Extended Materials and Methods. 3

Supporting Material for “Domestic heating from forest logging residues: environmental risks and benefits.” by Giuntoli J., Caserini, S., Marelli L., Baxter D. and Agostini A.

Contents

Goal and scope 3

GWP factors described in the IPCC WG I, 5th Assessment Report (2013) as used in this work. 3

Conversion factors for the Particulate Matter/Respiratory Inorganics Impact (as calculated by ILCD (2012) and implemented in Gabi 6.3). 4

Collection and seasoning 4

Transport 6

Wood pellets manufacturing 6

Reference system: forest carbon 7

Climate metrics model 8

Near-term climate forcers (NTCF) 11

Sensitivity analysis 12

Sensitivity of the additional CO2 emissions from forest floor carbon pool change with the time-horizon chosen for annualization. 12

Sensitivity of Surface Temperature Response (cumulative) to decay rates. 13

References 14

Figure S 1: Cumulative CO2 emission profiles for both cases analysed. The figure illustrates the baseline case of pellets produced from branches (of diameter between 10 and 30 mm) burned in a domestic stove replacing a natural gas boiler. Case 1 considers sustained emissions until the end of the century. Case 2 considers the production of 1 MJ useful heat per year until the 20th year. After that, the forest system is not affected while combustion of biomass and natural gas is stopped. The difference between immediate release of CO2 from biomass combustion and the gradual release from the forest floor is also represented as "Bio-Net" curve. 8

Figure S 2: Additional CO2 emissions per MJ heat produced deriving from missed accumulation of forest floor carbon stock for the three feedstocks considered. The emissions are calculated at 20 years and 100 years time horizons and by annualizing the steady-state value by 20 or 100 years. 12

Figure S 3: Surface Temperature Response (cumulative) to a sustained emission profile for fossil systems (NG, fuel oil and coal) and pellets(PS) pathway. (a) SRT(c) for a system with emission profiles relative to the production of 1 MJth. per year (Case 1); (b) SRT(c) for a system operating for 20 years (Case 2). The grey-filled area represents the range of responses when different decay rates for the biomass feedstock are considered. The solid-green curve represents the baseline case of branches (11.5%*yr-1), the dashed-green curve represents fast-decaying residues and the dotted-green curve represents a "critical" decay rate for which the SRT(instantaneous) at year 2100 equal between bioenergy and NG system (see Fig. 5). 13

Figure S 4: Surface Temperature Response instantaneous for NG and coal pathway for a system producing 1 MJth. per year for 20 years (Case 2). Solid lines represent the impact of climate forcers for the NG system, while the dotted lines illustrate the impacts from the coal system. 13

Table S 1: Physico – chemical properties of fresh wood, wood pellets and wood chips as considered in this work. 3

Table S 2: Environmental impacts analysed and characterization methods used. 3

Table S 3: Global Warming Potentials at a time horizon of 100 years as defined by the IPCC – AR5 (Myhre et al., 2013). The GWP factors include climate feedbacks. 4

Table S 4: Characterisation factors for the Particulate Matter/Respiratory Inorganics Impact as calculated by ILCD (2012) and implemented in Gabi 6.3. 4

Table S 5: Main input and output flows for the processes of collection of forest logging residues, seasoning and chipping. For original sources see (JRC, 2014). The basis for calculation is the Lower Heating Value (LHV) of the dry fraction of the wood. 5

Table S 6: Main emission factors from forestry machinery. Values are given per MJ diesel combusted. Values are obtained from a mix of sources (Winther & Nielsen, 2006; Winther & Samaras, 2010; EEA, 2013). 6

Table S 7: Fuel consumption and emission factors for a 40 t diesel truck. For original sources see (JRC, 2014). 6

Table S 8: Process for the production of pellets from fresh woodchips. For original sources see (JRC, 2014). The basis for calculation is the Lower Heating Value (LHV) of the dry fraction of the wood. 7

Table S 9: Decomposition rates for various logging residues. 8

Table S 10: Parameters used for the calculation of IRF and RF in Eq. S3 to S6. 9

Table S 11: Parameters used for the calculation of IRF of CO2 (Eq. S2). 10

Table S 12: Parameters used for the calculation of the climate response function (Eq. S9). 10

Table S 13: Parameters used for the calculation of the climate metrics for ozone precursors (Eq. S16-S17). 11

Extended Materials and Methods.

Table S 1 presents the wood characteristics used for the calculations in this study.

Table S 1: Physico – chemical properties of fresh wood, wood pellets and wood chips as considered in this work.

Property / Amount
Moisture fresh wood [% f.m.]* / 50
Moisture seasoned wood [% f.m.]* / 30
Moisture wood chips [% f.m.]* / 30
Moisture wood pellets [% f.m.]* / 10
Energy [MJ / kgd.m.]** / 19
Carbon content [kg/kgd.m.] / 0.5
Biomass C emissions [gCO2/MJd.m.] / 96.5***

*on a fresh matter basis;

** on a dry matter basis.

*** When calculating CO2 emissions from biomass combustion, for simplicity all carbon is considered to be fully oxidized to CO2. Emissions of C as CO, CH4 and NMVOC account only for about 0.1-0.2% of the total C.

Goal and scope

The analysis focuses on the environmental impacts described in Table S 2 and using the methods recommended by the ILCD (2012) and implemented by PE International in Gabi 6.3.

Table S 2: Environmental impacts analysed and characterization methods used.

Impact category / Characterization Model / Category indicator result
Global Warming / IPCC 5th Assessment report, 2013. Global Warming potentials (GWP) at 100-year time horizon. Climate feedbacks included. / kg CO2 eq.
Acidification / Accumulated exceedance / mol H+ eq.
Particulate matter / RiskPoll / kg PM2.5 eq.
Photochemical Ozone Formation / LOTOS-EUROS model / kg NMVOC eq.

GWP factors described in the IPCC WG I, 5th Assessment Report (2013) as used in this work.

Contrary to the calculations in the JRC report (2014), we have used updated GWP(100) values according to the latest IPCC 5th Assessment Report.

EU legislation requires only well-mixed GHG (CO2, CH4, N2O) to be included in the calculations of supply-chain emissions.

Table S 3: Global Warming Potentials at a time horizon of 100 years as defined by the IPCC – AR5 (Myhre et al., 2013). The GWP factors include climate feedbacks.

Molecule / GWP 100-year
Well-mixed GHG
CO2 / 1
CH4 / 34
N2O / 298

CO2 emissions from fossil fuels combustion are considered net of CO emissions. Thus, the impact of methane conversion to CO2 is excluded from the GWP factor. Also, carbon in other non-CO2 pollutants is smaller by orders of magnitude and thus included in the total CO2 emissions. Indirect emissions of N2O deriving from NOx and NH3 emissions are included in the analysis based on the IPCC (2006) recommendations: 1% of the nitrogen in NOx and NH3 is emitted as N2O from atmospheric deposition on soils and water surfaces.

Conversion factors for the Particulate Matter/Respiratory Inorganics Impact (as calculated by ILCD (2012) and implemented in Gabi 6.3).

Table S 4: Characterisation factors for the Particulate Matter/Respiratory Inorganics Impact as calculated by ILCD (2012) and implemented in Gabi 6.3.

Molecule / 1 kg (molecule) = x kg PM2.5 eq.
NH3 / 0.0667
CO / 3.56E-04
PM10 / 0.23
PM2.5 – PM10 / 0.6
PM2.5 / 1
NO2 / 7.2E-03
NO / 0.0111
SO2 / 0.0611

Collection and seasoning

All the inputs to the processes and GHG emissions are the same as in JRC (2014). We refer to that document for all the underlying assumptions and original sources. Other pollutants' emissions are referenced in the tables.

Residues collection (all pathways) / Seasoning (only AS and DH pathways) / Comminution (only DH and PS pathways)
Input / Value / Unit / Input / Value / Unit / Input / Value / Unit
Loose residues (at 50% moisture) / 1 / MJ / Collected residues (at 50% moisture) / 1.05c / MJ / Collected residues (at 30% moisture (AS-DH) – 50% (PS)) / 1.025 / MJ
Diesel / 0.012a / MJ / Diesel / 0.0034 / MJ
Output / Value / Unit / Output / Value / Unit / Output / Value / Unit
Collected residues (at 50% moisture)b / 1 / MJ / Collected residues (at 30% moisture) / 1 / MJ / Wood chips (at 30% moisture) / 1 / MJ

a This value includes stump lifting, forwarding to roadside, oil use in machinery, transport of machines to site, loading and unloading of the wood.

b The process uses average values for the collection of loose residues and bundles.

c This value is obtained for open air storage of residues with coverage from rain. Storage is usually for a period of 3 to 8 months.

Emission / Unit / Value
CO2 / g/MJdiesel / 72.8
N2O / g/MJdiesel / 0.003
CH4 / g/MJdiesel / 0.001
CO / g/MJdiesel / 0.27
NOx / g/MJdiesel / 0.68
SO2 / g/MJdiesel / 0.023
NMVOC / g/MJdiesel / 0.056
NH3 / g/MJdiesel / 0.0002
PM2.5 / g/MJdiesel / 0.034

Transport

Table S 7: Fuel consumption and emission factors for a 40 t diesel truck. For original sources see (JRC, 2014).

Unit / Amount
Fuel / MJ/tkm / 0.811
CH4 / g/tkm / 0.0034
N2O / g/tkm / 0.0015

Comments

The return voyage (empty) is taken into account in this value.

This process is commonly used for the transportation of logs, chips and pellets.

The fuel consumption corresponds to 30.5 l/100 km.

The fuel consumption and emissions are a weighted average of Tier 2 values among different Euro classes based on the fleet composition indicated in the COPERT model.

Other pollutants emissions are derived from PE Professional database (version 2013).

Wood pellets manufacturing

The most common feedstock for domestic wood pellets production is nowadays constituted of sawdust. New plants are increasingly using roundwood and logging residues chips as feedstocks (Cocchi et al., 2011; Sikkema et al., 2011).

The process of pellet manufacturing consists of a few basic sub-processes: drying, size reduction, pelletizing, cooling and screening (Mani, 2005). The process requires power for various operations such as the handling of the wood (including the screening of contaminants), the grinding of the raw material to a smaller size and, finally, the pelletization. This electrical power is commonly taken from the grid. In a future perspective process, an internal CHP plant fed with wood chips and bark could provide the process heat and power required, but this is not common nowadays as many small scale CHP engines are still at a pre-commercial stage (Obernberger & Thek, 2010). In this study the EU-27 electricity grid mix is used (PE professional database, version 2013).

The wood chips are delivered to the mill with moisture content of 50% but in order to comply with standards (EN 14961-1, 2010) this value should be lower than 10% for the final domestic pellets produced. In this study it is considered that the chips are dried in a rotary drum drier and that the hot flue gases are provided by a large wood chips furnace (5 MWth.). The necessary heat is calculated based on the value of 1100 kWh/tonne of water evaporated (Obernberger & Thek, 2010). No additives and binders are considered in this analysis.

Input / Unit / Amount
Woodchips / MJ / 1.01
Electricity / MJ / 0.050
Heat / MJ / 0.185
Diesel / MJ / 0.0020
Output / Emissions / Unit / Amount
Wood pellets / MJ / 1.00
CH4 / g / 1.53E-06
N2O / g / 6.40E-06

Reference system: forest carbon

For the case of residues from logging operations, we assumed that if they were not used for energy production, they would be left on the forest floor to contribute to forest floor litter, to recirculate part of the nutrients and to be slowly incorporated as soil organic carbon.

For the purpose of this study it was assumed that the wood left in the forest would decompose with a single exponential behaviour with the kinetics of decomposition varying depending on the wood type and wood size. Based on available data, we defined a baseline decay rate which has been found to apply for the decomposition of branches in boreal and temperate conditions (Table S 9). We have studied a broad range of possible decay rates to analyse the sensitivity of the surface temperature response to this parameter.

We assume, as in other experimental and modelling studies, that the residues decompose following a single exponential decay, as shown in Equation S1:

(Eq. S1)

Where M(t) represents the mass of the residue at time t, M(0) the initial mass and k is the decay rate. This same equation can be used to account for the decrease in the carbon pool of forest litter and the associated carbon emissions from the reference forest system in absence of bioenergy demand.