CO₂ Emissions and Economy of Co-Firing Carbonized Wood Pellets at Coal-Fired Power Plants: The Case of Overseas Production of Pellets and Use in Japan

Abstract

CO₂ emissions reduction from coal-fired power plants is an urgent issue in Japan, as well as around the world. The purpose of this study is to estimate the CO₂ emissions and economy of using carbonized wood pellets produced overseas and co-fired at coal-fired power plants in Japan. We examined carbonized wood pellets produced in Canada and Vietnam, since those countries are major exporters of wood pellets for Japan. The results obtained are as follows: (1) The CO₂ emissions and calculated cost per calorific value of carbonized wood pellets (CP25), which have a fixed carbon content of 25 wt.%, are lower than those of wood pellets at the port of import in Japan. When the fixed carbon of carbonized biomass is controlled at 25 wt.% or more via a carbonizer, sufficient pyrolysis gas (the heat source used for drying and carbonization without auxiliary fuel) can be obtained. (2) Carbonized wood pellets manufactured in Vietnam are more economical than those manufactured in Canada, since the resource of wood is less expensive and the transportation distance is shorter from Vietnam compared to Canada. (3) When carbonized wood pellets at CP25 are co-fired in coal-fired power plants, they do not affect the cost of the electricity generated, even if the carbonized pellets are blended at a high ratio.

1. Introduction

Coal-fired power generation accounts for about 30% of the total amount of power generated by general power utilities and is one of Japan’s main sources of power [1]. CO₂ emissions reduction from coal-fired power plants is an urgent issue in Japan, as well as around the world. One of the measures used to reduce CO₂ emissions is the co-firing of wood biomass [2,3,4,5,6,7]. However, the calorific value and bulk density of wood biomass are lower than those of coal, so transportation and storage are inefficient. In addition, since wood biomass is fibrous, the grindability of coal is reduced when wood biomass is blended and pulverized with coal. For this reason, the biomass co-firing rate at existing pulverized coal-fired power stations is about 3–4% on a calorific basis [8]. The carbonization of wood biomass serves to improve energy density [9], grindability [10,11,12], and combustibility. Carbonized wood pellets (CP) can be used at high co-firing rates at existing coal-fired power plants without structural modifications, and they could reduce CO₂ emissions from existing coal-fired power plants drastically [13,14,15]. However, as the resource potential of wood biomass in Japan is limited, an effective approach would be to produce CP overseas using wood biomass, and then import them for use at coal-fired power plants in Japan. In previous studies, it has also been reported that the use of carbonized biomass for energy is environmentally friendly and economical [16,17,18]. In order to further clarify the of this approach feasibility in Japan, it is necessary to design an actual-scale carbonizer and estimate the optimum operating conditions based on carbonization experiment results [19]. Furthermore, based on this information, it is essential to estimate the CO₂ emissions and costs related to producing CP overseas and transporting them to Japan.

The purpose of this study was to estimate the CO₂ emissions and economy of CP produced overseas and co-fired at coal-fired power plants in Japan. We assumed that CP are produced in Canada and Vietnam, since they are the major exporters of wood pellets for Japan. In addition, we compared CP with wood pellets (WP) that had already been imported and examined the differences in CO₂ emissions and fuel costs.

2. Methodology

2.1. Assumptions of Fuel Production Process

Figure 1 shows a flow chart of WP and CP production and transportation processes. We assumed that lumber residue from sawmills was used as the raw material for WP and CP. The torrefaction process using wood pellets as a raw material was beyond the scope of this study. Bunker A was used to dry the raw material, and electricity was used in the dryer, crusher, and pelletizer. However, in CP, the combustion heat of the pyrolysis gas generated in the carbonization process was used as the carbonization heat source and the drying heat source. If the combustion heat is insufficient, Bunker A was added for auxiliary fuel. Canada and Vietnam, two countries which export large volumes of WP to Japan [20], were selected as the countries of production of WP and CP. WP and CP were transported overland in containers from a local mill near the port of export, and then via a bulk carrier to a Japanese port.

2.2. Assumptions of Transportation Conditions of WP and CP

We assumed that forty-foot containers (with a maximum load capacity of 26 tons) would be used for the land transportation of the produced WP and CP, and that light oil-fueled trucks would carry them. Meanwhile, Bunker C would be used as fuel for marine transportation.

Table 1. Fuel production sites in Canada and Vietnam and specifications of transportation to Japan.

Country	Canada	Vietnam
Sawmill location	Merritt, British Columbia	Thanh Tam Commune Chon Thanh Dist., Binh Phuoc
Loading port	Vancouver	Cai Mep
Unloading port	Yokohama, Japan
Land transportation distance, km	274	128
Marine transportation distance, km	7912	4476

shows the fuel production sites in Canada and Vietnam and the specifications of the transportation to Japan.

The energy consumption during land transportation was calculated using Equation (1) [21].

Energy consumption (L) = Cargo weight (t) × Transportation distance (km) × Fuel consumption intensity (L/t/km)

(1)

As derived from Reference [21], 0.0285 (L/t/km) was used as the fuel consumption intensity, with a maximum load capacity of 12–17 tons (at a loading rate of 100%).

2.3. Cost Estimation

For cost estimation, the levelized cost of fuel (LCOF) given by Equation (2) was used. This equation of LCOF is the same concept as the levelized cost of electricity (LCOE) method adopted by the IEA report [22].

$$LCOF={\displaystyle \sum }_{n=1}^N\frac{{\sum }_{t=0}^{t=tmax}\left(I_{n, t}+O_{n, t}+T_{n, t}\right){\left(1+r\right)}^{-t}}{{\sum }_{t=0}^{t=tmax}\left(P_t{\left(1+r\right)}^{-t}\right)}$$

(2)

Here, I is the initial cost, O is the operation cost, T is the transportation cost, P is the fuel production amount, r is the discount rate, and tmax is the number of years of operation. We set r = 5% and tmax = 15 years.

The initial costs of the facilities for WP and CP production are shown in

Table 2. Assumption of initial cost of facilities for WP and CP production (in million USD).

Case Name	WP	CP20 and CP25
Stockyard	5	5
Dryer	4.5	3.6
Carbonizing unit	-	13
Crusher	2	-
Pelletizer	4	3.1
Silo	1	-
Construction	3	4.3
Total	19.5	29

Facility Scale: The raw material processing amount is approximately 280,000 tons per year.

, with reference to the values in Reference [23]. The assumed O&M cost is shown in

Table 3. Assumption of O&M cost.

Country	Canada	Vietnam
Lumber residue, USD/t	11.83 [24]	2 [25]
Maintenance	3%/year of plant cost
Electricity, USD/kWh	0.0875 [26]	0.07 [26]
Oil bunker A, USD/L	0.987 [26]	0.74 [26]
Personnel expenses, USD/Month	2817 [26]	242 [26]

.

3. Results and Discussion

3.1. Estimation of Energy Consumption during Production of WP and CP

We conducted several wood biomass carbonization experiments with Mitsubishi Heavy Industries Environmental and Chemical Engineering Co., Ltd., Yokohama, Kanagawa, Japan and thus had already obtained energy balance data [27]. The heat/mass balance of the carbonizer can be organized by fixed carbon of CP even if the carbonization conditions such as the type of biomass, temperature, and residence time are different [19]. The amount of heat of the pyrolysis gas generated by the carbonizer also correlates with the fixed carbon of CP [27]. Fixed carbon is a term for analytical items defined in “JIS M8812 Coal and coke—Methods for proximate analysis”. Fixed carbon does not indicate the total carbon content of the sample, but rather the weight ratio of solid combustible residue to the sample material that consists of solid combustible residue, volatile matter, and ash.

Figure 2 shows a conceptual diagram of CP and pyrolysis gas generation by carbonization, and the auxiliary fuel consumption obtained from the heat/mass balance analysis of the carbonizer. As shown in Figure 2, as the fixed carbon of CP increases, the amount of heat from the pyrolysis gas produced by the carbonizer increases. Figure 2 also shows the amount of heat required by the carbonizer. When the fixed carbon of CP is about 25 wt.%, the amount of heat of the pyrolysis gas exceeds the required amount and Bunker A is not required as auxiliary fuel. When fixed carbon is over 25 wt.%, auxiliary fuel is not required, and a thermally independent operation of the carbonizer can be achieved.

Based on the above results, we selected WP and two types of CP with 20 wt.% fixed carbon (CP20) and 25 wt.% fixed carbon (CP25).

Table 4. Estimation of energy consumption during the fuel production of WP and CP.

Case Name	WP	CP20	CP25
Production Fuel	Wood Pellet	Carbonized Wood Pellet
Proximate Analysis (*1), Dry Basis
Ash, wt.%	0.1	0.6	0.6
Volatile matter, wt.%	87.7	79.4	74.4
Fixed carbon, wt.%	12.2	20	25
Fuel production rate, t/day	464	336	305
Moisture of fuel, wt.% (wet basis)	8.0	5.0	5.0
HHV of fuel, MJ/kg (dry basis)	20.00	21.94	22.80
Energy Consumption
Oil bunker A, L/day	28,331	6513	0
Electric power of dryer, kW	338	338	338
Electric power of carbonizing unit, kW	-	211	204
Electric power of pelletizer, kW	3123	2147	1948
Annual Energy Consumption
Availability, %	90
Fuel production rate, t/year	152,467	110,327	100,087
Oil bunker A, kL/year	9307	2140	0
Electric energy, MWh/year	27,291	21,257	19,636

Information on raw biomass: (1) HHV (dry basis): 20.0 MJ/kg; (2) input amount (wet basis): 280,539 t/year; (3) moisture: 50.0 wt.%; (*1): according to JIS M 8812.

shows the estimation of energy consumption during the production of WP and CP. We assumed the amount of raw biomass input to be approximately 280,000 tons per year (wet basis, approximately 100,000 tons per year for pellet production) based on the typical scale of pellet production facilities in Canada [28].

3.2. Estimation of Energy Consumption during Transportation

Table 5. Energy consumption during transportation.

	Case Name	WP	CP20	CP25
Canada	Land, kL/year	1191	862	782
	Marine, kL/year	1850	1338	1214
Vietnam	Land, kL/year	556	402	365
	Marine, kL/year	4618	3342	3031

shows the energy consumption of the fuels during transportation. According to discussions between the authors and the operator of a wood biomass power generation company, bulk carriers are used to transport wood chips and WP. Bulk carriers of the 20,000 to 60,000-ton class are used on the North American route, and those of the 10,000-ton class are used on the Southeast Asian route. As per [29], the CO₂ emission intensity was 4.6 g-CO₂/t/km in Canada (ship size: 60,000–99,999 DWT (deadweight tonnage)), which was calculated by considering the navigation distance of the ship. For Vietnam, it was 20.3 g-CO₂/t/km (ship size: <9999 DWT). The energy consumption (Bunker C consumption) for marine transportation was calculated backwards from the CO₂ emission intensity of bulk carriers and the CO₂ emission of Bunker C during transportation.

3.3. CO₂ Emissions

Table 6. Calorific value of fuel and CO₂ emission intensity.

	Items	Values
Calorific value of fuel	Oil bunker A, MJ/L	39.1
	Light oil, MJ/L	37.7
	Oil bunker C, MJ/L	41.9
CO2 emission intensity	Oil bunker A, kg/L	2.71 [23]
	Light oil, kg/L	2.58 [23]
	Oil bunker C, kg/L	3.00 [23]
	Electricity in Canada, kg/kWh	0.15 [30]
	Electricity in Vietnam, kg/kWh	0.8649 [31]

shows the calorific value of the fuel used for the production and transportation of WP/CP and the CO₂ emission intensity of fuel and electricity, respectively. Figure 3 shows the calculated CO₂ emissions of WP and CP based on the data in Table 6.

When comparing the CO₂ emissions for WP and CP, the CO₂ emissions for CP25 were less than half those of WP. This lower value of CO₂ emission for CP25 is due to the zero consumption of Bunker A during the drying process of the raw material. However, in overseas pellet factories, the combustion heat of sawmill residues is often used for the drying process instead of Bunker A. In this case, the CO₂ emissions from Bunker A are zero; thus, CO₂ emissions are almost the same as those for CP.

When comparing CP20 and CP25, CP25, which has zero consumption of Bunker A, was superior. In addition, as energy density increased due to carbonization, the energy consumption for transportation was reduced, thereby slightly lowering CO₂ emissions.

Comparing Canada and Vietnam, the CO₂ emission intensity of grid power in Canada (0.15 kg/kWh) is lower than that in Vietnam, at 0.8649 kg/kWh (see Table 6). In addition, as it was assumed that the size of bulk vessels was 60,000–99,999 DWT on the Canada route and less than 9,999 DWT on the Vietnam route, the CO₂ emissions in Canada were lower than those in Vietnam due to the difference in the fuel efficiency of bulk vessels. Consequently, the CO₂ emissions of Canadian pellets with a long transportation distance were smaller than those of Vietnamese pellets with a short transportation distance.

3.4. Economic Efficiency

LCOF values at the port of import in Japan, which includes the initial cost, operation cost, and transportation cost in each case, were evaluated (Figure 4). CP have a higher equipment cost compared to WP (see Table 2). However, as Bunker A is used in the drying process for WP, the operation cost for WP is higher than that for CP. As a result, CP have a lower total cost compared to WP. Comparing CP20 and CP25, CP20 uses Bunker A as an auxiliary fuel for the carbonization process heat source (see Figure 1 and Figure 2), resulting in a slightly higher operating cost than for CP25. Comparing Canada and Vietnam, pellets from Vietnam can be imported at a lower cost due to the difference in operating costs, such as the costs of biomass raw material (sawmill residue), the fuel unit price, labor costs, and transportation costs (see Table 3).

3.5. Co-Firing at a Coal-Fired Power Plant

We estimated the CO₂ emissions and procurement costs of co-firing WP or carbonized biomass (CP20, CP25) at a coal-fired power plant. These results are shown in Figure 5 and Figure 6, respectively. Both CO₂ emissions and procurement costs are shown as dimensionless numbers based on coal alone. The horizontal axis is organized by the blending ratio of biomass with coal. For the import price of coal, the CIF (Cost Insurance and Freight) price (USD120.54/t) of steaming coal in 2018 was used [32].

The blending ratio of WP with coal reaches about 4%, even at the most modern coal-fired power plant [8,33]. The main reason for this is the increased vibration and power required when WP are pulverized with a roller mill for coal. Contrastingly, CP are easy to pulverize and can thus have an increased blending ratio with coal [19]. It was observed that if the blending ratio was increased to 40% (10 times), CO₂ emissions could be reduced by about 35–40% in all cases.

The cost of generating electricity using WP from Canada increased by a factor of about 1.4–1.5 compared to that of coal alone, but in the case of Vietnam, the cost of generating electricity using WP was stable at any blending ratio of the pellets. On the other hand, CP25 was the best in terms of both CO₂ emissions and fuel costs at the same ratio of blending biomass with coal. The reason for this is that CP25, which allows the thermally self-sustaining operation of the carbonizer, consumes less energy during production.

4. Conclusions

The CO₂ emissions and economic efficiency of manufacturing CP (carbonized pellets) overseas and using them at a coal-fired power plant in Japan were estimated in this study. The following conclusions were drawn.

(1) We found that the CO₂ emission intensity and the procurement cost per calorific value of CP were equal to or less than those of WP (wood pellets). This is mainly because the consumption of Bunker A that is required for CP production can be curtailed using the pyrolysis gas generated in the carbonization process as fuel. (2) The equipment cost for manufacturing CP is higher than that of WP. However, the operation cost and CO₂ emissions of the former are lower than those of the latter. This is because the manufacturing conditions for CP25 (fixed carbon ratio of 25 wt.%) were adopted, and the thermally independent operation of the carbonizer was achieved using the pyrolysis gas generated in the carbonization process. (3) The procurement cost involved in manufacturing CP in Vietnam was significantly lower than that in Canada. The use of CP manufactured in Vietnam would not affect the cost of the generated electricity, even if the carbonized pellets were blended at high ratio; thus, it was very cost effective as a means of reducing CO₂ emissions from coal-fired power plants.