layout of a typical plant showing receiv-
ing, screening, storage, and conveying.
Major considerations for the design of feeding and handling systems are
transportation, sealing, and injection. The feed should be transported smoothly
Fuel conveyor
system
Gasification
plant
Fuel storage
silos
Screening
Unloading
station
f
Igure
8.2
Plant layout for biomass gasification. The fuel, delivered by truck (or rail car), is
cleaned of foreign materials before it is stored in silos.
272
chapter
|
8 Biomass Handling
from the temporary storage to the feed system, which must be sealed against
the gasifier’s pressure and temperature. The fuel is then injected into the gas-
ifier. Metering or measurement of the fuel feed rate is an important aspect of
the feed system, as it controls the entire process.
The following subsections discuss the individual components of a solids-
handling system for biomass. They assume the biomass to be solid, although
some biomass, such as sewage sludge, is in slurry or semisolid form.
8.3.1 receiving
Biomass is brought to the plant typically by truck or, sometimes, by rail car.
For large biomass plants, unloading from the truck or rail car is a major task.
Manual unloading can be strenuous and uneconomical except in very small
plants. This is why large plants use truck hoisters, wagon tipplers, or bottom-
discharge wagons. Figure 8.3 shows a typical system where a truck hoister
unloads the biomass. The truck drives onto the hoist platform and is clamped
down. The hoister tilts to a sharp angle, allowing the entire load to drop into
the receiving chute under gravity. This method is fast and economical.
A bottom-discharge wagon may be used for rail cars. The wagon drops its
load into a large bin located below the rail. An alternative is a standard open-
top wagon and a tippler to rotate it 180 degrees to empty its contents into a bin
underneath. Such units are procured from the suppliers of various bulk material-
handling equipment. Their capacities depend on a number of factors, including
plant throughput and frequency of truck and/or rail arrival.
8.3.2 storage
The primary purpose of storage is to retain the biomass in a good condition
and in a position convenient for easy transfer to the next stage of operation,
fIgure 8.3 Biomass delivery truck tilted to unload at the gasification plant. (Source: Photo-
graph by the author.)
273
8.3 Biomass-Handling System
such as drying or feeding into the gasifier. The stored biomass should be pro-
tected from rain, snow, and infiltration of groundwater.
Once unloaded, the biomass is moved by belt conveyers to the storage yard,
where it is stored in piles according to usage patterns. If the biomass is from
several sources and is to be mixed before use, the piles are arranged in such a
way that they can be mixed conveniently into the desired proportions. Because
of the large volume of biomass, indoor storage may not be economical. Open-
air storage is most common, though it can cause absorption of additional
moisture from rain or snow and produce dust pollution. Storage can be of two
types: above ground, for large-volume biomass, or enclosure in a silo or bunker.
Figure 8.2 showed the general arrangement of the solids-handling system
in a typical biomass-fired plant. A truck-receiving station unloads into an under-
ground hopper from which a belt conveyor takes the biomass to a screening
station. After removal of foreign materials, the biomass is crushed and screened
to the desired size range and then transported into silos for covered storage.
From there, it is taken to the plant as required. Figure 8.4 is a photograph of
receiving, size-screening, and above-ground outdoor storage.
Underground bunker storage is very convenient and cost effective from a
fuel delivery standpoint, as it protects the biomass from rain and snow. However,
because it needs good ventilation and drainage for safety and environmental
protection, its capital cost is higher than that of above-ground storage. The
hygroscopic nature of biomass is a major issue, as it causes the prepared
biomass to absorb moisture even if stored indoors. Moreover, long-term storage
can cause physical and chemical changes in the biomass that might adversely
Conveyors from fuel
receiver
Conveyor to
plant
Storage piles Scraper
fIgure 8.4 Biomass is conveyed to the storage pile; the scrapper collects it when needed and
transfers it to conveyors that take it to the fuel preparation plant. (Source: Photograph by the author.)
274
chapter
|
8 Biomass Handling
affect its flow and gasification properties. For these reasons, it is desirable to
occasionally turn the biomass. A simple and practical way of doing this is to
draw it at a rate higher than that required and return the excess to the top of
the pile.
Moving or retrieving the biomass from the storage piles to the gasifier plant
requires careful design, because interruptions or delays can have a major effect
on the operation of the plant. Generally, it is desirable to withdraw biomass
from the bottom of the pile such that the first in–first out principle is followed
to allow a relatively uniform shelf life.
The properties of the biomass determine the ease with which it is retrieved
or handled. Oversized materials, frozen chunks (in cold countries), and compac-
tion can lead to poor or interrupted fuel flow. If the fuel bin is not filled uni-
formly, erratic operation can result, creating problems for hydraulic scrapers
and bridging over the unloaders. Sticks, wires, and gloves, for example, can
jam augers. Mobile loaders normally achieve uniformity in above-ground
storage buildings or in live-bottom unloaders and augers in bins and silos. For
large plants, a scraper connected to a conveyor, as shown in Figure 8.4, is more
efficient for reclamation.
The following are some common methods for retrieval of biomass from
storage:
Simple gravity feed or chute
Screw-type auger feed
Conveyor belt
Pneumatic blower
Pumped flow
Bucket conveyor
Frontloader
Bucket grab
Walking beams are sometimes used on the floors of large bunkers or storage
buildings to facilitate the movement of biomass to the discharge end of the
storage.
Above-Ground Outdoor Storage
In large-scale plants, above-ground outdoor storage is the only option (Figure
8.4). Indoor storage is usually too expensive. Biomass needs to be piled in
patterns that allow maximum flexibility in retrieval as well as in delivery.
Furthermore, it is necessary to ensure the first in–first out principle. In some
cases, an emergency or strategic reserve is kept separate from the regular flow
of biomass. This is a special consideration for long-term storage.
Good ventilation is important in storage design. Biomass absorbs moisture.
Ventilation prevents condensation of moisture and the formation of moulds that
can pose serious health hazards. It also prevents composting (formation of
275
8.3 Biomass-Handling System
methane), which not only reduces the energy content of the biomass, but also
may run the risk of fire. Because tall storage piles are difficult to ventilate, the
maximum height of a wood chip storage pile should not exceed 8 to 10
m
(Biomass Energy Centre, 2009). For an indoor facility, water or moisture accu-
mulation may occur inadvertently. Unless moved periodically, the biomass may
form fungi and cause a health hazard. Drainage is an important issue, especially
for outdoor storage.
Silos and Bins for Storage of Biomass
Improper storage not only makes retrieval difficult, it also can adversely affect
the quality of the biomass. Retrieval or reclamation from storage is equally
important, if not more so. It represents one of the most trouble-prone areas of
biomass plant operation. The handling system and its individual components
must be designed to ensure uninterrupted flow to the gasifier at a measured rate.
Bunkers, silos, and bins provide temporary storage in a protective environ-
ment. Bunkers are a type of large-scale storage. Although the term bunker is
generally associated with underground shelter, here it refers to the indoor
storage of fuel in power or process plants that is not necessarily underground.
Silos could be fairly large in diameter (4–10
m) and are very tall, which is good
for storing grain-type biomass. For example, Figure 8.5 shows a tower silo for
cattle feed. Bins are for smaller-capacity temporary storage.
fIgure 8.5 Typical grain silo for storing cattle feed. (Source: Photograph by the author.)
276
chapter
|
8 Biomass Handling
Hopper Design
Hoppers or chutes facilitate withdrawal of biomass or other solids from tem-
porary storage such as a bunker. Major issues in their design include (1) mode
of solids flow, (2) slope angle of discharge, and (3) size of discharge end.
Funnel flow is characterized by an annular zone of stationary solids and a
moving core of solids at the center. In this case, the solids flow primarily
through the core of the hopper. Solids in the periphery either remain stationary
(Figure 8.6c, left) or move very slowly (Figure 8.6c, right). Fine particles tend
to move through the core while coarser particles stay preferentially in the
annulus. The particles from the top surface can flow into the funnel, thus violat-
ing the doctrine of first in–first out. If that does not happen, a stationary annulus
is formed and the discharge stops, causing a rat hole to form through the hopper
that becomes void and stops the flow. The rest of the solids in the hopper stay
in the annulus (Figure 8.6a, right), which prevents the hopper from emptying
completely. The only positive thing about a funnel-flow hopper is that it requires
a lower height.
Mass flow (Figure 8.6b) is the preferred flow mode because the solids flow
across the entire hopper cross-section. Though there may be some difference
in velocity, this allows an uninterrupted and consistent flow with very little
radial size segregation, which permits the hopper to effectively follow the first
in–first out norm. However, because of the solids’ plug-flow behavior, there
can be more wear on the hopper walls with abrasive solids. Therefore, the
required height of a mass-flow hopper must be greater than that of a funnel-flow
hopper. The steeper the cone angle of a hopper, the higher the probability of a
mass flow of solids through it. Some common operating problems with hoppers
are
Rat holing
Funnel flow
Arching
Arching
(a) (b) (c)
Rat-holing
Stagnant
material
Flowing material
Mass flowNo flow Funnel flow
f
Igure
8.6
Schematic of three types of solids flow through a hopper: (a) no flow, (b) mass flow,
and (c) funnel flow.
277
8.3 Biomass-Handling System
Flushing
Insufficient flow and incomplete emptying
Caking
Two of the most common problems experienced in an improperly designed
silo or bin (hereafter referred to as silo) are no flow and erratic flow. No flow
from a silo can be due to either arching or rat holing (Figure 8.6a).
Rat holing (Figure 8.6a, right) most often happens in the flow of biomass
with particles that are cohesive and rough. This is a serious problem in hoppers.
To facilitate solids flow, the rat hole must be collapsed by proper aeration in
the hopper or by vibrations on the hopper wall.
Arching occurs when cohesive particles form an obstruction over the exit
(Figure 8.6a, left), usually in the shape of an arch or a bridge above the hopper
outlet that prevents further discharge. The arch can be interlocking, with the
particles mechanically locking to form the obstruction, or it can be cohesive.
Coarse particles can also form an arch while competing for an exit, as a
traffic jam results from a large number of vehicles trying to pass through
a narrow road in an unregulated manner. By making the outlet size at least 8
to 12 times the size of the largest particle, this type of arching can be avoided
(Jacob, 2000).
Flushing results in the uncontrolled flow of fine solids—Geldart’s group A
or group C particles (Basu, 2006, p. 443)—through the exit hole. It it is uncom-
mon in relatively coarse biomass, but it can happen if the hopper is improperly
aerated in an attempt to collapse a rat hole.
Another problem influenced by hopper design is inadequate emptying. This
can happen if the sloped base of the hopper is improperly inclined, causing
some solids to remain on the floor that cannot flow by themselves.
Erratic flow from an inappropriately designed hopper often results from
alternating between an arch and a rat hole. A rat hole may collapse because of
an external force, such as vibrations created by a plant pulverizer (mill), a
passing train, or a flow-aid device such as an air cannon or vibrator. Some
biomass discharges as the rat hole collapses, but the falling material can compact
over the outlet and form an arch. The arch may break because of a similar
external force, and the material flow will resume until the flow channel is
emptied and a rat hole is once again formed (Hossfeld and Barnum, 2007).
Material discharge problems can also occur if the biomass stays in the
bunker for a very long time, forming cakes because of humidity, pressure, and
temperature. This easily results in arching or rat holes. To avoid this, renewal
of solids in the hopper is necessary.
There are some special problems in fuel-handling systems. For example,
spontaneous ignition of coal can occur if fine coal particles stay stagnant in a
bunker for too long. Even in an operating silo, a stagnant region can be a
problem for fuels like coal, which are prone to spontaneous combustion. Fine
dust in the silo may lead to dust explosion. If the fuel flows through a channel