Estimation of net carbon sequestration potential with farmland application of bagasse charcoal: life cycle inventory analysis through a pilot sugarcane bagasse carbonisation plant.
According to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report, enriching soil carbon storage is regarded as a viable option for mitigating greenhouse gas (GHG) emissions in the agricultural sector (IPCC 2007).
Biochar, charcoal produced from biomass pyrolysis, is highly stable against microbial decomposition (Baldock and Smemik 2002), and most of it is likely to be stable for at least hundreds of years when added to soil (Lehmann et al. 2009). In addition, applying biochar to farmland as a soil amendment improves the physical and chemical properties of soil and stimulates plant growth (Glaser et al. 2002; Lehmann et al. 2006; Fowles 2007; Chan et al. 2008). Therefore, applying biochar to farmland has the potential to mitigate C[O.sub.2] emissions in addition to improving soil properties and enhancing crop yields (Lehmann 2007; Laird 2008; Lehmann and Joseph 2009; McHenry 2009).
When assessing C[O.sub.2] mitigation effects achieved through farmland application of biochar on a regional scale, the estimation of net carbon sequestration potential (net avoided GHG emissions) by considering C[O.sub.2] emissions throughout the biochar life cycle, including pyrolysis, transportation, and farmland application, is important (Fig. 1). Although the importance of assessing the biochar life cycle has been recognised, little research has been published concerning this topic (Gaunt and Lehmann 2008; Gaunt and Cowie 2009; Roberts et al. 2010) because information on C[O.sub.2] emissions from the pyrolysis and biochar processes is still limited. Thus, we installed a pilot carbonisation plant for sugarcane bagasse on Miyako Island in Japan and collected operational data from the plant (Shinogi and Kameyama 2007; Ueno et al. 2007). Sugarcane bagasse is the residue obtained after pressing sugarcane stalks to extract juice in sugar factories. In addition, field experiments on farmland application of bagasse charcoal, produced from sugarcane bagasse carbonisation within the plant, were conducted on the island (Komiya et al. 2006; Chen et al. 2007). From the results of these experiments, it was noted that applying bagasse charcoal to calcareous soil on the island was effective in improving water availability to crops, enhancing crop yields, and reducing nitrate leaching.
The objective of this study was to estimate the net carbon sequestration potential of farmland application of bagasse charcoal produced by the pilot plant. To this end, the operational properties of the carbonisation device were examined, and C[O.sub.2] emissions and carbon stabilised as charcoal within the life cycle of bagasse charcoal were calculated. Furthermore, the C[O.sub.2] mitigation potential was estimated on the basis of inventory data from the pilot plant.
[FIGURE 1 OMITTED]
Outline of Miyako Island
Miyako Island is a subtropical island in south-western Japan. This island consists of coral limestone of high permeability. This limestone is covered with calcareous soil known as 'Shimajiri-Mail'. The soil is classified as Typic Paleudalfs in USDA Soil Taxonomy (Soil Survey Staff 2010). The soil has a clay content of 54.3%, average pH 8.0, carbon content 17.4 g/kg, and cation exchange capacity (CEC) 21.07 cmol/kg (Okinawa Prefecture 1984). Although this soil is clay-rich, the permeability is relatively high. In addition, the soil has low fertiliser-retaining capacity and limits the availability of water for crops (Kubotera 2006).
Because agriculture is the main industry and sugarcane is cultivated on ~80% of the farmland, the main biomass resource on the island is sugarcane bagasse (Kanri and Shinogi 2006). About 60 000 t/year [moisture content (MC) on wet basis: 50%] of sugarcane bagasse is generated by the sugar factories on Miyako Island and ~80% of the bagasse is currently used as a substitute for fuel in sugar factories, while the remainder (~12000t/year) is directly or indirectly applied to farmland (Okinawa Prefecture 2004).
Outline of the pilot carbonisation plant
The pilot plant is composed of a carbonisation device (Meywa Co. Ltd, Japan) housed within a steel building and a greenhouse for storage and drying of feedstock. The carbonisation device is externally heated; the necessary heat for carbonisation is provided by kerosene and syngas combustion (Fig. 2). The designed operating potential of the device is 0.1 t dry weight (DW)/h. Carbonisation reactions occur while the bagasse moves slowly through the screw conveyor for 20-30 min (Ueno et al. 2007). All of the syngas produced during the carbonisation reaction is combusted in a burner of the device as a substitute for kerosene to maintain temperatures in the carbonisation furnace and reduce kerosene consumption. An exhaust gas processing facility is used for separation of dust and syngas by a centrifuge. Tap water is used for scatter prevention of the charcoal and quenching the charcoal.
Kerosene and electricity consumption, as well as temperature changes in the central part of the carbonisation furnace during operation, are shown in Fig. 3. Electricity is constantly consumed throughout the operation. A large volume of kerosene is used to heat the carbonisation furnace during the start-up period; however, little kerosene is consumed during the carbonisation period because syngas is utilised to maintain the furnace temperature. Therefore, sequential operation of the device for long periods reduces kerosene consumption.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Analysis of the pilot plant operational data
Operational data on the carbonisation process were collected at the pilot plant during 2005 and 2006. Controlled carbonisation temperatures and feedstock MC were in the ranges 500-700[degrees]C and 17.5-60%, respectively. Energy consumption and charcoal yields were influenced by carbonisation conditions, i.e. carbonisation temperatures and feedstock MC (Ueno et al. 2007). Thus, we investigated the effects of carbonisation conditions on energy consumption and charcoal yields during the carbonisation process and collected inventory data for a life cycle inventory analysis.
Life cycle inventory analysis of farmland application of bagasse charcoal
As mentioned above, energy consumption and carbon yields are influenced by carbonisation conditions (Ueno et al. 2007). As a consequence, the net carbon sequestration potential is also probably influenced by carbonisation conditions. Thus, we investigated the following on the basis of inventory data collected from the pilot plant: (1) the effects of carbonisation conditions on C[O.sub.2] emissions from the carbonisation process and the net carbon sequestration potentials of farmland application of bagasse charcoal; (2) comparison of C[O.sub.2] emissions from each process and carbon stabilised as charcoal within the life cycle of applying bagasse charcoal to farmland; (3) sensitivity analysis of net carbon sequestration potential for some process parameters; and (4) C[O.sub.2] mitigation potential by applying bagasse charcoal to farmlands on Miyako Island.
Systems and system boundaries for analysis
The systems and system boundaries for analysis are shown in Fig. 4. The reference system comprised of the following: (i) truck transport of sugarcane bagasse from the sugar factory to the farmlands, and (ii) applying bagasse to the farmlands directly with a tractor. The biochar system comprised the following: (i) truck transport of sugarcane bagasse from the sugar factory to the pilot plant; (ii) construction of the pilot plant (carbonisation device, steel building, and greenhouse); (iii) drying the sugarcane bagasse in a greenhouse and then converting it into bagasse charcoal using the carbonisation device; (iv) truck transport of bagasse charcoal from the pilot plant to the farmlands; and (v) applying bagasse charcoal to the farmlands using a tractor.
[FIGURE 4 OMITTED]
Bagasse production (sugarcane compression), transport of construction materials, truck production, and agricultural machine production were not included in the present analysis. Also, fertilizer application and [N.sub.2]O emission from the soil in farmlands after the bagasse and bagasse charcoal application were not included. Assumptions for the analysis and calculation methods are described in the following section.
Avoided emissions from reference system
The transportation distance from the sugar factory to the farmlands was 8 km, according to a questionnaire survey of a sugar factory on Miyako Island. The bagasse was directly applied to the farmlands by hand and ploughed with a rotary plough. The application rate of bagasse to the farmlands was 20 t/ha (MC 50%).
Emissions from biochar system
The distance from the sugar factory to the pilot plant was 4km, based on actual conditions. The life span of the carbonisation device, steel building, and greenhouse were 5, 10, and 10 years, respectively, based on the respective legal lifetimes of an incinerator, steel building, and greenhouse (National Tax Agency 2002). C[O.sub.2] emission from the construction of the pilot plant was calculated from the C[O.sub.2] emission factor of materials (Architectural Institute of Japan 2003) used for construction of the pilot plant. The carbonisation device was continuously active from 0900 hours on Monday to 1700 hours on Friday (260 working days for the device and 5408 working hours per year, including start-up). The rate of feedstock input to the carbonisation device was 0.1 t DW/h, based on the designed potential of the device. The carbon content of the bagasse charcoal was 71%, based on observed results with 600[degrees]C controlled carbonisation temperature (Ueno et al. 2007). The transportation distance from the pilot plant to the farmlands was 8 km, based on the average distance from the sugar factory to each sugarcane field. Bagasse charcoal was applied to farmlands by hand and ploughed twice with a rotary plough, based on questionnaire surveys from farmers. The rate of applying bagasse charcoal to the farmlands was 60 t DW/ha (3.0% w/w in the 0-0.20 m soil surface layer), based on previous works of Miyako Island (Komiya et al. 2006; Chen et al. 2007).
Carbon stabilised as charcoal
Some 75% of the carbon contained in bagasse charcoal is stable in the soil over a 10-year period, which was similar to that stated in Gaunt and Cowie (2009). The assumed rate of turnover corresponded to a half-life of 80 years (time that elapses before half of the biochar decomposes), and a mean residence time of 115 years. The mean biochar residence time is from hundreds to thousands of years when added to soil (Cheng et al. 2008; Lehmann et al. 2008; Liang et al. 2008; Kuzyakov et al. 2009) and is influenced by feedstock, pyrolysis condition, aging, soil type, and climatic conditions (Lehmann et al. 2009). Therefore, this parameter needs to be further investigated in the future.
Results and discussions
Operational properties of the carbonisation device
Figure 5 shows the volume of kerosene consumed in the carbonisation device during the start-up period at 500[degrees]C, 600[degrees]C, and 700[degrees]C. The carbonisation device required a large volume of kerosene at the start of the operation (Fig. 3). Once the device was heated, kerosene consumption during the carbonisation period was considerably lower.
[FIGURE 5 OMITTED]
It was expected that a larger volume of kerosene would be required to attain a high carbonisation temperature. However, the total kerosene consumption during the start-up period was not significantly different for the 3 carbonisation temperatures, demonstrating that kerosene consumption during the start-up period was not influenced by carbonisation temperature in the range 500-700[degrees]C. The average volume of kerosene consumption was 31 L.
The relationship between volume of kerosene consumed during the carbonisation period (C) and feedstock MC at the 3 carbonisation temperatures is shown in Fig. 6. The volume of kerosene consumed was strongly dependent on the initial feedstock MC, whereas it was not influenced by the carbonisation temperature. A linear regression equation between C and MC for the 3 carbonisation temperatures was obtained:
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
C = 1.7MC - 17; [r.sup.2] = 0.61 (1)
When feedstock was <20% MC, almost no kerosene was required to maintain the temperature in the carbonisation furnace. When feedstock MC was >20%, more kerosene was consumed because heat is required to evaporate the water contained in the feedstock. Hence, the initial dryness of the feedstock is an important factor in estimating the volume of kerosene consumed.
Electricity consumption was constant during the operation of the carbonisation device (Fig. 3). Because electricity was consumed by the motors and fans of the device, the quantity was not influenced by the feedstock MC or carbonisation temperature.
Observed charcoal yields were not significantly influenced by carbonisation temperatures in the range 500-700[degrees]C (Fig. 7). The average charcoal yield at 500-700[degrees]C was 0.21 t DW/t DW, which was almost the same as that in the report on sugarcane bagasse carbonisation (Zandersons et al. 1999; Katyal et al. 2003), and was lower than for green waste (0.35 t DW/t DW), cattle manure (0.42 t DW/t DW), and wheat straw (0.38 t DW/ t DW), as reported by Gaunt and Cowie (2009). The operational properties of the carbonisation device are summarised in Table 1.
[FIGURE 8 OMITTED]
C[O.sub.2] emissions from each process and carbon stabilised as charcoal
Calculated C[O.sub.2] emissions from each process and carbon stabilised as charcoal using dry feedstock (MC 20%) and wet feedstock (MC 50%), respectively, are shown in Fig. 8. For these calculations, the feedstock input and charcoal output were 530 and 110 t DW/year, respectively, from the above-mentioned assumptions and inventory data (Table 1).
Carbon stabilised as bagasse charcoal estimated from the inventory data was 0.41 t C[O.sub.2]/tDW. This was lower than the values for other biomass products reported by Gaunt and Cowie (2009) because of lower charcoal yield for sugarcane bagasse, as mentioned above.
The net carbon sequestration potentials, including all C[O.sub.2] emissions and carbon stabilised as charcoal, for the pilot plant using dry and wet feedstock were 0.3 and 0.2t C[O.sub.2]/tDW, respectively. These values corresponded to 70% and 50% of the carbon stabilised as bagasse charcoal. The estimated net carbon sequestration potential (net avoided GHG emissions) was generally lower than the values reported by Roberts et al. (2010) and Gaunt and Cowie (2009). This difference was mainly due to the use of syngas and oil as fossil fuel alternatives outside of pyrolysis facilities and reduction of soil [N.sub.2]O emissions, which are not included in this analysis. As described in the Methods (Outline of the pilot carbonisation plant), all syngas produced during the carbonisation reaction is combusted in a burner of the device as a substitute for kerosene within the pilot plant.
The results from the life cycle inventory analysis show that the carbonisation process was the greatest contributor to C[O.sub.2] emissions. C[O.sub.2] emissions from the carbonisation process with dry and wet feedstock contributed 30% and 60% of all positive C[O.sub.2] emissions, respectively. Furthermore, kerosene consumption was the major contributor to C[O.sub.2] emissions during the carbonisation process. C[O.sub.2] emission from kerosene consumption for wet feedstock was 0.1 t C[O.sub.2]/t DW more than for dry feedstock. Therefore, the difference in net carbon sequestration potential using dry and wet feedstocks was related to the volume of kerosene consumed during the carbonisation process. Thus, reducing kerosene consumption greatly contributes to enhancing the net carbon sequestration potential, and dryness of the feedstocks is the most important factor for estimating net carbon sequestration potential.
A sensitivity analysis of net carbon sequestration potential with 20% of feedstock MC for charcoal yield, stable C for charcoal, and the distance of charcoal transportation is shown in Table 2. The charcoal yield varied from 14% to 28%w/w, based on variability of experimental results in the pilot plant (Fig. 7). If the charcoal yield was varied from 14% to 28% w/w, with a baseline of 21% w/w, the net carbon sequestration potential changed by -48% and 45%, respectively. For the stable C sensitivity analysis, the portion of stable C in the charcoal varied from 50% to 90%, with a baseline of 75%. When 50% of the C in the charcoal was stable, the net carbon sequestration potential was decreased by 48%. When 90% of the C was stable, 28% more carbon was sequestered. They showed that charcoal yield and stable C in charcoal significantly influenced the net carbon sequestration potential. Variation in the distance of charcoal transportation from 0 to 20 km did not significantly change the net carbon sequestration potential.
C[O.sub.2] mitigation potential
On Miyako Island, 60 000 t/year of sugarcane bagasse comes from sugar factories. Since ~80% of the bagasse is currently used as a substitute for fuel in sugar factories (Okinawa Prefecture 2004), bagasse available for biochar is 12000t/year. Thus, 1260tDW/year of bagasse charcoal can be produced according to our inventory data (Table 1). Because the appropriate rate of application of bagasse charcoal to farmlands for enhancement of sugarcane yields is 60 t DW/ha (3.0%w/w in the 0-0.20m soil surface layer) from previous work on Miyako Island (Komiya et al. 2006; Chen et al. 2007), there would be enough for 21 ha/year of farmland. Therefore, the C[O.sub.2] mitigation potential with farmland application of bagasse charcoal on Miyako Island would be 1200-1800 t C[O.sub.2]/year and 60-90 t C[O.sub.2]/ha, based on net carbon sequestration of 0.2-0.3 t C[O.sub.2]/t DW, if the available bagasse was applied to farmlands on the island as bagasse charcoal.
On the other hand, applying bagasse charcoal to calcareous soil on Miyako island increases the available water and nitrogen fertiliser utilisation efficiency for crops (Komiya et al. 2006; Chen et al. 2007). This enables a reduction in the volume of irrigation water and the quantity of nitrogen fertiliser applied to crops. In addition, [N.sub.2]O emissions from soils may be significantly reduced by biochar application (Yanai et al. 2007; Spokas et al. 2009). Consequently, reduction of [N.sub.2]O emissions from the soil surface after applying nitrogen fertiliser is expected. Therefore, it will be important to estimate net avoided GHG emissions by considering these effects in the future.
The net carbon sequestration potential of farmland application of bagasse charcoal was estimated on the basis of inventory data from a pilot plant. The results are summarised as follows:
(1) The estimated net carbon sequestration potentials for the pilot plant using dry and wet feedstock were 0.3 and 0.2 t C[O.sub.2]/t DW, respectively. These values corresponded to 70% and 50% of the carbon stabilised as bagasse charcoal.
(2) Within the life cycle of applying bagasse charcoal to farmland, the carbonisation process was the greatest contributor to C[O.sub.2] emissions. Moreover, kerosene consumption was the major contributor to C[O.sub.2] emissions during the carbonisation process.
(3) When feedstock had <20% MC, almost no kerosene was required for carbonisation. Hence, the initial dryness of feedstock is an important factor for estimating net carbon sequestration potentials.
(4) The C[O.sub.2] mitigation potential with farmland application of bagasse charcoal on Miyako Island would be 1200-1800t C[O.sub.2]/year if the available bagasse was applied to farmlands on the island as bagasse charcoal.
We thank Dr Yoshihito Shirato (National Institute for Agro-Environmental Sciences, Japan) for his review of an earlier version of this manuscript. This study was supported by a grant from the Ministry of Agriculture, Forestry and Fisheries of Japan (Rural Biomass Research Project, BUM-Cm6200).
Manuscript received 5 January 2010, accepted 29 April 2010
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Koji Kameyama (A,C), Yoshiyuki Shinogi (A), Teruhito Miyamoto (A), and Koyu Agarie (B)
(A) Department of Land and Water Resources, National Institute for Rural Engineering, National Agriculture and Food Research Organization, Kannondai 2-1-6, Tsukuba, Ibaraki 305-8609, Japan.
(B) NPO Subtropical Biomass Research Center, Ueno-Nobaru 1190-204, Miyakojima, Okinawa 906-0201, Japan.
(C) Corresponding author. Email: firstname.lastname@example.org
Table 1. Inventory data for carbonisation device Charcoal yield Carbon content (carbonisation temperature 600[degrees]C) (A) During starting-up Kerosene Electricity During carbonisation Kerosene (MC 20%) (B) Kerosene (MC 50%) (B) Electricity Charcoal yield 0.21tDW/tDW Carbon content (carbonisation temperature 600[degrees]C) (A) 71 During starting-up 31 L 11 kWh During carbonisation 17 L/t DW 68 L/t DW 70 kWh/t DW (A) Ueno et al. (2007). (B) MC, Moisture content. Table 2. Sensitivity analysis of net carbon sequestration potential for charcoal yield, stable C, and charcoal transportation distance (feedstock moisture content 20%) Charcoal yield (% w/w) 14% 21% 28% (baseline) Net carbon sequestration 0.15 0.29 0.42 (t C[O.sub.2]/tDW bagasse) Change -48 0 45 Stable C in charcoal 50% 75% 90% (baseline) Net carbon sequestration 0.15 0.29 0.37 (t C[O.sub.2]/tDW bagasse) Change -48 0 28 Charcoal transport distance 0 km 8 km 20 km (baseline) Net carbon sequestration 0.30 0.29 0.27 (t C[O.sub.2]/tDW bagasse) Change 3 0 -7
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|Author:||Kameyama, Koji; Shinogi, Yoshiyuki; Miyamoto, Teruhito; Agarie, Koyu|
|Publication:||Australian Journal of Soil Research|
|Date:||Sep 1, 2010|
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