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Bio-energy with carbon capture and storage (BECCS):1
Opportunities for performance improvement2
Mai Buia,b, Mathilde Fajardya,b, Niall Mac Dowella,b,∗3
aCentre for Process Systems Engineering, Imperial College London, South Kensington,4
London SW7 2AZ UK5
bCentre for Environmental Policy, Imperial College London, South Kensington, London6
SW7 1NA UK7
Abstract8
This study evaluates the performance of a 500 MW pulverised fuel BECCS sys-9
tem. A performance matrix is developed to assess the opportunities for BECCS10
performance improvement in terms of: energy eciency, carbon intensity, and11
pollutant emissions. The eect of fuel properties was analysed for variable (i)12
coal type (high/medium sulphur content), (ii) biomass type (wheat straw and13
wood chips), (iii) moisture content, and (iv) biomass co-ring proportion %. It14
was observed that the co-ring of biomass increased the quantity and quality15
of waste heat available for recovery from the exhaust gas. The opportunities to16
improve energy eciency in the BECCS system include enhancing heat recovery17
and using high performance solvents for CO2 capture, such as biphasic mate-18
rials. Implementing these approaches increased the power generation eciency19
from 31%HHV (conventional MEA system) to 38%HHV (using new solvent with20
heat recovery). Furthermore, power generation eciency was found to inuence21
the carbon intensity on an annual basis and annual capacity (load factor) of22
the BECCS system. Signicant reductions to SOX emissions were achieved by23
increasing biomass co-ring % or using low sulphur coal.24
Keywords: Biomass, bio-energy, BECCS, carbon capture and storage (CCS),25
eciency, co-ring, co-combustion, greenhouse gas removal (GGR)26
∗Corresponding author, Email: [email protected]
Preprint submitted to Fuel April 20, 2017
1. Introduction27
1.1. Bio-energy with carbon capture and storage (BECCS)28
Carbon capture and sequestration (CCS) and negative emissions technolo-29
gies will play an essential role in achieving deep reductions in atmospheric CO230
concentration [1]. There is growing interest in bio-energy with carbon capture31
and storage (BECCS) as a promising negative emissions technology, and as a32
means to meet global warming targets of below 2°C target [1] and 1.5°C set33
by COP21 [2]. This highlights the importance of having BECCS as a CO234
mitigation option.35
The BECCS technology was rst introduced for hydrogen production [3],36
before the concept was adapted for negative emissions electricity generation37
[4]. Over the lifetime of biomass growth, there is a net transfer of atmospheric38
CO2 into the biomass. The CO2 arising from the combustion of this biomass is39
captured and stored in geological formations, enabling the permanent removal40
of CO2 from the atmosphere [5, 6], and potentially achieving an overall negative41
carbon balance [5, 6, 7, 8, 9, 10, 11, 12]. Biomass is generally considered a CO242
neutral substitute for fossil fuels, where co-combustion has also been shown43
to reduce the emissions of pollutants SOX, NOX and particulates [13, 14, 15].44
Another important advantage of bio-energy with CCS is that it provides reliable45
rm low carbon electricity, unlike intermittent renewable energy sources (IRES)46
such wind or photovoltaic [16, 17]. The economic loss caused by power outages is47
two orders of magnitude greater than the cost of electricity [17]. Therefore, this48
emphasises the value of having rm capacity technologies to balance the use of49
IRES in an electricity system. Bio-energy with CCS is recognised as a practical50
and immediate approach to mitigating the use of coal and decarbonising the51
electricity sector.52
1.2. Approaches to improve eciency53
1.2.1. Enhancement of fuel properties54
The fuel composition and properties of biomass dier signicantly from coal55
[18]. Biomass typically has lower HHV (higher heating value) and higher mois-56
2
ture content in comparison with coal. Furthermore, co-ring biomass tends57
to increase fouling and slagging in the boiler furnace [19, 20, 21, 22]. Conse-58
quently, biomass co-ring tends to reduce the energy eciency of the power plant59
[22, 23, 24]. The CO2 capture process imposes an additional energy penalty due60
to heat requirements for solvent regeneration [25, 26]. Enhancements to energy61
eciency are required to minimise the marginal cost of electricity generation,62
enabling operation of the power plant at higher load factor [27, 28]. Therefore,63
BECC performance improvements would improve commercial viability of the64
technology and encourage large scale deployment.65
Coal blending to meet power plant requirements is conventional practice66
[29, 30, 31]. Until recently, the main motivation for coal blending has been to67
reduce cost and to utilise more readily available coal resources, i.e., indigenous68
coal. However, as the fuel markets change, the availability of indigenous coals is69
declining, increasing the use of imported coals [31]. Fuel quality has an impact70
on almost every aspect of power plant operation and performance. Boilers are71
typically designed based on a fuel specication, where there is normally an72
allowable range for important fuel properties [29] (e.g., HHV, moisture content,73
ash content and composition, grindability). Table 1 shows typical fuel property74
requirements for a pulverised coal-red power plant. Within these property75
limits, the power plant is expected to produce full load [30]. However, deviation76
from this design fuel specication can be detrimental to plant performance as77
eciency would reduce outside the recommended property limits. The use of78
alternative fuels in power plants requires detailed evaluation of the resulting79
impacts on performance and cost [30]. The following lists the impact of fuel80
properties on certain plant performance [29, 30, 31]:81
Fuel handling and storage: heating value, moisture content, volatile82
matter content, ash content and composition, grindability;83
Pulverising/milling: heating value, moisture content, volatile matter84
content, ash content and composition;85
Combustion performance in the boiler furnace: moisture, ash,86
3
volatile matter, heating value, particle size distribution;87
Ash management/handling and inuence on heat transfer: heat-88
ing value, moisture, ash content and composition;89
Pollutant emissions: sulphur content, nitrogen content.90
Depending on how performance is impacted, modications to equipment and91
operations may be required. In the case of biomass co-ring, various power92
plant congurations and modications have been developed to improve biomass93
co-ring performance [32, 33, 34, 35, 36]. To minimise the risk to normal boiler94
operation during biomass co-ring, necessary plant modications may include95
separate fuel feeding systems, the addition of biomass-dedicated burners and96
ash handling systems. For appropriate design of the combustion facility, it is97
essential that the fuel characteristics (e.g., moisture, ash, heating value, ignition98
temperature) of the biomass are well understood [37].99
Fuel blending to meet power plant specications and requirements enables100
the use of alternative fuels, whilst maintaining energy output and preventing101
damage to the boiler. Additionally, fuel blending can be used as a means of com-102
plying with emission regulations, notably sulphur or mercury emissions [31]. For103
instance, SOX emissions during the combustion of high sulphur bituminous coals104
have been shown to reduce signicantly by co-ring with fuels of low sulphur105
content, e.g., biomass [14, 38, 39, 40], or low sulphur coal [41]. Although cer-106
tain biomass fuel properties reduce plant eciency (e.g., high moisture) [22, 24],107
some fuel properties can be used to improve the overall combustion performance.108
In comparison to pure coal, biomass typically has higher oxygen content and109
greater volatile matter [23, 35], which improves the reactivity and ignition char-110
acteristics of the fuel blend [31, 42], and reduces unburnt carbon [43].111
Another approach to enhance performance is the development of biomass112
pretreatment techniques which improve biomass fuel properties, thus reduc-113
ing the negative impacts of fuel properties that cause slagging/fouling and re-114
duced combustion performance. It is desirable to alter the physical properties115
of biomass to match those of coal, minimising the need to modify fuel handling116
4
Table 1: Typical coal quality requirements for a pulverised coal-red power plant [30]. ar
= as received, mf = moisture free, daf = dry and ash free. The `Typical limits' are ranges
commonly reported from both literature and survey of power plant operators, where those in
the brackets indicate the outer limits that are acceptable under certain circumstances.
ParameterDesired
valuesTypical limits
Heating value (MJ/kg ar) High Min 2425 (23)
Moisture content (% ar) 48 Max 12
Volatile matter (% mf) 2035 Min 20 (side-red furnaces)
1520 Max 20 (down-red furnaces)
Ash content (% mf) Low Max 1520 (max 30)
Sulphur content (% daf) Low
Dependent on local pollution
regulations and ue gas
desulphurisation capacity
Nitrogen content (% daf) Low
(0.81.1), dependent on local
pollution regulations and NOX
control measures
Chlorine content (% daf) Low Max 0.10.3 (max 0.5)
Hardgrove Grindability Index High Min 5055 (min 39)
5
and combustion equipment [42]. Biomass pretreatment can involve mechanical,117
thermal, chemical or biological processes.118
The densication process increases the bulk density of biomass to improve119
the eciency of downstream steps, including storage, handling, supply/feed [44].120
Biomass densication can be achieved through pelletising mills [45], briquette121
presses [46], screw extruders or agglomeration (i.e., binding powder particles)122
[47]. The options for pretreatment include:123
Leaching or washing: removes undesirable chemical components that124
cause ash deposition issues (i.e., the silicates, chlorides and sulphates of125
potassium and calcium), or corrosion (i.e., acidic compounds formed from126
chlorine and sulphur) [42].127
Dry torrefaction: generates biomass fuel with similar properties to coal,128
e.g., reduces moisture content, increases heating value, reduces propensity129
to reabsorb H2O, improves grindability/milling characteristics, particles130
are more spherical, also size is smaller and more uniform [42, 44, 48]. Dur-131
ing torrefaction, raw biomass is heated up to temperatures between 200132
and 300°C[49]. The energy requirement is a function of the inlet biomass133
moisture content at the inlet, torrefaction temperature and reaction time.134
For an inlet biomass moisture content of 15%, the energy requirement has135
been found to be around 90 kWh per ton of biomass [50]. However, this136
value typically increases signicantly with higher moisture content as the137
drying heat requirement constitutes a large fraction of the total heat duty138
for torrefaction.139
Hydrothermal carbonisation or wet torrefaction: end-product is140
similar to dry torrefaction but wet torrefaction produces a solid with141
greater energy density [51].142
Steam explosion: pressurisation with saturated steam causes physical143
and chemical changes, the end-product has high density and low moisture144
reabsorption [44]. During the process, heat is required to heat the biomass145
6
and generate the steam at the selected temperature. The heat duty varies146
with the temperature of the process and biomass type. For instance, the147
heat duty for steam explosion of switchgrass is 150 kWh per ton of biomass148
at 140°C, whereas wheat straw requires 210 kWh per ton of biomass at149
180°C [52].150
Each densication and pretreatment process vary in terms of energy consump-151
tion, impacts on chemical and physical properties, as well as end-product quality152
[47]. From an energy eciency perspective, there will be a compromise between153
the energy consumption of the pretreatment process and level of fuel enhance-154
ment. To determine the value of biomass pretreatment, the impact of enhancing155
specic fuel properties on the overall power plant performance needs to be as-156
sessed carefully.157
Blending or co-ring biomass together with coal can provide a fuel that meets158
power plant requirements. The biomass reduces the emissions of pollutants159
SOX and NOX, whereas coal increases the heating value to improve combustion160
performance. Further improvements to fuel properties can be achieved through161
biomass pre-treatment (e.g., drying, densication). Detailed evaluation of the162
eect of blended fuel properties and fuel enhancement is necessary to understand163
biomass co-ring combustion behaviour, as well as the subsequent eect on164
power plant performance.165
Due to limitations on fuel specication (as seen in Table 1), thus, the166
biomass-coal co-ring ratio is restricted in existing coal pulverized boilers. In167
a recent report on biomass co-ring, the IEA Bioenergy recommended a maxi-168
mum biomass co-ring ratio of 10% of the energy input, beyond which, negative169
impacts on combustion performance and ash management may occur. However,170
the co-ring ration can increase up to 40% if the biomass has been milled to171
the right particle size [53]. Using the specications of a relatively low moisture172
biomass, this would translate in a maximum co-ring of around 60% in terms of173
mass basis. This value can be considered as the maximum co-ring ratio on a174
mass basis, beyond which, adjustments (e.g. move the ame detection system)175
7
or design modications (e.g. bring the ignition plane back to its initial position,176
ash deposition protection measures) must be performed on the boiler technology177
[53].178
1.2.2. Process design for eciency improvements179
The CO2 capture process requires low grade thermal energy for solvent re-180
generation, typically sourced as saturated steam at ∼3 bar [25, 54]. The steam181
is predominantly supplied through steam extraction from the stream cycle of the182
power plant, which imposes an eciency penalty on the system [55, 56]. Several183
options for steam extraction and steam cycle design have been developed to184
minimise the eciency penalty incurred due to CO2 capture, these include:185
Steam extraction at the cross-over pipe between the intermediate pressure186
(IP) and low pressure (LP) turbines [57, 58, 59, 60, 61];187
Extraction of steam from within the LP steam turbine at a suitable point188
[62];189
Optimised designs for steam cycle retrots [54, 63].190
An alternative approach is to modify the process conguration of the CO2191
capture process to reduce the energy penalty and improve eciency. Some stud-192
ies focus on cost-eective stripper congurations to improve energy eciency,193
e.g, Karimi et al. (2011) [64], Van Wagener & Rochelle (2011) [65]. Various194
congurations for CO2 removal from natural gas-based power plant have also195
been evaluated [66, 67, 68]. Amrollahi et al. (2012) combined lean vapour com-196
pression with absorber inter-cooling to decrease energy consumption from 3.74197
to 2.71 MJ/kg CO2 [68], whereas Øi et al. (2014) reduced heat consumption198
from 3.26 MJ/kg CO2 (baseline) to 2.90 MJ/kg CO2 by only using the vapour199
recompression conguration [66]. In contrast, Zhang et al. (2016) focussed on200
evaluating congurations that minimised capital expense. There are several201
extensive reviews on conguration modications for CO2 capture in coal-red202
power plants [69, 70, 71, 72]. Of these process modications, the split-ow203
8
congurations shows promise, providing a 8.3% reduction to reboiler duty (full204
load), thus, reducing the eciency penalty substantially [73].205
Process modications can be implemented to use waste heat recovery and206
further improve the overall energy eciency [74]. There are three potential207
approaches to recover waste energy:208
1. Heat recovery or heat integration within the power plant to improve e-209
ciency;210
2. Waste heat recovery from the CO2 capture process to use in the power211
plant and improve eciency;212
3. Heat recovery from the power plant to use in the CO2 capture process,213
reducing the energy penalty.214
Examples of the rst approach include ue gas heat recovery, which can be215
utilised for fuel drying [75], or applied in a low pressure economiser to heat the216
steam cycle condensate [76, 77, 78, 79]. Pfa et al. (2010) [74] investigated217
approaches to recover energy from the CO2 capture process to enhance power218
plant eciency. Heat recovered from the stripper overhead condenser and the219
CO2 compressor intercoolers in the capture process were used to pre-heat the220
power plant steam cycle condensate and combustion air [74]. Alternatively, ue221
gas heat recovery can supply energy for solvent regeneration in the CO2 capture222
process [80, 81, 82, 83, 84]. For this approach, the amount of heat that is recover-223
able from ue gas depends on: (i) the point along the pollution control pathway224
(e.g., exit of boiler, electrostatic precipitator, ue gas desulphurisation); and225
(ii) fuel type and quality [55]. In a coal-red power plant, the measured ue226
gas temperature at the economiser outlet is ∼345°C [85], whereas the solvent227
regeneration temperature is typically 120°C [86, 87, 88]. All of these studies228
investigated the implementation of eciency enhancements for applications in229
fossil fuel power plants. The moisture content of biomass can change signi-230
cantly, aecting the ue gas heat transfer rate [89] and ue gas volume ow231
rate [30], which in turn inuences power plant thermal eciency [82]. There-232
fore, an extensive performance assessment of biomass-specic power systems is233
9
necessary.234
1.3. Study objectives235
To improve the energy eciency of BECCS in this study, heat recovery from236
boiler exhaust ue gas is used to supply heat for the CO2 capture process. This237
paper evaluates the performance of a 500 MW dedicated BECCS system under238
dierent operating conditions (e.g., variable fuel properties, dierent capture239
solvents, and eect of heat recovery). The eects on combustion performance240
from dierent: (i) coal types (high and medium sulphur content), (ii) biomass241
types (wheat straw and clean wood chips with variable moisture content), and242
(iii) biomass co-ring proportions percentage was demonstrated. Blending coal243
with biomass provides the benet of reducing pollutant emissions. Also, by vary-244
ing fuel moisture content, the impact of drying biomass on plant performance245
was investigated. Additionally, the eect of solvent technology (e.g., dierent246
heat duty) and biomass co-ring proportion on the system energy eciency247
and carbon intensity was evaluated. These results lead to the development of a248
performance matrix which summarises the eect of key process parameters.249
The remainder of the paper is structured as follows: (i) First the methodol-250
ogy and modelling procedure is presented. (ii) The performance of a 500 MW251
BECCS plant is evaluated, assessing the inuence of fuel quality and biomass co-252
ring proportion on the adiabatic ame temperature, SOX and NOX emissions,253
heat recovery, plant eciency and carbon intensity. (iii) A performance matrix254
is developed to assess the opportunities for BECCS performance improvement255
in terms of minimising pollutant emissions, enhancing power eciency, reducing256
carbon intensity. Lastly, (iv) the paper concludes with a discussion of future257
directions for this research eld.258
2. Methodology259
The modelling procedure used to assess performance of the 500 MW pul-260
verised fuel BECCS system has been summarised below. Detailed equations261
and descriptions of the models is available in Bui et al. (2017)[83].262
10
2.1. Power plant and CO2 capture model263
The fuel types selected for this analysis included (i) high sulphur coal, (ii)264
low sulphur coal, (iii) clean virgin wood chip, and (iv) wheat straw. The mois-265
ture content of biomass fuels varied between 5% to 50%. The properties and266
composition of these fuels is provided in Table 2.267
Three CO2 capture solvents were considered:268
1. High heat duty scenario: MEA with a regeneration energy requirement of269
3600 MJ/tCO2[90], and temperature of 120°C;270
2. Medium heat duty scenario: industrial solvent Cansolv, which requires271
2300 MJ/tCO2and 120°C in the reboiler [86, 87];272
3. Low heat duty scenario: uses new solvent that operates at 2000 MJ/tCO2273
and 80°C [83].274
The new solvent heat duty is based on performance data for biphasic sol-275
vent systems, which can achieve overall heat duty reduction of 30% compared276
to conventional MEA systems [93]. The heat duty of 2900 MJ/tCO2has been277
reported to be the attainable limits for MEA systems [88]. Therefore, a 30%278
reduction to an energy requirement of 2000 MJ/tCO2 is in accordance with the279
state-of-the-art systems in literature [94, 95].280
The ultra-supercritical 500 MW coal-red power plant with 90% post-combustion281
CO2 capture was modelled in the Integrated Environment Controlled Model282
(IECM) software [90]. The fuel and solvent data was implemented in the IECM283
model to calculated the fuel ring ow rate and the net power output, which was284
subsequently used to determine the power plant eciency (%HHV) and carbon285
intensity (kg CO2 emitted per MWh generated).286
2.2. Thermochemical combustion model287
The thermochemistry of the co-combustion of coal with biomass was mod-288
elled in the software FactSage 7.0 [96, 97]. Table 3 summarises the coal and289
biomass fuel blending scenarios modelled in FactSage, where the biomass co-290
ring proportion was increased from 0% to 50% at increments of 5%. The fuel291
11
Table 2: Fuel properties and composition for the coal and biomass types used for this analysis.
High sulphur
coal
Medium
sulphur coal
Clean wood
chipsWheat straw
Reference [90] [91] [91] [14, 92]
HHV (MJ·kg−1 dry) 27.14 27.06 19.16 19.22
Fuel composition wt % wt % dry wt % dry wt %
C 63.75 64.6 50 48.7
H 4.5 4.38 5.4 5.7
O 6.88 7.02 42.2 39.1
Cl 0.29 0.023 0.02 0.32
S 2.51 0.86 0.05 0.01
N 1.25 1.41 0.3 0.6
Moisture 11.12 9.5
Ash 9.7 12.2 2.0 5.5
Ash composition % ash % ash % ash % ash
SiO2 46.8 50 43.1 56.2
Al2O3 18.0 30.0 8.9 1.2
Fe2O3 20.0 9.8 3.9 1.2
CaO 7.0 4.0 28.0 6.5
MgO 1.0 0.5 4.2 3.0
Na2O 0.6 0.1 2.0 1.3
K2O 1.9 0.1 5.5 23.7
TiO2 1.0 2.0 0.4 0.06
P2O5 0.2 1.8 2.2 4.4
SO3 3.5 1.7 1.8 1.1
MnO 0 0 0 1.34
12
ring ow rates from IECM in tonnes per hour was used as the mass basis for292
the calculations in FactSage. To ensure complete combustion, the excess air293
coecient (λ) of 1.3 was used, which maintain an O2 concentration of ∼5 6%294
in the ue gas (in accordance with industrial practice).295
Table 3: Coal and biomass co-combustion scenarios modelled in FactSage.
Scenario Coal type Biomass type
A Medium sulphur coal Wheat straw 5% moisture
B Medium sulphur coal Wheat straw 16% moisture
C Medium sulphur coal Wood chip 5% moisture
D Medium sulphur coal Wood chip 50% moisture
E High sulphur coal Wheat straw 5% moisture
F High sulphur coal Wheat straw 16% moisture
G High sulphur coal Wood chip 5% moisture
H High sulphur coal Wood chip 50% moisture
The combustion of each fuel blend was simulated from 200°C to the adiabatic296
ame temperature (the AFT was calculated in FactSage). The subsequent ue297
gas mixture was cooled from the AFT to 370°C (predicted ue gas temperature298
at the boiler exit in IECM). The objective was to calculate the following proper-299
ties for each biomass co-ring scenario: (i) AFT, (ii) SOX and NOX emissions,300
and (iii) exhaust gas properties at 370°C. These calculated exhaust gas prop-301
erties, i.e., AFT, gas ow rate and specic heat capacity, were subsequently302
implemented in the heat recovery model.303
2.3. Heat recovery model304
A heat recovery model was developed in MATLAB; it was based on a heat305
exchanger system consisting of a heater, an evaporator and a super-heater [98].306
Using the input data from the FactSage analysis, the heat exchanger model307
(shown in Figure 1) was used to calculate the recoverable heat, which is the308
percentage of solvent heat duty recoverable from the boiler exhaust gases.309
13
Figure 1: Schematic of the heat recovery model consisting of the super-heater (SH), evaporator
(E) and heater (H).
3. Performance evaluation of a 500 MW BECCS system310
3.1. Adiabatic ame temperature (AFT)311
Figure 2 illustrates that AFT generally increased with higher biomass co-312
ring proportions. For all fuel blends, the overall higher heating value (HHV) of313
the blended fuel reduced as biomass co-ring % increased, thus, the fuel ring314
rate increased in order to meet the specied power plant capacity (500 MW).315
The biomass moisture content had a signicant inuence on the degree at which316
AFT increased. The coal co-red with biomass of low 5% moisture achieved the317
highest AFT increase. Compared to ring medium sulphur coal alone, 50%318
co-ring with 5% moisture wood/straw increased the AFT by 136°C (Scenario319
A and C). The straw with moderate moisture content of 16% also provided a320
substantial increase of 108°C in AFT at 50% biomass co-ring. In contrast,321
there was a marginal 4 5°C increase in AFT when co-ring coal with biomass322
of 50% moisture. The medium sulphur coal had higher ash content compared to323
high sulphur coal (shown in Table 2), consequently, co-ring the same biomass324
with medium sulphur coal generated lower AFT compared to high sulphur coal.325
The fuel moisture and ash content had a strong impact on the combustion326
performance (i.e., AFT), which is concordant with previous research [99]. The327
selection of fuels that reduce the moisture and ash content of the overall blend328
14
Figure 2: Adiabatic ame temperature (AFT) for dierent co-ring scenarios with coal and
biomass.
can provide higher AFT and potentially achieve greater ue gas temperatures.329
3.2. Pollutant emissions330
The SOX and NOX emissions have been represented as concentration of the331
exhaust ue gas at 370°C (from the FactSage analysis) in terms of parts per332
million (ppm) and parts per billion (ppb), respectively. Typically, the reduction333
in SOX emissions during biomass co-ring are due to: (i) reduction in fuel334
sulphur content, (ii) presence of chemical compounds in ash that can absorb335
SO2 (e.g., alkali oxides) [14].336
Figure 3 (left) shows that SOX emissions reduced as the biomass co-ring337
proportion increased. Furthermore, the SOX emissions from high sulphur coal338
co-ring were signicantly greater than the medium sulphur coal scenarios. The339
scenarios involving high sulphur coal, Figure 4 (left), demonstrate SOX emissions340
decreased linearly with higher biomass co-ring %, which can be attributed to341
the biomass (both the wood chip and wheat straw) having signicantly lower342
15
Figure 3: Emissions of SOX (left) and NOX (right) during the co-combustion of coal with
biomass at dierent co-ring percentages. Refer to Table 3 for fuel blends in each Scenario.
sulphur content compared to coal. In contrast, biomass co-ring with medium343
sulphur coal resulted in a non-linear decrease of SOX emissions, shown by Figure344
4 (right). As discussed by Bui et al. (2017) [83, 84], this non-linear (step-change)345
behaviour may be the result of equilibrium reaction shifts that occur at low fuel346
sulphur content. The FactSage analysis revealed that the alkali metal oxides in347
the ash also contributed to the reduction in SOX emissions, specically CaO,348
MgO [14, 100], Na2O and K2O [101, 102].349
Some experimental studies suggest increased biomass co-ring % reduced350
NOX emissions [103, 104]. However, other studies have indicated high propor-351
tions of biomass co-ring can lead to NOX emissions increasing [40, 105] or352
remaining unchanged [106] compared combustion of coal alone. The variation353
in the eect of biomass co-ring on NOX emissions across dierent studies is354
due to the combustion conditions having a signicant eect on the level of NOX355
emissions during biomass co-ring [14, 15, 40, 107]. In particular, temperature356
and the presence of N2 can signicantly inuence NOX emission levels [108]. At357
combustion temperatures >1300°C, thermal and prompt NOX reaction path-358
ways can occur leading to NOX formation from the N2 in air [105, 109, 110].359
Figure 3 (right) shows AFT was above 1300°C, resulting in higher NOX emis-360
sions due to increased formation from N2 in air. In practice, combustion condi-361
16
Figure 4: The SOX emissions when biomass is co-red with coal of high sulphur content (left)
and medium sulphur content (right). Refer to Table 3 for fuel blends in each Scenario
tions are optimised and controlled to ensure NOX emissions are minimal (e.g.,362
temperature maintained below 1300°C, air staging, fuel staging) [105]. Future363
modelling work will need to consider such combustion control mechanisms to364
ensure the prediction of NOX emissions are an accurate representation of an365
actual power plant combustion system.366
3.3. Heat recovery367
The heat recovery evaluation was conducted for co-combustion scenario of368
high sulphur coal with 16% moisture wheat straw (Scenario F). Figure 5 illus-369
trates the inuence of solvent heat duty and biomass co-ring proportion on370
the recoverable heat (percentage of heat duty compensated by ue gas heat371
recovery). Of the three solvent scenarios at 0% biomass co-ring (i.e., ring372
coal alone), heat recovery can only full 100% of the regeneration energy re-373
quirements for the new solvent scenario (i.e., heat duty of 2000 MJ/tCO2). To374
meet the specied power plant capacity, the fuel ring rate had to increase as375
the biomass co-ring % increased, as a consequence of biomass having a lower376
HHV compared to coal. As a result, the ow rate of the exhaust gas leaving377
the boiler increased with higher biomass co-ring proportions, thereby increas-378
ing the amount of recoverable heat. Once biomass co-ring proportion reaches379
17
Figure 5: The fraction of solvent heat requirement that can be supplied from ue gas heat
recovery as a function of the solvent heat duty (HD) and biomass co-ring percentage.
40%, heat recovery is capable of supplying all of the energy requirements in all380
three solvent scenarios. Therefore, ue gas heat recovery could enable operation381
of a BECCS plant without an eciency penalty from CO2 capture. However,382
this would be at the expense of some capital investment.383
3.4. Plant eciency384
The plant eciency for dierent scenarios at 50% biomass co-ring is illus-385
trated by Figure 6. The MEA system (baseline) had an eciency of 31%HHV.386
A signicant increase in BECCS plant eciency was achieved by using a more387
advanced CO2 capture solvents and ue gas heat recovery. The scenario using388
new solvent and heat recovery achieved 38%HHV eciency, which is 8% higher389
than the world average of 30% (LHV, or slightly <29%HHV) for coal-red power390
plants [111]. Thus, these eciency enhancements address the energy penalty391
issues caused by co-ring biomass and solvent regeneration.392
18
Figure 6: Eciency (%HHV) of a 500 MW BECCS plant using dierent capture solvents and
HR at 50% biomass co-ring, compared with average power plant eciencies worldwide. NS
= new solvent, HR = heat recovery, BAU = business as usual.
3.5. Carbon intensity393
As the biomass co-ring proportion is increased, the BECCS plant be-394
comes more carbon negative (Figure 7). Additionally, the carbon negativity395
of the system increased as the plant eciency decreased. As shown in Table396
4 at 50% biomass co-ring, the low eciency MEA system captured -296 kg397
CO2/MWh, whereas the `new solvent' system with heat recovery captured -244398
kg CO2/MWh. Although counter-intuitive, the rationale is that a system with399
higher eciency would burn less fuel per MWh of electricity generated, thus,400
less CO2 is captured from the atmosphere. This highlights the importance of401
identifying an appropriate performance matrix for the evaluation of BECCS.402
It is important to note that this conclusion holds as long as the CO2 emissions403
associated with biomass supply chain or biomass carbon footprint (BCF) 404
do not oset the amount of CO2 removed by the power plant. There is a405
maximum carbon footprint value, which is a function of the biomass carbon406
content, co-ring proportion and capture rate, beyond which, the facility is no407
19
Figure 7: System carbon intensity as a function of the biomass co-ring proportion %.
Table 4: Eect of solvent type on the carbon intensity and eciency of a 500 MW BECCS
plant co-ring 50% biomass with 90% CO2 capture.
SystemEciency
(%HHV)
Carbon intensity
(kg CO2/MWh)
MEA 31.3 -296
Cansolv 33.7 -275
New solvent 34.3 -271
New solvent + heat recovery 38.0 -244
longer carbon negative. In this conguration, facilities that are more ecient408
at converting biomass into power will emit less CO2 than their less ecient409
counterparts. The eect of biomass carbon footprint in kgCO2/MWhHHV and410
co-ring proportion on the power plant carbon intensity is illustrated for wheat411
straw at 90% capture rate in gure 8a (low eciency MEA process) and 8b412
(high eciency new solvent process).413
As can be observed in both gures, less ecient plants are slightly more car-414
bon negative for values below the BCF upper limit, and more carbon positive415
20
Figure 8: System carbon intensity (CI) as a function of the biomass co-ring proportion %
and biomass carbon footprint (BCF), for a MEA solvent (left) and a "new solvent" (right).
for values above the BCF upper limit compared to the more ecient plants.416
The BCF upper limit increases with co-ring, from 93 kgCO2/MWhHHV at 20%417
co-ring, to 301 kgCO2/MWhHHV at 100% co-ring. To put this in context,418
bioelectricity facilities are required to report emissions 60% lower than the EU419
mean electricity carbon intensity, according to the UK Bioenergy strategy [112].420
This translates into a limit of 285 kgCO2/MWhe for embedded GHG emission421
in the biomass supply chain, or a limit of 128 kgCO2/MWhHHV when assuming422
a maximal biomass conversion eciency of 45%HHV. These limits are over the423
BCF upper bound at 20% co-ring, which underlines the importance of consid-424
ering the biomass carbon footprint during identication of BECCS performance425
indicators.426
4. BECCS performance matrix427
The results are summarised into a BECCS performance matrix in Table 5,428
which illustrates the eect of key properties on the plant performance. The429
measures for performance in a BECCS system include: (i) pollutant reduction,430
(ii) energy eciency, (iii) CO2 negativity, or (iv) a combination of these.431
21
Table5:Perform
ance
matrix
foraBECCSsystem
,dem
onstratingtherelationship
betweenprocess
properties
andperform
ance
indicators.∗Fora
pow
erplantofagiven
capacity,theuse
ofafuelwithlower
HHV
willrequirehigher
fuelringrates,whichwillincrease
theow
rate
ofuegas,
leadingto
higher
heatrecovery.
Perform
ance
parameter
Ash
content
Sulphur
content
Moisture
content
HHV
Biomass
co-ring
%
Solventheat
duty
NO
X↓
dependent
oncombustionconditions,e.g.,temperature<1300
°C
SO
X↓
↑↓
↓
↑
AFT↑
↓
↓
↑
Heatrecovery↑
↓
↓↓∗
↑↓
Eciency↑
↓
↓↑
↓↓
CO
2negativity↑
↑
↑↑
↑↑
Exhaustgasowrate↑
↑↓
↑
22
4.1. Reduction of pollutant emissions432
The reduction of pollutant emissions from a power plant will be necessary for433
two possible reasons: (i) satisfy stringent emission regulations [43], or (ii) meet434
requirements of downstream processes for air pollution control [113, 114, 115].435
Blending coal with biomass can be used as a means to reduce pollutant emissions436
of SOX and NOX [32], as the results from this study have demonstrated. Signi-437
cant reductions to SOX emissions can be achieved by increasing the biomass co-438
ring proportion (decrease in overall sulphur content). Fuel selection is another439
important factor; selecting coals that have low sulphur content can signicantly440
reduce SOX emissions. The SOX levels ranged from 420 1180 ppm when high441
sulphur coal was co-red with biomass, whereas the use of medium sulphur442
coal reduced SOX emissions to <10 ppm (satisfying tolerance requirements of443
amines). Further reductions to SOX can be achieved by utilising biomass with444
ash that contains alkali metal oxides (e.g., CaO, MgO, Na2O and K2O). How-445
ever, biomass ash containing silicates, chlorides and sulphates of potassium and446
calcium are undesirable as they increase slagging and fouling formation [42].447
Although co-ring with fuels of low nitrogen content will contribute to re-448
ductions in NOX emissions [103, 104], NOX formation is mainly inuenced by449
combustion conditions. To minimise NOX emissions for each specic fuel blend,450
the combustion conditions require optimisation or reconguration of the burn-451
ers. Some approaches used to reduce NOX formation include fuel/air staging452
[110], or replacing N2 gas with CO2 (reduces NOX formation from the N2 in453
air) [41].454
4.2. Improvement to power eciency455
To improve the eciency of the BECCS systems and reduce the marginal456
cost of electricity generation, it is important to consider the following:457
Fuel selection for ideal combustion properties;458
High performance solvents (low heat duty and regeneration temperature);459
Heat recovery (increases with higher AFT and ue gas ow rate).460
23
The fuel properties that would improve eciency include higher HHV, lower461
moisture and ash content. For instance, selecting coals with lower ash content,462
whereas low moisture biomass with low tendency for ash deposition is ideal.463
To minimise the energy penalty from CO2 capture, utilise a high performance464
solvent with low heat duty and low regeneration temperature.465
Although the heat in the exhaust gas at the boiler exit is considered waste466
from the perspective of electricity generation, this is useful energy, in terms of467
quality and quantity, for solvent regeneration in the CO2 capture process. The468
implementation of a heat recovery system would require capital investment.469
However, the advantage would be that heat recovery could allow BECCS to470
operate without the eciency penalty associated with CO2 capture. Thus, en-471
hancing heat recovery conditions can indirectly improve plant eciency. How-472
ever, some factors that improve heat recovery tend to reduce eciency. For473
instance, co-ring fuels with low HHV (e.g., biomass) reduces plant eciency,474
however, requires higher fuel ring rate and increases the ue gas ow rate,475
which in turn increases recoverable heat.476
4.3. Enhancement of CO2 negativity477
The systems with lower eciency will be more carbon negative (per MWh478
basis) as they consume more biomass fuel, which captures and permanently479
stores more CO2 from the atmosphere. Consequently, enhancing the CO2 nega-480
tivity per MWh for a BECCS system would require implementing measures that481
reduce eciency. This may involve co-ring biomass with low fuel quality (e.g.,482
high moisture and ash) or using the least ecient CO2 capture system (e.g., sol-483
vents with high heat duty such as MEA). The use lower quality fuels can reduce484
fuel costs. Also, the least ecient subcritical plants generally will have lower485
capital costs compared to the high eciency supercritical or ultrasupercritical486
power plants [116], further cost reductions are possible by retrotting the cur-487
rent eet of power plants for BECCS, most of which use subcritical technology488
[117, 118]. However, it is essential to consider the impact of plant eciency on489
the annual dispatch load of the system.490
24
Figure 9: Annual negative carbon emissions (ktCO2/yr), i.e., avoided CO2 emissions, as a
function of the system eciency (%HHV) and load factor (%) of a BECCS system.
The load factor (annual capacity) of a power plant is a function of the plant491
eciency. Systems that have higher eciency will have lower marginal costs492
for electricity generation, which enables them to be economically competitive493
with other power generation technology. Therefore, enabling these plants to494
operate at higher load factors (higher annual dispatch factor) [27, 28]. Figure 9495
illustrates the inuence of plant eciency and annual capacity (load factor %)496
on the annual avoided CO2 emissions. The low eciency MEA system captures497
0.66 MtCO2 annually at 60% capacity. However, the high eciency system using498
new solvent and heat recovery capture the same amount of CO2 annually but499
will operate at a capacity factor of 72%. Thus, increasing the CO2 negativity500
(e.g., decreasing plant eciency) of a BECCS plant on a MWh basis will not501
necessary increase the annual CO2 mitigation potential.502
5. Conclusion503
BECCS has the potential provide negative emissions, whilst providing reli-504
able rm power generation capacity. However, marginal costs are higher com-505
25
pared to other power generation technologies due to the energy penalty incurred506
from the CO2 capture process and use of biomass fuel. This study evaluates507
opportunities to improve the performance of BECCS through: (i) pollutant508
reduction, (ii) eciency enhancement, and (iii) CO2 negativity.509
Biomass co-ring with coal has been reported to provide signicant reduc-510
tions in SOX and NOX emissions. The thermochemical analysis demonstrates511
that increasing biomass co-ring proportion reduced SOX emissions due to the512
decrease in fuel sulphur content. Fuel selection is also an important factor in513
reducing SOX formation. For instance, co-ring low sulphur coal reduced SOX514
by up to two orders of magnitude. Although some alkali metals are associated515
with ash deposition problems, these ash component have a role in SOX reduc-516
tion. The NOX emissions were mainly dependent on combustion conditions,517
therefore, conditions may need to be calibrated to minimise NOX formation.518
Factors that enhance eciency in a BECCS system include the use of high519
performance solvents (low heat duty) and using heat recovery to supply energy520
for solvent regeneration. The eciency can be improved indirectly by increasing521
the recoverable heat from the ue gas, e.g., greater AFT or higher ue gas ow522
rate. By using a high performance solvent with heat recovery, the BECCS523
system could achieve an eciency of 38%HHV, higher than the current eet of524
coal-red power plant with eciencies ranging from 26 35%.525
On the other hand, to achieve greater carbon negativity (on a per MWh526
basis), a low eciency system is more desirable due to increased consumption527
of biomass fuel, which results in more CO2 being captured and permanently528
stored. Increasing the CO2 negativity per MWh would involve using biomass529
with high moisture and ash content or utilising the least ecient capture solvent,530
e.g., MEA. However, increasing CO2 negativity on a MWh basis (e.g., low531
eciency) will not equate to higher CO2 mitigation. It is important to consider532
the impact of eciency on the dispatch rate of the system in the electricity533
market. Typically, reducing the eciency of a power plant will decrease its534
annual capacity factor, lowering its dispatch rate in comparison to the high535
eciency systems.536
26
The major barrier for commercial deployment of BECCS is the lack of eco-537
nomic and political drivers, rather than technical (assuming fossil-fuel CCS is538
now a proven technology) [26]. The potential role of BECCS in the electricity539
grid is unique as it will provide reliable rm capacity, whilst also providing a540
means to decarbonise the electricity sector [16, 17]. To enhance the economic541
viability and encourage deployment of BECCS plant with higher CO2 negativ-542
ity, nancial incentives for electricity generation with negative CO2 emissions543
would be necessary. To improve the commercial potential of BECCS, there is a544
need to develop sustainable biomass supply chains and establish suitable CO2545
sequestration sites [26].546
6. Acknowledgements547
The authors would like to acknowledge funding from the EPSRC under548
grants EP/M001369/1 (MESMERISE-CCS), EP/M015351/1 (Opening New Fu-549
els for UK Generation) and EP/N024567/1 (CCSInSupply). Mathilde Fajardy550
thanks Imperial College London for funding a PhD scholarship.551
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