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Grate-firing of biomass for heat and power production
Chungen Yin , Lasse A. Rosendahl, Søren K. Kær
Institute of Energy Technology, Aalborg University, DK-9220 Aalborg East, Denmark
a r t i c l e i n f o
Article history:
Received 20 December 2007
Accepted 9 May 2008
Available online 27 June 2008
Keywords:
Biomass
Grate-fired boiler
Pollutant emission
Particulate matter
Deposit formation
Corrosion
CFD
Fluidized bed
a b s t r a c t
As a renewable and environmentally friendly energy source, biomass (i.e., any organic non-fossil fuel)
and its utilization are gaining an increasingly important role worldwide. Grate-firing is one of the main
competing technologies in biomass combustion for heat and power production, because it can fire awide range of fuels of varying moisture content, and requires less fuel preparation and handling. The
basic objective of this paper is to review the state-of-the-art knowledge on grate-fired boilers burning
biomass: the key elements in the firing system and the development, the important combustion
mechanism, the recent breakthrough in the technology, the most pressing issues, the current research
and development activities, and the critical future problems to be resolved. The grate assembly (the
most characteristic element in grate-fired boilers), the key combustion mechanism in the fuel bed on
the grate, and the advanced secondary air supply (a real breakthrough in this technology) are
highlighted for grate-firing systems. Amongst all the issues or problems associated with grate-fired
boilers burning biomass, primary pollutant formation and control, deposition formation and corrosion,
modelling and computational fluid dynamics (CFD) simulations are discussed in detail. The literature
survey and discussions are primarily pertaining to grate-fired boilers burning biomass, though these
issues are more or less general. Other technologies (e.g., fluidized bed combustion or suspension
combustion) are also mentioned or discussed, to some extent, mainly for comparison and to better
illustrate the special characteristics of grate-firing of biomass. Based on these, some critical problems,
which may not be sufficiently resolved by the existing efforts and have to be addressed by future
research and development, are outlined.& 2008 Elsevier Ltd. All rights reserved.
20% wood chips and 80% straw 33 MWfuel CHP unit (producing 8.3 MWe and 20.8MJ/s
heat)
[71]
Water-cooled vibratinggrate
Wheat straw 108 MWfuel CHP unit (producing 35 MWe and 50MJ/sheat)
[53]
a A unified description of the boiler capacity would be more helpful. Different descriptions (e.g., main steam parameters, feeding rate of biomass, thermal megawatts or
electrical megawatts) are used here due to the lack of the unified information in the references.
Fig. 2. A laboratory-scale water-cooled vibrating grate [59].
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754730
rical temperature and flow temperature) [157]. Most of these
indices were originally proposed for coals, and different limitsare suggested for coal quality parameters relating to deposition
ARTICLE IN PRESS
Fig. 9. Deposits on super-heaters during firing straw or straw/coal in boilers. (a) Deposits on superheater in upper furnace during firing straw at Masnedø CHP plant [146].
(b) Deposit build-up on superheaters after 1 week of co-firing coal and straw at Amager power plant [156].
Table 8
Characterization of aerosol particles (o1mm) in flue gas or fly ash particles
Grate-fired boilers Sampling point Concentration of
aerosols in flue gas
Particle size distribution of
aerosols (or fly ash particles)
Composition of aerosol particles
(or fly ash particles)
25MWfuel wheat straw-
fired grate boiler
[136]
The flue gas particles are
sampled upstream the
electrostatic precipitator
(ESP), where gastemperature is 120 1C
and O2 concentration
fluctuates 8%.
The concentration of the
aerosols is 300–480 mg/
N m3 (cascade impactor)
or2 106–1.8 107 N cm3
(scanning mobility
particle sizer).
The number geometric mean of the
submicron peak is 0.19–0.30mm.
Small particles (0.1–0.3mm) are
close to spherical. Large particles(0.3–0.8mm) vary from almost
spherical to aggregates consisting
of 5–10 distinct primary units with
dp 0.1–0.2mm.
The aerosol particles mainly
consist of KCl and K2SO4 with
minor amounts of P. Elements Ca,
Mg and Si are detected in largerparticles only (41 mm).
6 MWth moist forest
residue-fired grate
boiler at 85% load
(The dominant
species of the fuel
are spruce and pine.
The moisture content
is 40%.) [110,137]
The flue gas particles
were sampled
downstream the cyclone
and upstream the ESP,
where the gas
temperature is about
1901C.
The concentration of the
aerosol particles is close
to those of the coarse
particles (1–10mm): both
are 79mg/Nm3
(related to 13% CO2 dry
gas).
Bimodal particle size distribution is
detected: aerosol particles have a
peak at about 0.2 mm; while coarse
particles are peaked at 2 mm.
The dominant species (mass ratio
410%) in the aerosol particles are
K, S, and Cl. Minor elements are Zn
and Ca. Cr, Fe, Ag, Cd, Mn, P, etc. are
present as traces. The dominant
species in the coarse particles are
Ca, K, and S.
Waste wood fired grate
boiler (nominal
capacity 40 MWth)
[135]
From the flue gas duct
behind the economizer
(mean flue gas
temperature 1781C).
The mean concentration
of aerosols is 84.2mg/
N m3 (related to 13 vol.%
O2 and dry gas).
The aerosol particles are mainly in
the range of 0.088–0.707mm with
the peak at 0.354mm.
The aerosols mainly consist of Cl, K,
and Pb. Ca, Na, S, Si, and Zn are
present in minor amounts. Fe, Mn,
and P are present as traces.
450 2 tonnes/day MSW(90% household
waste and 10%
business waste)
grate incinerator
[140]
Fly ash was sampledfrom the ash pit under
the bag filter, which is
located just before the
stack.
No particleconcentration data.
Over 95% of the fly ash particleswere o149mm, in which 49.4%
were in between 53–75mm. The fly
ash is characterized by a more
uniform distribution of concaves
and agglomeration on its surface.
The fly ash has highly complexmineralogy. The main crystalline
compounds detected include KCl,
NaCl, and SiO2. The fly ash requires
further treatment before final
disposal since the leaching
concentration of Pb exceeds the
regulatory level.
In this table, N m3 and Ncm3 mean m3 and cm3 under normal conditions (1 atm and 273K), respectively.
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754738
soot blowers, are often used to mitigate the deposits on super-
heater tubes. However, they could make the high-temperature
corrosion even worse by effectively removing the corrosion
products from the tubes while exposing them to new corrosive
fly ash deposits.
4.3. Modelling and CFD simulations for diagnosis, optimization, and
new design
Not all the relevant phenomena in a combustion system are
described and understood in full details, but CFD calculations give
an impression of reciprocal relationships. Mathematical model-
ling and CFD simulations form a helpful tool to improve the
understanding of the details, probe the problems, and optimize
the plant operation, as well as aid in a new design. Compared to
modelling of pulverized coal boilers, CFD modelling of biomass-
fired grate furnaces is inherently more difficult due to the complex
biomass conversion in the fuel bed on the grate, the turbulentreacting flow in the freeboard, and the intensive interaction
between them. Fig.12(a) sketches the sub-processes in grate-fired
boilers, i.e., thermal conversion of biomass in the fuel bed, and
primary combustion and burnout in the freeboard, which interact
with each other. Fig. 12(b) shows the most popular methodology
in modelling of grate-firing biomass, in which a separate model
is used to solve the thermal conversion of biomass in the fuel
bed, CFD is used for the freeboard simulation, and they are
coupled by the heat and mass transfer at the interface (i.e., the top
surface of the fuel bed). More precisely, the bed model provides
the inlet conditions (e.g., distribution of gas species concentration,
velocity, and temperature along the grate) for freeboard simula-
tion, whilst the freeboard simulation returns the heat flux
released from the flame and furnace walls onto the fuel layer tothe bed model.
However, there are different modelling methodologies for
grate-firing systems, for instance [3,63,181], in which the wholefuel bed is included as a part of the CFD simulation domain and
there is no need for a separate bed model to provide inlet
conditions. In this methodology, the precise size distribution of
the biomass particles fed into the boiler plays a decisive role in
final CFD simulation results.
Because it is difficult and expensive to carry out comprehen-
sive experimental studies on grate-fired boilers, many modelling
and CFD simulation efforts have been made instead. Table 9 lists
some of the representative modelling works on grate-firing of
biomass in the literature, in which the main purposes and findings
of the works are highlighted. As shown in the table, the modelling
and simulation works can be classified into five groups and the
first two groups’ efforts dominate. Most of the modelling and CFD
works have been validated, to different extents, by experimental
results, particularly the modelling works on biomass conversion
in the fuel bed on the grate.
4.3.1. Modelling of biomass conversion in the fuel bed on the grate
The processes in the biomass bed on the grate may be the most
grate-specific area for grate-fired boilers. The behaviour of
biomass conversion on the grate significantly affects the incom-
pletely burned char in the bottom ash, the distribution of the
combustibles released into the freeboard, and the precursors of
NO x, SO x, PCDD/PCDF, and particle formation. Therefore, biomass
conversion on the grate affects not only the combustion efficiency
of the grate-fired boiler but also the deposition and corrosion
tendency and pollutant emissions from the boiler. As seen in Table
9, there are quite some efforts on this issue, mainly by modelling.Three different approaches on how to model the fuel bed may be
found in literature.
Approach I . Fluent’s porous-media model is used to investigate
the solid-refuse bed on top of a roller grate. The results obtained
from this modelling are used as the inlet conditions for the
modelling of the freeboard region [201,202].
Approach II . More typically, freeboard modelling treats the fuel
bed by using inlet conditions based on experience or measure-
ments. When the combustion rate is prescribed as a function of
the position on the grate, inlet conditions (e.g., temperature,
velocity and individual species concentration) can be calculated
from the overall heat and mass balances of fuel components and
primary air, see, for example, [39,57,61,80,95,106,192,194,195,203].
The experience- or measurements-based bed models have beenproven to be quite robust and useful in studying biomass
ARTICLE IN PRESS
Fig. 12. Grate-firing of biomass and modelling methodology. (a) Biomass conversion in fuel bed, gas combustion in freeboard and their interaction. (b) Modelling concept:
coupling CFD and bed model.
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754742
Summary of the modelling of grate-firing of biomass: the main purpose and the main findings
Purpose of work Main conclusions or findings Reference
Group 1—Modelling of biomass conversion in the fuel bed on the grate
Aerodynamics study of chain link stoker mats by CFD,
visualization and tests to improve the under-grate primary air
(PA) distribution.
CFD is used to aid in redesign of a traditional link design which shows
improvement in PA distribution. Pollutant emission from chain grate
furnaces may be mainly attributed to poor PA distribution.
[182]
To develop a model to characterize and quantify the mixing of biomass fuels on a grate.
(1) The existing grate systems do not mix the refuse sufficiently. (2) Modelpredictions show good agreement with measurements from three 1/15
scaled models of industrial grates.
[183]
To derive methods on a statistical basis to describe quantitatively
the mixing process of a packed bed on a forward acting grate.
(1) Two methods are introduced, based on particles’ velocities and
trajectories, to quantify the mixing process. (2) The trajectory-based method
is believed to be more accurate and suitable.
[184]
To study the effect of fuel mixing on waste bed combustion in
MSW grate incinerators by model development and
experiments.
(1) Improvement of the combustion intensity by fuel mixing is observed. (2)
Control of the primary air supply could further enhance the waste
combustion.
[185]
To study the effect of particle mixing caused by grate movement
on the ignition, burning rate, unburnt carbon (UBC) in ash,
and so on.
(1) Increasing bed mixing from low to medium level significantly increases
the burning rate and reduces UBC in ash. (2) Excessive mixing may cause
significant delay in ignition or even extinction.
[186]
To evaluate the residence time of a moving bed on a forward-
acting grate by numerical approaches.
Discrete element method is applied to describe the motion of a moving bed
(e.g., its particles). Tracking particles can obtain better predictions of the
residence time of a moving bed.
[187]
To understand MSW combustion in a grate incinerator by
modelling and tests of wooden particles combustion in a
fixed bed testrig.
(1) A 1D bed model is developed. (2) Radiation in fuel bed is important in
initiating flame front and transferring heat to the cold bed. (3) Air supply
rate, LHV, and particle size are important parameters.
[79]
To develop a fuel bed model and incorporate it in CFD for
modelling of coal combustion in 60 MWfuel grate boiler.
(1) The properties of different bed zones determine the conditions in the gas
phase above the bed. (2) The channelled flows in furnace do not seem to be
sensitive to the details of bed model.
[188]
To develop a bed model on a travelling grate by incorporating
sub-process models and solving governing equations for gas
and solid.
(1) A 2D bed model (FLIC) is developed. (2) Model predictions without
considering channelling effect agree with experiments in total mass loss but
show big discrepancy in temperature and gas composition.
[189]
To study the effect of air preheating on fuels combustion on a
grate by pot fixed-bed experiments and real-scale MSW plant
observations.
(1) Preheating of PA acts as a catalyst for the ignition on a grate rather than
only drying of the waste; (2) pot furnace experiments have only a limited
value in studying grate boiler combustion.
[190]
To study numerically the effects of fuel properties (i.e., LHV,
density, particle size, and packed bed porosity) on biomass
combustion characteristics on a grate.
(1) Average burning rate is mostly affected by fuel particle size. (2)
Combustion stoichiometry is equally affected by LHV and particle size. (3)
Density has the strongest effect on solid temperature. (4) LHV and particle
size have the strongest effect on CO; LHV and density have the dominant
effect on CH4; and particle size has the greatest effect on H 2 concentration at
the bed top.
[78]
To investigate the importance of particle size and density of fuel
on biomass conversion in a packed bed by a 1D model andtests.
(1) Particle density has a very small effect on the conversion rate in a packed
bed. (2) Particle size has a significant influence on the conversion of a packedbed. In a bed of large particles (30mm 30mm 30 mm), a clear
temperature difference exists between gas and solid. In a bed of small
particles (3 mm 3 mm 3 mm), the temperature difference is small and
could be neglected in modelling.
[55]
To study the conversion of biomass on the grate of a 25 MWe
forward reciprocating grate boiler by modelling and
experiments.
(1) By varying grate speed to obtain constant bed height, the furnace can
achieve 499% of conversion efficiency with 40–50% of normal PA supply. (2)
Char conversion is much slower than devolatilization.
[65]
A sensitivity analysis of the uncertainty of model parameters
related to heat and mass transport, reaction rates, and
composition of volatiles.
(1) Prediction of ignition rate and temperature peak is relatively insensitive
to the uncertainty in the parameters; (2) composition of volatiles affects
greatly gas concentration in the bed and gas ignition.
[191]
Group 2—CFD modelling of mixing and combustion in the freeboard
To study the effect of advanced secondary/over-fire air Ecotube
system in a 15 WMth biomass waste-fired grate boiler using
CFD.
New Ecotube air system generates a considerable improvement in efficiency
for biomass combustion in the grate boiler.
[80]
To model and validate a 50 MWth wood chips-fired grate boiler,
and then to predict the effect of an ‘ECO’ air system on NO x
emissions.
With an improved SA supply (by ‘ECO’-tubes), 30% NO x reduction can be
achieved.
[57]
To evaluate the effect of an ‘Ecotube’ air supply system on
combustion and emission from a 25 MWth biomass-fired
grate boiler using CFD.
The new SA system can result in a higher furnace flame occupation
coefficient, a more uniform heat release, a longer life of combustion
chamber, a lower level of pollutant emissions, and combustion noise.
[39]
To investigate the effect of air staging and flue gas recirculation
on flue gas burnout, mixing, and temperature distribution
using CFD.
Appropriate air staging and flue gas recirculation have a considerable
potential to optimize the mixing and improve the temperature distribution
and control to prevent slagging in biomass grate boilers.
[192]
Combined simulation of a 1 MWth wood chips-fired sloping grate
hot-water boiler by interactively employing a 1D bed model
and CFD.
(1) The predicted results of the fuel bed model are rather insensitive to the
freeboard conditions and more likely to be affected by fuel bed properties.
(2) Minor changes in SA largely reduce CO emission.
[56]
To study a 12T/h MSW-fired grate boiler by FLIC/fluent combined
simulation, as well as measurements.
(1) MSW on the grate is ignited at 1.8–2.0 m away from the waste entrance;
the measured maximum bed temperature is 1000–11281C with big
fluctuations up to 800 1C. (2) Improving SA is necessary to reduce particle
carry-over to boiler tubes and to increase the heat transfer.
[48]
To evaluate MSW combustion process in a pilot grate furnace
using CFD and experiments, under centre-flow operation.
(1) CFD and experiments show good agreement on flue gas burnout in
combustion chamber. (2) CFD is a valuable tool for further study on the
effect of PA supply, fuel mass flow, and grate movement.
[49]
To present a CFD analysis of a 33 MWfuel straw-fired grate boiler. (1) Model predictions show a good agreement with available measurements(temperature and species). (2) Poor mixing between bulk flow and SA jets is
partly responsible for high CO and UBC in fly ash.
[193]
C. Yin et al. / Progress in Energy and Combustion Science 34 (2008) 725–754 743
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