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: niall@imperial.ac.uk

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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