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An energy-biochar chain involving biomass gasification and rice cultivation in Northern Italy

Authors


Correspondence: Emanuele Lugato, tel. +390553033711, fax +39055308910, e-mail e.lugato@ibimet.cnr.it

Abstract

The competing demand for food and bioenergy requires new solutions for the agricultural sector as, for instance, the coupling of energy production from gasification technology and the application of the resulting biochar as soil amendment. A prerequisite for the implementation of this strategy is the scale-specific assessment of both the energetic performance and of the impacts in terms of greenhouse gases (GHG) emission and crop responses. This study considered the gasification process developed by Advanced Gasification Technology (AGT, Italy), which is a fixed-bed, down-draft, open core, compact gasifier, having 350 kW of nominal electric capacity (microgeneration); this gasifier uses biomass feedstock deriving from agricultural/forest products and byproducts. In this study, the resulting biochar, derived from conifer wood chips of mountain forestry management in North-western Italy, was applied to a nearby paddy rice field, located in the largest rice agricultural area of Europe. We performed a Life Cycle Analysis (LCA) adapting the BEAT2 model specifically focusing on the GHG balance of the supply chain, from the forestry management to the field distribution of the resulting biochar. The results indicated that the gasification stage had the highest impact in the supply chain in terms of emissions, but net emissions allocated to biochar were always negative (ranging between −0.54 and −2.1 t CO2e t−1 biochar), hypothesizing two scenarios of 32% and 7.3% biochar mineralization rate in soil, over a time period of 100 years. Finally, biochar had a marginal but positive effect on rice yield, thus increasing the sustainability of this energy-biochar chain.

Introduction

Natural gas and oil prices are rapidly increasing together with the demand of energy. Global carbon dioxide (CO2) emissions from fossil fuels are causing a rapid increase in atmospheric CO2 concentration, which is already dangerously affecting the climate, by increasing radiative forcing. Nevertheless, the energetic utilization of biomass derived from industrial organic wastes and agricultural residues is also increasing at multiple scales, thus reducing the carbon footprint of human activities (Ericsson et al., 2009). The thermochemical conversion of biomass is one of those technologies, potentially allowing energy production at highly distributed scales where industries and farmers can become producers of energy, then leading to very short production chains with limited long-range transport of biomass. Pyrolysis and gasification are the main processes for the thermochemical conversion of biomass. In pyrolysis systems, the biomass is heated in the absence of oxygen with different biomass residence times, whereas in gasification the biomass is partially oxidized by heating at high temperatures (<1.000 °C). Both processes produce: noncondensable gases (syngas), condensable vapors/liquids (bio-oil, tar), and solids (char, ash) (Brewer et al., 2009). In the gasification process the primary product is syngas, a mixture of carbon monoxide (CO) and CO2, hydrogen (H2), methane (CH4), and nitrogen (N2). Gasification can be applied on a large range of biomass with different energy content, using gasifier with different bed and air or oxygen injection systems generally distinguished into three main types: updraft, downdraft, and crossdraft. The production of charcoal may be substantial, especially in downdraft gasifiers (Antal & Grønli, 2003). Such subproduct is normally a fine-grained, highly porous material that may significantly vary in its chemical and physical properties depending on the process parameters and the feedstock. As it contains a large fraction of carbon (C) often bound in recalcitrant aromatic molecules and H/C and O/C ratio <0.6 and <0.4 respectively (Schimmelpfennig & Glaser, 2012), this substance can be assimilated to biochar that has been recently proposed as one of the most promising strategies to sustainably sequester atmospheric CO2 in agricultural soils (Lehmann, 2007; Woodward et al., 2009; Sohi et al.,2010). Mitigation potentials of biochar were estimated to be as high as 12% of current anthropogenic CO2 emissions (Woolf et al., 2010), something that might possibly be achieved under a win-to-win framework leading to a substantial enhancement of soil fertility (Sohi et al., 2010), increased crop yields and renewable energy production (Lee et al., 2010). The long-term stability of biochar was demonstrated to be greater compared with nonpyrolyzed organic matter that was incorporated into soils in the same environmental conditions (Baldock & Smernik, 2002; Cheng et al., 2008; Liang et al., 2008). Biochar has a mean residence time in the soil ranging from 293 to 9259 years (Hammes et al., 2008; Lehmann et al., 2009) and this long-term stability is a fundamental prerequisite to consider biochar as a suitable method for carbon sequestration. Such an optimistic scenario requires, however, more detailed and reliable assessment of direct biochar effects on crops and on the environment as well as an evaluation of socioeconomic implications. Many studies proved that biochar application to soil leads to yield increases at different latitudes and on different soil types (Kimetu et al., 2008; Sinclair et al., 2009; Van Zwieten et al., 2008; Gaskin et al., 2010; Noguera et al., 2010; Vaccari et al., 2011). Although positive effects have been observed on plant productivity and soil quality (Glaser et al., 2002), the majority of published articles agree that further studies are needed before recommending large-scale biochar application as soil amendment: the long-term stability of biochar in soil has not been fully demonstrated, the safety for human health not fully assessed, while the impact on the radiative balance due to changes in soil albedo has only been partially addressed (Genesio et al., 2012). On the other hand, the energy-biochar chain associated with gasification has not yet received much attention, and there are no specific Life Cycle Assessment (LCA) studies done so far. LCA (ISO, 2006a,b) is today widely used for the quantification of the environmental impacts at each stage of the life cycle of a product system, enabling the identification of the stages, processes, and/or materials that are most environmentally advantageous or damaging. Currently, few LCA studies have been done in the biochar field and, of those conducted, most focus on the carbon/greenhouse gases (GHG) aspect (Roberts et al., 2010; Hammond et al., 2011; Ibarrola et al., 2012). The specified focus is justifiable as the main driver behind biochar is its potential to sequester atmospheric CO2 and store the carbon long term in the soil and, in this sense, LCA can help to identify the optimal scale and to develop guidelines for its implementation and to design appropriate agricultural practices and support policies (Hammond et al., 2011).

This study describes a full energy-biochar production chain that involves the availability of a woody feedstock, its use in a commercial-scale downdraft gasification prototype, the production of electricity and the use of the biochar as a soil amendment applied to a paddy rice field. Our study attempts to consider the most important aspects of such a chain by means of a Life Cycle GHG Assessment, by investigating the net effects of biochar application on rice productivity and by estimating the net mitigation potential of the supply chain. Finally, the agronomic performance of biochar from gasification is compared in a field experiment with biochar from slow pyrolysis. Paddy rice is undoubtedly the most important and diffuse crop in the world, covering about 160 Mha in 2009 (FaoSTAT – http://faostat.fao.org). The interest in biochar application in agriculture, including paddy fields, is increasing due to the positive effects on crop yield and soil fertility (Jeffery et al., 2011). However, agronomic studies in temperate paddy rice areas are still lacking and biochar effect on crop yield need to be evaluated to boost the development of such biomass-energy chain.

Materials and methods

Gasification plant and biochar production

This study considered the gasification technology and operational plants developed by Advanced Gasification Technology (AGT, Cremona, Italy), a private company working in the field of research, development, and industrialization of biomass gasification systems for electricity generation, heat or alternative fuels for multiple uses. AGT is the exclusive owner of a fixed-bed, down-draft, open core, compact gasifier, having 350 kW of nominal electric capacity (microgeneration); this gasifier uses biomass feedstock deriving from agricultural/forest products and byproducts. The gasifier consists of a reactor, scrubbers system, electrostatic filters, regenerative blowers, and system cabinet (Fig. 1). The reactor consists of a robust, refractory-lined high temperature vessel, where the biomass fuel is conveyed and subsequently broken down and converted to synthesis gas. An automatically controlled belt drive system loads the feedstock into the chamber. The scrubbers system is a multistage liquid spray cooling apparatus that washes out impurities and progressively cools the synthesis gas. Closed-loop processes recover tar and other impurities and continuously return these back to the reactor. The gas flowing from the reactor is hot (approximately 650 °C) and rich in impurities, containing charcoal dust and tars in vapor phase. The scrubbers consist of a vertical pipe in which water is injected at high pressure and mixed with the gas, obtaining dust removal and a primary cleaning of tars. The tars, due to cooling, partially condense forming an aerosol consisting of micrometer-sized droplets, which must be completely removed to ensure operation of an internal combustion engine. To this end, syngas is filtered by means of a wet electrostatic system that is the final stage of gas cleaning. It removes tar and liquid mists, yielding a clean synthesis gas that will fuel electrical generation engines without fouling. Closed-loop reactor return processes are employed here too. Two regenerative blowers suck air into the reactor from the top, move the synthesis gas through the various processes, and supply pressurized synthesis gas output into the genset engines or turbines. The cabinets house the programmable logic controller-based control system and its computer interfaces, along with the various sensor transmitters, electrical wiring and safety interfaces. During the reduction process, charcoal is consumed generating dust, which prevents the regular syngas flow in the reactor; this dust is removed by means of a suitable system. It is aimed at facilitating the fall of charcoal dust in a reactor lower chamber from which it is conveyed outside by means of a screws system. The gasifier generally produces a quantity of biochar between 5% and 15% of starting incoming material (on dry matter basis), depending mainly on biomass typology and characteristics.

Figure 1.

Advanced gasification technology gasifier.

For conifer wood, biochar yield is close to 10%. Schematics of the gasification process are provided in Fig. 2.

Figure 2.

General flow chart of the advanced gasification technology process.

Feedstock and biochar characteristics

The biochar produced by gasification (BCg) at the AGT plant derived from conifer wood chips, resulting from the mountain forestry management in the North Italian Apennines (Valle Staffora, Pavia, Lombardy – 44°45′N; 9°13′E). The tree species were Larch (Larix decidua Mill.), Scots pine (Pinus sylvestris L.), Black pine (Pinus nigra A.), Silver fir (Abies alba M.), and Spruce (Picea excelsa L.). Moreover, as a part of a wider agronomic experiment, another biochar obtained by slow pyrolysis in conventional kilns from orchard prunings (BCp) was tested in the paddy rice field as comparison. The characteristics of the two biochars are reported in Table 1.

Table 1. Chemical and physical characteristics of biochar obtained from pyrolysis (BCp) and gasification (BCg)
ParameterBCpBCg
FeedstockOrchard pruningTree conifer
Temperature of production (°C)5501000
Total C (g kg−1)680.0760.8
Total N (g kg−1)7.13
C/N ratio95.8256
H/Cna0.47
P (g kg−1)23.32.9
K (g kg−1)13.92.95
Ca (g kg−1)254.9
pH (1:2.5 H2O)7.810.4
Bulk density (Mg m−3)0.630.19

The carbon and nitrogen contents were determined using a CHN Elemental Analyzer (Carlo Erba Instruments, mod 1500 series 2). Biochar samples were screened through a 2 mm sieve and finally oven dried at 105 °C for 24 h. The dry samples were acid digested with a microwave oven (CEM, MARSXpress) according to the EPA method 3052 (USEPA, 1995). The solutions obtained after mineralization were filtered (0.45 μm PTFE) and diluted. Total contents of P, K, and Ca were determined by an ICP optical spectrometer (Vista MPX; Varian Inc., Palo Alto, CA, USA) using scandium as internal standard. The pH was measured in a soil/water solution at a 1 : 2.5 ratio. Bulk density was calculated gravimetrically.

Field experiment

A field experiment was made on rice (Oryza sativa L.) in 2010 in a rice production area of northern Italy where this crop is cultivated under flooding conditions in monoculture. The local climate is subhumid, with annual rainfall of about 990 mm and average annual temperature of 11.6 °C (maximum and minimum average monthly temperatures in July and January are 28 and −3 °C, respectively). The soil has a sandy-loam texture (3.8, 27.5, and 68.7% of clay, silt, and sand, respectively), pH (in H2O) of 5.6 and absence of carbonates along the profile. Soil organic carbon in the Ap horizon (0–30 cm) is 0.76%, with a C/N ratio of 8. This soil, cultivated as paddy rice since 1800, presents an anthraquic diagnostic horizon and is classified as Aquic Hapludalf, coarse-loamy, mixed, nonacid, mesic (I.P.L.A. – Regional Pedological Office). A fully randomized experimental layout was designed within a field of 350 × 120 m. The following treatments were considered: C = control; BCp = application of 40 t ha−1 of biochar from slow pyrolysis; BCg = application of 40 t ha−1 of biochar from gasification. The experimental plots were circular (50 m2 area) and replicated four times. Biochar was applied manually on April 13th, 2010 and incorporated into the topsoil layer by a rotary hoeing the next day, prior to flooding. Apart from biochar treatments, all plots were managed with the same management practices (Table 2).

Table 2. Main management practices applied in the paddy rice
ManagementDateTypeAmount
  1. BCp, biochar from pyrolysis; BCg, biochar from gasification; between I and II flooding periods the paddy field was drained to allow a chemical treatment against weeds; between II and III the paddy field was drained to promote the rice rooting.

Tillage10/11/2009Moldboard plow 
14/04/2010Rotating hoeing 
Biochar application13/04/2010BCp/BGp40 t ha−1
Sowing16/05/2010Var. Loto 
FertilizationPre-sowingUrea60 kg N ha−1
Pre-sowingOrganic40 kg N ha−1
JuneUrea60 kg N ha−1
Mid-AugustN-K (23, 30)30 (40) kg N (K2O) ha−1
Flooding20/04 – 3/05I – weeds controlWater level of 10 cm
05/05 – 20/05II – rice sownWater level of 10 cm
15/06 – 2/08III – rice growingWater level of 10 cm
Harvest28/09/2010  

Crop aboveground biomass was sampled at harvest from four sampling areas of 1 m2 in the central part of each plot on September 28th 2010. The collected biomass was dried at 65 °C in a forced draft oven for 36 h to measure the dry weight; then, the grain (grain + husk) was manually separated from the rest of the plant. Data were analyzed with ANOVA and significantly different means were differentiated with the Student–Newman–Keuls test at P = 0.05 (R version 2.12.1).

Life cycle assessment (LCA)

Using the ISO guidelines (ISO, 2006a,b) as a reference the Life Cycle GHG emissions were calculated for the biochar supply chain produced via gasification. The BEAT2 v2.1 model of the generation of electricity via the gasification of wood chips derived from forest residues (Defra/Environment Agency, 2010) was used. The model includes the following life cycle stages: regeneration, harvesting, extraction, transportation (roundtrip) (3% loss, fresh weight basis), chipping (5% loss, fresh weight basis), drying, and gasification. Detailed information on what is included in the BEAT2 model can be found by accessing the BEAT2 model online: NE Master file ‘FRim_c_g_e’. In addition to these stages, a ‘biochar application’ module was added to account for the emissions associated with the application of biochar to agricultural land. The assumption is made that the biochar is applied with the same agricultural machinery as used for fertilizer application. The ‘biochar application’ unit process was therefore based on the BEAT2 model of GHG emissions originating from the machinery used for the fertilizer application; BEAT2 NE Master file ‘Str_g_e’ worksheet ‘cultivation’, using the emissions associated with ‘machinery’ and ‘maintenance’ of fertilizer application. With regard to the gasification plant, the ‘gasification’ unit process of the BEAT2 model was adjusted by incorporating primary AGT data regarding the start-up electricity required to ignite the gasifier as well as the waste gases from the combustion engine. In contrast with the BEAT2 gasification plant, which requires a significant amount of natural gas as start-up fuel, the AGT gasification plant uses the syngas produced to start the gas engine, and a small electrical heating element to ignite the biomass in the gasifier, therefore requiring a very small amount of start-up energy. The ‘drying’ unit process was also adjusted as the surplus/waste heat from the AGT gas engine is used to dry the conifer wood chips from 40% down to 10% moisture content and therefore no fossil fuels are used to dry the feedstock.

The Functional Unit (FU) is 1 tonne of biochar. The final unit of measurement is therefore GHG emissions (kg CO2e) per tonne of biochar; where methane (CH4) and nitrous oxide (N2O) are converted to CO2e using the IPCC Global Warming Potentials of 25 and 298, respectively (IPCC, 2007). The system boundary, illustrated in Fig. 3, shows the biochar supply chain as used in this analysis. The effects of biochar application on crop yield and of forest residue removal on forest carbon stocks and fluxes are outside the scope of this LCA. As indicated by the transport, chipping and drying occurs at the plant. A generalized roundtrip transportation distance of 100 km is assumed and ‘regeneration’, ‘harvesting’, and ‘extraction’ are all included under ‘forestry management’.

Figure 3.

System boundary of the biochar supply chain. The solid box represents what is included within the system boundary, while the dashed lines at either end and below indicate the omissions. ‘T’ signifies transportation.

The conifer-based residue feedstock results from mountain forestry management and has no alternative commercial use in Italy. As at least 90% of the forest residues are left on-site when not used for biochar production, the reference system considers 100% carbon mineralization (decaying of the wood left on the forest floor) within 100 years. As 19.467 t of forest residues at plantation (50% moisture content, 48% carbon dry weight) are required to produce 1 t of biochar, the reference system of 100% mineralization would generate 17.131 t of CO2.

With regard to the life cycle of the biochar (BCg), the emissions and impacts that occur along the supply chain have to be partitioned or shared, as multiple products are generated. The gasification process produces both electricity and biochar and because biochar is considered to be a coproduct, the emissions resulting from the feedstock supply and conversion have to be allocated between these two products. Two different methods for allocation are considered here based on economic value and energy content of the products. For the latter, the energy content of biochar used is 25 MJ kg−1 (6000 kcal kg−1). The economic allocation is based on the price the Italian government paid to sustain renewable energy in 2009: 0.28 € per kWh. Because no commercial market currently exists for biochar, an opportunity cost has been calculated based on the energy content and conversion efficiency of converting the biochar to electricity. Therefore, we calculate the minimum economic value of biochar to be 0.42 € kg−1; biochar, when gasified, generates about 1.5 kWhe kg−1. We acknowledge that this indirectly links the economic allocation to the energy content, but we still consider this to be a reasonable alternative as it sets a minimum value, with current Italian green energy policies, at which it would make sense to produce biochar rather than electricity. Higher values would result in more of the supply chain emissions being allocated to the biochar compared with the electricity.

Considering the allocation methods, two scenarios are calculated, containing only the proportion of emissions allocated to the biochar according to the economic value and energy content allocation methods, respectively: ‘BCg (economic allocation)’ and ‘BCg (energy content allocation)’.

The net GHG emissions for the biochar supply chain are also calculated. The carbon content of the conifer biochar (BCg) is 76.1%, resulting in the production of 2790 kg CO2 t−1 biochar when fully oxidized. However, the net fraction of carbon that may be effectively sequestered depends on the residence time of biochar in the soil, which is known to be very variable depending on the feedstock and the production process. Several studies have shown that the recalcitrance of biochar increases with increasing production temperature, as pyrolysis temperature increases its aromaticity (Baldock & Smernik, 2002; Nguyen et al., 2010). In general, higher biochar recalcitrance is associated with longer residence times and lower H/C ratios. Harvey et al. (2012) observed a consistent relationship between their “recalcitrance index” measured thermogravimetrically and H/C ratio, thus suggesting that for a H/C value equal to 0.5 (as for BCg), the amount of carbon mineralized after 1 year in the soil is no more than 1%. Other studies have proposed other ways to account for biochar residence time: Shackley et al. (2012) assumed that the Carbon Stability Factor of rice husk biochar over 100 years is equal to 7.3% whereas Cheng et al. (2008) and Lehmann et al. (2009) estimated, on average, that 15% of the carbon contained in biochar is degraded within 1 year, while one-fifth of the remaining fraction (another 17%) is lost after a 100 year period, thereby equally 32% over a century. The exact residence time of the particular type of the biochar from gasification used in this study (BCg) is not yet known but, to encompass a wide range of degradation rates over a 100 year period, our LCA considers two scenarios: a fast mineralization rate equal to 32% of carbon lost over a century (Lehmann et al., 2009) (Scenario 1), and a slower rate of 7.3% over a century (Shackley et al., 2012) (Scenario 2). Scenario 1 is very consistent with both pyrogenic carbon turnover models recently proposed by Singh et al. (2012); the exponential turnover models fitted to literature data considered different types of pyrogenic carbon, pooling low, and high pyrolysis temperature. It is known, however, that the degree of condensation and biochar recalcitrance increases with temperatures (Schneider et al., 2010), thus justifying to a large extent the choice made for Scenario 2. It is also assumed that over the production cycle the biomass carbon stocks in the feedstock production area remain constant. It can be expected that biochar acts as a long-term carbon sink transferring the carbon from a relatively short cycle system (forestry) into a very long-cycle system as char in the soil. However, to verify this expectation, further analysis and better data are needed to analyze the changes in forest stocks and the recalcitrance of the biochar.

Results

Electric energy production

The electrical productivity of a gasifier is the function of the chemical–physical characteristics of the incoming materials, such as the elemental composition, the content of dry matter and ash, the calorific power, and the bulk density. In general, it is assumed a specific electric productivity (kWhe kg−1 d.m.) equal to 1 for wood matrices.

Table 3 summarizes the principal technical parameters relative to a gasifier with an electrical power equal to 350 kW, fueled by wood chips (conifer wood) with a humidity of 10% in weight.

Table 3. Principle technical parameters of an AGT gasifier of an electrical power equal to 350 kW fueled by conifer wood chips
FuelWood (conifer) (moisture = 10%W/W)
Fuel flow to gasification350 kg h−1
Production of syngas840 Nm3 h−1
Syngas low calorific value1.075 kcal Nm−3
Nominal electric power350 kW
Nominal thermal power≈1.050 kW
Starting temperature of syngas650 °C
Final temperature of syngas50 °C
Electrical self-consumption≈50 kW
Production of charcoal≈10% W/W (on dry matter basis)

The composition of syngas is the function of the process temperature, the typology and characteristics of the ingoing biomass. The typical composition of wood base syngas of an AGT gasifier is 53.1% nitrogen, 13.1% carbon dioxide, 16.6% carbon monoxide, 14.0% hydrogen, and 3.2% methane, with 12% of moisture.

Biochar effects on rice productivity

The application of biochar from gasification (BCg) increased rice grain production by 12%, but such effect was only significantly different at P = 0.16. The observed effect was inferior to that of BCp application that caused instead a significant increase in rice yield (36%; P = 0.05) (Fig. 4). This behavior was not surprising as BCp was richer in macronutrient compared with BCg (Table 2), as low temperature pyrolysis maintains a higher proportion of labile material (Peng et al., 2011; Pereira et al., 2011), which could increase the nutrient availability (e.g., especially P) during its decomposition.

Figure 4.

Grain yield and rice straw production in the treatments considered. Values followed by the same letters are not statistically different at P = 0.05 by the Student–Newman–Keuls test, for grain (lower case) and straw (upper case).

Priming effects of biochar on soil organic matter have been repeatedly reported (Zimmerman et al., 2011; Hamer et al., 2004) thus suggesting that biochar addition may favor rapid nitrogen mineralization and the subsequent uptake by plants (Nelson et al., 2011). As for the specific case of rice, our results basically agree with previous observations made by Zhang et al. (2010), that applied 40 t ha−1 of biochar from wheat straw pyrolyzed at 350–550 °C and finally observed a rice yield increase of 14 and 12% in unfertilized and fertilized paddy soils, respectively.

LCA

The Life Cycle GHG emissions indicate that the life cycle stage with the greatest environmental GHG impact is gasification (Fig. 5); for detailed numbers across the supply chain and the scenarios please see Table S1 of the Supplementary Information. The drying process is usually also a significant factor in biomass supply chains due to the energy inputs needed to evaporate the water in the feedstock. However, as surplus/waste heat from the AGT gas engine is used to dry the conifer wood chips from 40% down to 10% moisture content, no fossil fuels are required for drying and therefore no emissions are produced.

Figure 5.

Greenhouse gases (GHG) emissions (kg CO2e t−1 biochar) along the BCg supply chain according to the two allocation methods.

To calculate the net GHG emissions for the BCg supply chain, two different mineralization rates were considered; Scenario 1 (32%) and Scenario 2 (7.3%). Considering these scenarios and the two allocation methods, all scenarios result in negative net carbon emissions to the atmosphere, i.e., representing a carbon sink (Fig. 6). The results of the LCA are sensitive to the choice of allocation method. However, as the main product of the gasification plant is electricity, biochar could also have been considered a waste product. Waste products are usually not responsible for any of the environmental impacts as they are produced unintentionally (Chen et al., 2010) and therefore all the emissions are allocated to the main product, which in this case is electricity. As a waste product, biochar would not be assigned any of the emissions produced along the supply chain except for, depending on the reference system, potentially the emissions resulting from the transportation and application of biochar, both of which are minimal. If the total emissions from biochar production and use were zero, or close to zero, the full sequestration potential, i.e., the amount of carbon in the biochar, would determine, along with the mineralization rate, the net GHG emissions.

Figure 6.

Net greenhouse gases (GHG) emissions (kg CO2e t−1 biochar), considering a 76.1% biochar carbon content, for the BCg supply chain, including both allocation methods and the two mineralization rates: 32% (net emissions – Scenario 1) and 7.3% (net emissions – Scenario 2). Negative net emissions indicate a carbon sink.

Discussion

The energy-biochar chain associated with biomass gasification is certainly of interest both from an economic and an environmental point of view. LCA data presented in this study clearly outline the net positive effects of such a chain, both in terms of economic return and mitigation potentials. However, there are numerous unknowns that still require extensive research and a greater collaboration among researchers working in different fields. It is well understood that the biomass feedstock and its composition as well as the operating conditions of the gasification process do affect the characteristics of the biochar and, with this, its subsequent behavior in the soil. This study did not explore all the effects of biochar variability that may potentially affect the net mitigation potentials (carbon content and residence time) as well as the overall amendment effect on agricultural soils (nutrient retention capacity and hydrological properties). In fact, as indicated by the system boundary (Fig. 3), the biochar effect on crop productivity was not included in the GHG calculations. A positive effect of increased yield would result in a greater amount of CO2 being removed from the atmosphere by the crop through photosynthesis, thereby reducing the supply chain's overall GHG emissions. In this case the indirect impacts resulting from the increased productivity should also be considered. For example, increased rice production could reduce the demand on another rice field, potentially resulting in the farmer switching to another crop or even converting the land, i.e., indirect land use change. However, as the article takes an attributional approach, the consequential impacts, including the effect on crop productivity, are outside the scope of this article's LCA. Similarly, the effect of the removal of forest residues on the forest's above and below ground carbon stocks is not included and further research is needed on the net impacts of the increased removal of residues from forestry systems as feedstock for biochar. The removal could have various impacts: it could reduce the risk of forest fires, it could change the productivity or it could also result in a loss of soil nutrients and carbon, thereby reducing the soil carbon stock and potentially having negative effects on the productivity of the forest ecosystem. The overall effect of the impact of the removal of forest residues is linked to the retention time of the biochar carbon in the soil, assumed in this article to be a long-term carbon sink.

Despite these approximations, the biochar obtained from gasification is certainly of interest as it is supposed that an increase in peak temperature during carbonization leads to the generation of biochar with higher aromaticity and therefore higher stability in the soil (Manyà, 2012). Further studies analyzing the effect of high temperatures on biochar stability and nutrient retention capacity are, however, required.

Although rice yield was stimulated by the addition of biochar from gasification, the overall amendment effect was relatively small and not statistically significant. In a field experiment in Northern Laos with a biochar application rate ranging from 4 to 16 t ha−1 Asai et al. (2009) reported increased rice yield, especially in the lower soil fertility conditions, as well as in the experiment done in China, by Zhang et al. (2010), where biochar application of 10–40 t ha−1 raised yield by about 15%. Other studies have instead reported conflicting results on rice (Asai et al., 2009; Noguera et al., 2010; Haefele et al., 2011), thus suggesting that the feedstock and type of biochar used may lead to substantial difference in yield stimulation effects or even cause yield reductions. When compared with other studies made with other types of biochar from gasification, this result reinforces the idea that not all biochars are equal (McLaughlin et al., 2009) and have variable effects on soils and crops. This poses a serious problem in biochar science, as consistent and replicable effects are required before biochar can be effectively used by the farmers to sequester carbon, improve their soils and enhance crop yields. The possibility to use biochar in rice monoculture in nontropical areas is certainly of interest and this may attract interest at the farming scale in the future. Finally, the LCA study which has been implemented clearly demonstrates that biochar from gasification has a large potential for climate change mitigation, even when not considering its yield stimulation potentials.

Acknowledgements

We are very grateful to Ing. E. Boggio Sella who made his farm available for this experiment and for his precious help during the field activities and Fondazione Minoprio for biochar characterization. This research contributes to the objectives of the EuroCHAR project (FP7-ENV-2010 ID-265179) and to ICHAR (Associazione Italiana Biochar).

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