Environmental performance of crop residues as an energy source for electricity production: The case of wheat straw in Denmark

Applied Energy 104 (2013) 633–641

Contents lists available at SciVerse ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Environmental performance of crop residues as an energy source for electricity production: The case of wheat straw in Denmark Thu Lan T. Nguyen ⇑, John E. Hermansen, Lisbeth Mogensen Department of Agroecology, Aarhus University, Tjele, Denmark

h i g h l i g h t s " This paper assesses the environmental performance of wheat straw as an energy source. " Coal and natural gas (NG) are selected as references for comparison with straw. " Straw has a better performance than coal and NG in some midpoint impact categories. " The single score results show that straw is better than coal but worse than NG. " Potential improvements lie in reducing NOx emissions and increasing power output.

a r t i c l e

i n f o

Article history: Received 14 May 2012 Received in revised form 16 November 2012 Accepted 20 November 2012

Keywords: Crop residues Straw biomass Midpoint impact assessment Single score

a b s t r a c t This paper aims to address the question, ‘‘What is the environmental performance of crop residues as an alternative energy source to fossil fuels, and whether and how can it be improved?’’. In order to address the issue, we compare electricity production from wheat straw to that from coal and natural gas. The results on the environmental performance of straw for energy utilization and the two fossil fuel references are displayed first for different midpoint categories and then aggregated into a single score. The midpoint impact assessment shows that substitution of straw either for coal or for natural gas reduces global warming, non-renewable energy use, human toxicity and ecotoxicity, but increases eutrophication, respiratory inorganics, acidification and photochemical ozone. The results at the aggregate level show that the use of straw biomass for conversion to energy scores better than that of coal but worse than natural gas. In order to investigate the question of whether and how a reduction in the single score per kW h of electricity produced from straw is feasible, we perform a scenario analysis where we consider two approaches. The first one is a potential significant reduction in emissions of nitrogen oxides (NOx) by implementing selective catalytic reduction technology and the second is a potential increase in power generation efficiency. The results of the scenario analysis show that both approaches are effective in enhancing the competitiveness of straw as an alternative energy source, though the second approach ‘‘increasing efficiency’’ is somewhat less attractive than the first ‘‘reducing NOx emissions’’. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Fossil fuel use for electricity and heat production is facing serious problems related to resource depletion and environmental degradation, notably climate change. Biomass fuels (e.g. wood waste, crop residues, energy crops), in contrast, are considered renewable and carbon neutral. Unlike fossil fuels that take millions of years to be available as an energy source, biomass can be regenerated relatively quickly through photosynthesis where sunlight is captured to convert atmospheric CO2 and water into organic matter. Biomass burning for energy releases CO2 back to the atmo⇑ Corresponding author. Tel.: +45 8715 7689; fax: +45 8999 1200. E-mail address: [email protected] (T.L.T. Nguyen). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.11.057

sphere but this biogenic CO2, considered to be part of the natural carbon cycle, is not counted as contributing to global warming. Apart from other biomass fuels, crop residues have recently received large attention as a potentially considerable source of renewable energy. On a global scale, crop residues of 3758  106 Mg/year, which is equivalent to 11  1015 kcal, are estimated to be available [1]. Out of this available amount, approx. three fourths are made up of cereal residues [1]. A clear advantage of using crop residues as an energy source is that it minimizes the impacts of land use changes since no additional agricultural land is taken into production [2]. In keeping with the cradle-to-grave concept of life cycle assessment (LCA), the upstream impacts of converting biomass to energy, i.e. in this case, the impacts associated with the removal of crop residues as well as the collecting,

634

T.L.T. Nguyen et al. / Applied Energy 104 (2013) 633–641

pre-processing and delivering of the biomass resource, however, need to be taken into account. There is an on-going discussion about the environmental implications of removing cereal straw from traditional cropping systems for energy purposes [3,4]. Another issue is that although biomass fuels generally have superior performance to fossil fuels in global warming and non-renewable energy, they may have inferior performance in other environmental impacts. Considering the above, we believe that a need exists for a thorough and comprehensive analysis to assess the sustainability of converting crop residues to energy. In this paper, we present as a case study the results of a life cycle analysis of the environmental performance of electricity generation from wheat straw in comparison with coal and natural gas. We also investigate the potential to improve the environmental performance of this biomass resource as an alternative energy source based on the findings of our hotspot analysis. The case study is supposed to take place in Denmark, where biomass fuelled combined heat and power plants have for many years been a common part of the national electricity and district heating supply [5]. The total amount of biomass resources available in Denmark – including lignocellulosic biomass (e.g. wood and straw), manure, grass and waste – has been estimated elsewhere at 182.3 PJ [6]. To achieve the objectives of the study, we aim to address the following questions: (1) What are the upstream impacts of converting crop residues to energy and what if they are included in the full chain analysis?; (2) In addition to global warming and non-renewable energy, what about other impact categories which are relevant for evaluating biomass as an alternative energy source, like acidification, eutrophication, respiratory inorganics, ecotoxicity, human toxicity, and photochemical smog; and (3) How to account for ‘‘trade-offs’’ among different impact categories? In relation to the last two questions, we find that it is not only necessary to take into account a set of relevant impact categories, but also to perform the analysis at a more aggregated level, i.e., translating environmental impacts in different midpoint categories into a single unit so that they can be weighted and added together to give a single score value. 2. Materials and methods 2.1. System boundary definition, process description and basic assumptions The LCA system boundary for straw-fired electricity generation is presented in Fig. 1, with a focus on the study area (Denmark). It includes three main system processes: straw removal, straw collection and pre-processing, and straw combustion at power plants. Briefly described, by the time of cereal crop harvest, straw is re-

Alternative use/fate: Incorporation into soil

Straw removal

straw

moved (instead of being ploughed into the soil), pre-processed and delivered to power plants where it is burned to produce electricity as the main product and heat as a co-product. The burning also produces bottom ash (or slag) and fly ash. The bottom ash is to be returned to natural ecosystems, as appropriate, to save fertilizers and to implement nutrient recycling. The value of fly ash as a resource, in contrast, has not been realized due to the presence of heavy metals in relatively high concentrations [7]. In many places, fly ash recycling is not acceptable but rather disposing in landfills [8,9]. Using a life cycle approach, apart from the impacts associated with the process of straw conversion to energy, we consider the upstream impacts associated with the removal, collection and pre-processing (including transportation) of the biomass resource. The logic behind basic assumptions for each of these upstream and downstream processes, straw removal, straw collection and preprocessing, and straw combustion is discussed below. 2.1.1. Straw removal Incorporation of straw into the soil builds up soil carbon as well as soil nitrogen, and returns valuable nutrients to the ecosystem. The removal of straw therefore loses the build-up of soil C and N and has to account for the environmental impacts resulting from the need for an extra input of mineral fertilizers to compensate for the nutrients removed with straw. Petersen and Knudsen [10] analyzed the effects of straw removal on carbon sequestration in agricultural soils under Danish climatic conditions. They found that the incorporation of 1 t of straw carbon into the soil would correspond to a carbon sequestration rate of 198 kg C (i.e. 19.8%) for loamy sand soil, in a 20-year perspective. The choice of the time perspective, 20 years, is based on the 2006 IPCC guidelines for estimating soil carbon changes [11]. The build up of organic nitrogen is assumed to follow carbon in the ratio of 1:10 [12]. Such build-up of soil C and N will not take place if the straw is removed from the soil. The removal of straw also results in the removal of nutrients which is assumed to be compensated by an additional input of mineral fertilizers. In order to estimate the amount of extra fertilizer inputs, it is necessary to determine the mineral fertilizer equivalent (MFE) of macronutrients in straw, nitrogen, phosphorus and potassium. In relation to nitrogen, only a portion of the nutrient removed with the straw is considered to be available for crop uptake if returned to the soil. Straw has a very high C/N ratio, which means that when it is incorporated into the soil, N will be immobilized instead of being mineralized to benefit the following crop at least for the first couple of years. Of course, the immobilized N will again be released, but the release (mineralization) occurs slowly over a period of several years. According to Petersen [13], about 30% of N in straw is available to crops (i.e., valued as

Soil C sequestration/N immobilization Nutrient (N, P, K) internal recycling

Straw collection and pre-processing

Disposing in landfills

Combustion at power plants

fly ash

Electricity Heat (co-product)

slag Returning to agricultural land

Fig. 1. LCA system boundary for straw-fired electricity generation.

T.L.T. Nguyen et al. / Applied Energy 104 (2013) 633–641

fertilizer N) over a 50-year perspective. Though the mineralization of N in straw over a shorter year time period, e.g., 20 years, is expected to be lower than 30%, we use this value as a proxy representation of the MFE of straw nitrogen. The value, while conservative, is nevertheless in close agreement with the lowest value of the range, 25–75%, proposed by Gabrielle and Gagnaire [3] to value fertilizer value of straw N based on the percentage crop yield loss as a result of straw removal. There are currently no general guidelines for determining the ‘‘fertilizer value’’ of P and K in straw. Here we use the highest theoretical value of 100% as it has been applied in practice as an approach to value crop residues [14]. We acknowledge that this is also a conservative assumption, but a reasonable one, considering that the extra environmental costs of adding P and K fertilizers due to straw removal is pretty much in balance with the benefits of resource recycling in relation to these two nutrient elements when the ash produced in the energy conversion process is returned to natural ecosystems. The removal of straw, which is assumed to be compensated by adding mineral N-fertilizers, will induce changes in emission profile of e.g. nitrous oxide, ammonia, nitrogen oxides and consequently soil nitrogen balance, compared to the alternative fate of leaving straw in the field. A summary of our estimation of environmental consequences of straw removal (instead of retention), based on the assumptions discussed above, is provided in Table 1. 2.1.2. Straw collection and pre-processing From literature survey, it is revealed that biomass supply chain, i.e., the collection, pre-processing and transportation of biomass, is an important component of the entire biomass-to-energy chain [19,20]. In considering this matter, in our case study, we find it necessary to categorize the environmental impacts associated with (1) energy use in straw pre-processing, e.g. pressing (baling) and handling in the field, and cutting into pieces at the power plant, and (2) straw transport from the field to the power plant. 2.1.3. Straw combustion and ash recycling In a conventional steam based power plant, only 40–45% of the energy input is converted into electrical power, whereas the remaining 55–60% is lost with the cooling water and with the hot flue gas up through the stack [4]. In order to enhance overall energy extraction from biomass resources, it is recommended that combined heat and power (CHP) be used [21,22]. In a CHP plant, the electrical power is generated basically in the same way as in a conventional power plant, but instead of discharging the cooling water into the sea, the steam is cooled by the return water in a dis-

trict heating system and thus used for heat generation. By recycling waste heat, CHP systems can achieve up to 60% heat efficiency [5]. The distribution of heat produced in CHP plants to district heating network would avoid the environmental burden (i.e., create environmental benefits) of district heat production. In contrast, various emissions from the burning of biomass (e.g. NOx, SO2, CO, particulate matters, etc.) create environmental costs and this undesirable consequence can be mitigated by implementing emission control technology like selective catalytic reduction (SCR), and selective non-catalytic reduction (SNCR). The environmental impacts of implementing emission control technology including production and transportation of manufacturing materials, installation and operation of the technology, however, need to be accounted for. In a recent published work, Liang et al. [23] assessed the environmental impacts of SCR and SNCR technology for NOx control, versus the case that no emission control technology was implemented. They concluded that SCR technology, as a highly effective method which can offer an 80% reduction in NOx emissions, is more environmentally friendly than SCNR. Apart from emission control, efficiency improvement is also of key importance for CHP plants. According to Beér [24], increasing the generating plant’s efficiency is a cost effective and readily available option to reduce not only resource use but also emissions per unit of electricity generated. As for coal fired plants, Beér reported a straightforward relationship between the percentage increase in plant efficiency and the percentage reduction in coal use and CO2 emissions. For resource conservation, the bottom ash as a by-product of straw combustion should be applied as appropriate to agricultural and forest soils to recycle nutrients particularly phosphorus and potassium. Upon ash recycling, no noticeable amounts of nitrogen will be re-circulated since most of the nitrogen is lost to the atmosphere during the burning of biomass. Biomass ash recycling saves energy and resource use (and hence related emissions) in producing fertilizer P and K. The savings will certainly help offset most of the costs of nutrient (P and K) removal with biomass. The practice thus contributes to the sustainable use of biomass as an energy source, provided that heavy metal contents of the ash to be recycled are within acceptable limits as mentioned earlier. The MFE of P and K in biomass ash is assumed to be 100% as that of P and K in straw (for explanation, see relevant discussion in section ‘‘straw removal’’). The ash, containing some portion of unburned carbon, also can be considered a biochar produced by heating biomass with limited or no air supply. Production and deposition of biochar into the soil has been recently recognized as a viable option for carbon sequestration [25,26] but the question on the stability of biochar

Table 1 Environmental consequences of straw removal concerning soil C sequestration loss, extra fertilizer costs and induced emissions (per tonne of straw wet weight). Environmental consequences

Unit

Amount

Comment (for more details, see text)

Soil C sequestration loss, 20 years perspective, loamy sand soil

kg

79.6

19.8% of the total C in straw

Extra fertilizer input N P K

kg 1.53 0.765 12.75

30% of the total N in straw 100% of the total P in straw 100% of the total K in straw

Emissions N2O–N from extra fertilizer-N application Avoided N2O–N from crop residues NH3–N from extra fertilizer-N application NO–N from extra fertilizer-N application N2–N from extra fertilizer-N application NO3–N, calculated as the change in the potential leaching due to straw removal Indirect N2O–N

kg 0.015 0.051 0.0306 0.0107 0.072 4.3

0.01  kg N in fertilizer [11] 0.01  kg N in crop residues [11] 0.02  kg N in fertilizer [15] 0.007  kg N in fertilizer [16] 0.047  kg N in fertilizer [16] Extra fertilizer-N input – N output in straw removed – N emissions from extra fertilizer-N application – (–N2O–N from crop residues – N build-up in soil) 0.0075  NO3–N + 0.01  (NH3–N + NOx–N) [11]

0.033

635

Remark: Straw composition (from which the values in the table are calculated). Dry matter content = 85%, Elemental composition (dry matter basis): C = 47.3% [17], N = 0.6%, P = 0.09%, K = 1.5% [18].

636

T.L.T. Nguyen et al. / Applied Energy 104 (2013) 633–641

over time needs to be considered. According to Laird et al. [27], about 17% of the carbon in biochar is lost to mineralization over a few months, and the remaining 83% is stable for millennia. In another study [28], it was found that the percentage of stable/recalcitrant organic carbon remaining in poultry litter biochar is 86.4% at high pyrolysis temperature (600 °C). Based on these references, in this study, we assume that the deposition of biochar into the soil would lead to a potential of carbon storage or carbon sinks of 85% of the carbon contained in the biochar. This process is an inverse of the process of straw removal, building up soil C and consequently soil N (i.e. creating C and N credits). Accounting for these carbon and nitrogen credits of biomass ash recycling to soil partially offsets the costs of straw removal in relation to the loss of soil carbon sequestration and soil nitrogen build-up. 2.2. Inventory for the two processes in the chain: straw collection and pre-processing, and straw combustion Key assumptions regarding inputs, outputs, environmental credits from co-products (ash and heat), and process emissions associated with 1 tonne of straw collected, pre-processed and burned at the power plant are summarized in Table 2. Data for the case study refer to heat and power production at a straw-fired steam turbine plant in Denmark, named Masnedø [29], with electrical efficiency scaled to the range reported for the same plant in an updated survey [30]. The electrical efficiency at Masnedø was around 20% in 1999 [29] but increased to 25% in the 2000s [30]. The two main sources of LCA data for process components of the studied system are LCAfood.dk [29] and Ecoinvent [31]. 2.3. Life cycle inventory data for fossil fuel references, coal and natural gas power production In relation to the two fossil fuel reference systems selected for the comparison with straw, a study by the International Energy Agency shows that natural gas when burned for electricity is more energy efficient than coal and this efficiency has been even increased significantly over time [35]. It is also pointed out that the widespread introduction of successively more efficient combined-cycle gas turbine (CCGT) plants has been the main driver of the increase. The latest CCGT plants are reported to have an efficiency level of up to 60%. In our study, life cycle inventory data for coal and natural gas power production process are obtained from Ecoinvent [31]. Stages in the life cycle of coal-fired power are coal extraction, transport, combustion, and ash disposal, which are not much different from those in the life cycle of natural gas-fired power, natural gas extraction, pipeline transport, regional distribution, combustion, and waste disposal. For a conservative evaluation of biomass versus fossil fuels, amongst different coal- and natural gas-to-electricity processes in Ecoinvent, we select those with high electrical efficiency and with advanced technologies for environmental protection, e.g., electricity from hard coal – 40% efficiency, SOx/NOx control technology, and electricity from natural gas – 58% efficiency, best technology. One may argue that this is an unfair comparison, but it makes sense in stressing the importance of improved energy efficiency and emission performance of biomass conversion to electricity. Scenario analysis is thus an important part of our work afterwards to put straw and fossil fuels into a reasonable comparison. 2.4. Functional unit, assessment parameters and method used The functional unit used in the assessment of straw as an alternative energy source and its references (i.e. coal and natural gas) is 1 kW h electricity produced at the power plant. Midpoint impact

categories considered are global warming, non-renewable energy, acidification, eutrophication, respiratory inorganics, ecotoxicity, human toxicity, and photochemical smog. All midpoint impacts are then translated into a single score expressed in monetary unit e.g. EUR. In order to calculate results at midpoints and at the aggregate level, we use a recently developed life cycle impact assessment (LCIA) method named Stepwise2006 [36]. The method aims at relating in a more consistent way (compared to previous LCIA methods) all midpoint impacts to damage categories and further to an aggregated single score. The core novelty of the method lies on the weighting procedure where different midpoint impact scores are weighted and expressed in a single unit. The weighted midpoint scores are then added together to provide a total single score. This step, weighting of the relative importance of environmental impacts, is necessary for handling trade-offs among impact categories. In Table 3 are presented aggregated (monetization) weighting factors for different midpoint impact categories which we obtain from Weidema [36]. The table also groups emissions listed in Table 2 into impact categories. The theoretical basis of the method is briefly described below. For calculating characterized results at midpoint level (e.g. global warming, acidification, eutrophication, human toxicity, ecotoxicity, energy use, etc.), Stepwise2006 combines the characterization models from the two recent LCIA methods, the IMPACT2002+v. 2.1 [37] and the EDIP2003 methods [38,39]. It is worth mentioning that both IMPACT2002+ and EDIP2003 are site-dependent LCIA models, related to a specific geographical context, the Western Europe. In the method, each midpoint impact category is classified into three damage categories, human health, ecosystem quality and resources with the measurement unit Quality Adjusted Life Year (QALY), Biodiversity Adjusted Hectare Years (BAHY), and EUR, respectively, using different characterization factors available in Weidema et al. [40]. Applying a weighting factor of 74000 EUR/QALY and 1400 EUR/BAHY, impact on human wellbeing and impact on ecosystem are further converted or translated into monetary unit, respectively, and this allows to aggregate these two impacts together with impact on resource productivity (in EUR) into a single score. Taking the impact category ‘‘respiratory inorganic’’ as an example, in Table 3 it shows that 1 kg PM2.5 eq has a monetary endpoint value of 68 EUR which is composed of 76.5% human health impact (52 EUR, converted from 7.0E 04 QALY) and 23.5% resource productivity impact (16 EUR) [36]. 3. Results and discussion Table 4 summarizes the LCA results on midpoint impacts and single score per kW h electricity produced from straw and the two fossil fuel references, coal and natural gas (NG). In addition, Fig. 2 illustrates our attempt to identify main contributing processes to the impacts of the entire straw-to-energy chain and main contributing midpoint impact categories. In identifying main contributing processes, we segregate the entire chain into six segments: straw removal, straw collection and pre-processing, straw combustion, fly ash disposal, heat recovery and ash recycling. Detailed discussions on the findings concerning the absolute value of the results presented in Table 4 as well as the main contributing sub-processes and main contributing impact categories illustrated in Fig. 2 are given below. 3.1. Midpoint impact results for electricity production from straw versus natural gas and coal The midpoint impact assessment shows that using straw as an energy source instead of fossil fuels would bring both

637

T.L.T. Nguyen et al. / Applied Energy 104 (2013) 633–641 Table 2 Inventory for the two processes in the chain, straw collection and pre-processing and straw combustion (values provided per tonne of straw, wet weight). Straw collection and pre-processing

Unit

Amount

LCA inventory data source

Input Straw Pressing (baling) and handling straw in the fielda plus cutting straw at the power plantb Transport to the power plant, by truck 16tc

t MJ tkm

1 65.4 100

[29] [31]

Output Straw

t

1

Straw combustion and ash recycling/disposal Input Straw (LHV = 14.5 GJ/t) Electricity (own product)b Heat (own product)b

t kW h MJ

1 110 40

Outputs (energy)b Electricity, 25% efficiency Heat (co-product), 60% efficiency Avoided product: district heat

kW h GJ GJ

1005 8.70 8.66

Bottom ash recyclingb Transport to proper sites (farmland, forest land), by truck 40tc Nutrient value Pd, avoided product: mineral fertilizer-P Kd, avoided product: mineral fertilizer-K Biochar Cd

kg tkm kg

54 5.4

Fly ash disposal in landfillsb

[31] [31] [31]

kg

0.78 8.64 1.84

kg

8.3

[31]

g g g g g g g g g

680 1900 670 20 7.25 911.5 57.4 1.92 1.48 0.32 2.23 30.4 11.6 23.1 24.6 8.7 89.7 30.4

e

Major emissions SO2 NOx HCl N2O CH4 CO TSP (total suspended particulate matters) PM10 (particulate matters with a diameter 610 lm) PM2.5 (particulate matters with a diameter 62.5 lm) Dioxin, measured as 2,3,7,8-tetrachlorodibenzo-p-dioxin PAH (polycyclic aromatic hydrocarbon), measured as benzo(a)pyrene Arsenic Cadmium Chromium Copper Mercury Lead Antimony

lg mg mg mg mg mg mg mg mg

a

Data source: Dalgaard et al. [32]. Data source: Nielsen et al. [29]. Transport distance from wheat farms to CHP plants and from CHP plants to sites where bottom ash is land-applied = 100 km. d Elemental (C, P and K) composition of slag or bottom ash: anonymous [33]. e Emissions: data are adapted from Nielsen et al. [29] and Nielsen and Illerup [34]. Note that at the plant, electrical filter is used for reduction of fly ash emissions but no SOx and NOx control technology is implemented. b

c

Table 3 Weighting factors for selected midpoint impact categories. Impact category

Unit of characterized values at midpoint

Weighting factor EUR/ characterized unit at midpoint

Emissions associated (only those listed in Table 2)

Human toxicity, carcinogens Human toxicity, non-carcinogens

kg C2H3Cl eq kg C2H3Cl eq

0.27 0.27

Respiratory inorganics Ecotoxicity, aquatic Ecotoxicity, terrestrial Global warming Acidification Eutrophication, aquatic Eutrophication, terrestrial Photochemical ozone, vegetation Non-renewable energy

kg PM2.5 eq kg TEG eq w kg TEG eq s kg CO2 eq m2 UES kg NO3 eq m2 UES m2  ppm  h MJ primary

68 7.0E 06 0.0011 0.083 0.0077 0.1 0.013 3.7E 04 0

Dioxin, PAH, arsenic, cadmium, chromium Dioxin, arsenic, cadmium, chromium, copper, mercury, lead, antimony CO, SO2, NOx, PM10, PM2.5, TSP (Dioxin, PAH, arsenic, cadmium, chromium, copper, mercury, lead, antimony) CO2, CH4, N2O SO2, NOx, HCl NOx NOx Dioxin, PAH, NOx, CH4, CO

TEG: triethylene glycol, UES: unprotected ecosystem.

638

T.L.T. Nguyen et al. / Applied Energy 104 (2013) 633–641

Table 4 Midpoint impacts and single score per kW h electricity produced from straw and the two fossil fuel references. Midpoint impact category

Unit of characterized values at midpoint

Human toxicity, carcinogens Human toxicity, non-carcinogens Respiratory inorganics Ecotoxicity, aquatic Ecotoxicity, terrestrial Global warming Acidification Eutrophication, aquatic Eutrophication, terrestrial Photochemical ozone, vegetation Non-renewable energy Sum (single score)

g C2H3Cl eq g C2H3Cl eq g PM2.5 eq kg TEG eq w kg TEG eq s kg CO2 eq m2 UES g NO3 eq m2 UES m2ppmh MJ primary EUR

Midpoint characterized score

Weighted score (EUR)

Straw

Coal

NG

Straw

Coal

NG

0.061 3.22 0.27 3.36 0.76 0.35 0.102 12.4 0.079 3.93 0.34

0.656 1.76 0.18 1.09 0.288 0.976 0.023 0.073 0.027 2.43 11.5

3.84 0.25 0.059 2.23 0.054 0.423 0.005 0.022 0.010 0.953 7.78

1.66E 05 8.73E 04 0.0182 2.35E 05 8.4E 04 0.029 7.85E 04 1250E 06 9.88E 04 14.6E 04 – 0.050

18E 05 4.8E 04 0.0123 0.76E 05 3.2E 04 0.081 1.8E 04 7.38E 06 3.33E 04 9.05E 04 – 0.097

104E 05 0.7E 04 0.004 2E 05 0.6E 04 0.035 0.4E 04 2.24E 06 1.3E 04 3.6E 04 – 0.041

global warming

others All impacts aggregated

respiratory inorganics

All impacts aggregated Non-renewable energy Ph. chemical ozone-veg Eutrophication, terrestrial Eutrophication, aquatic Acidification Global warming Ecotoxicity, terrestrial Ecotoxicity, aquatic Respiratory inorganics Human toxicity, non-carcinogens Human toxicity, carcinogens -320% ash recycling

heat recovery

-250% fly ash disposal

-180%

-110%

straw combustion

-40%

30%

collection and pre-processing

100% straw removal

Fig. 2. Breakdown of the impacts associated with the entire chain of electricity production from straw into contributing system processes and contributing impact categories.

environmental benefits and costs. The substitution reduces the impact on global warming, non-renewable energy, human toxicity and ecotoxicity, but increases respiratory inorganics, eutrophication, acidification and photochemical ozone. The superior performance of straw compared to fossil fuels especially in the two impact categories, non-renewable energy and global warming, is common for biomass fuels and the reasons discussed so far are due to the ‘‘uncounted’’ solar energy inputs to grow biomass and the ‘‘uncounted’’ carbon emissions in biomass fuel combustion, respectively. Clearly from Fig. 2, straw removal is an important contributor to ‘Eutrophication, aquatic’ (99%), ‘Global warming’ (88%), and Ecotoxicity, aquatic (60%). Straw combustion contributes a relatively large share to ‘Acidification’ (93%), ‘Photochemical ozone, vegetation’ (83%), ‘‘Respiratory inorganics’’ (78%), ‘Eutrophication, terrestrial’ (75%), ‘‘Human toxicity, non-carcinogens’’ (68%), and ‘‘Human toxicity, carcinogens’’ (60%). The recovery/recycling of the two co-products of the system, straw slag and especially heat, has a negative contribution to (i.e. a deduction from) the environmental burden of the energy product from straw in all impact categories. Straw collection and pre-processing has a small contribution to all impact categories except for ‘non-renewable energy’ where the contribution reaches 66%. Fly ash disposal can also be regarded as a minor contributor to all environmental impact categories; its contribution is too small to be distinguished in the graph.

3.2. Single score results for electricity production from straw versus natural gas and coal When all midpoint impact scores are weighted and summed, it shows that straw has a single score value of 0.05 EUR per kW h electricity produced, a significant part of which is accounted for by two system processes: straw removal (48%) and straw combustion (41%). The recovery/recycling of the two co-products heat and bottom ash results in a negative contribution of 25% and 4%, respectively. Though straw removal is identified as an environmental hotspot, reducing the environmental costs on a per mass unit basis (i.e. per tonne of straw removed) remains a challenge that needs to be resolved through future research. In contrast, it is possible to consider improvement potentials in the process of straw combustion by reducing emissions of major air pollutants (e.g. NOx, SO2, particulate matters, etc.) per unit of raw material input. The aggregation of the result of the single score analysis performed for straw shows that the impact on ‘Global warming’ makes the largest contribution (59%), followed by ‘Respiratory inorganics’ (37%). The ‘‘hotspot’’ analysis performed in this way shows that improvement can be achieved by reducing the global warming and respiratory inorganics impacts of the straw-based energy system. However, almost 90% of global warming impact is caused by straw removal, an upstream process, where measures to reduce its impacts have not been specifically identified. This suggests that improvement should be sought through reducing

639

T.L.T. Nguyen et al. / Applied Energy 104 (2013) 633–641

ash recycling

heat recovery

fly ash disposal

straw combustion

collection and pre-processing

straw removal

0.05

0.025

Natural gas

Sum ALT2

ALT2: Efficiency improvement

Sum ALT1

ALT1: NOx reduction

-0.025

Sum baseline

0

Baseline

Single score, EUR/kWh.

0.075

Fig. 3. Scenario analysis concerning approaches to enhance the competitiveness of straw as an alternative energy source to natural gas.

impact on respiratory inorganics which come predominantly from ‘‘straw combustion’’. This downstream process, as discussed earlier, is considered a source of various emissions, of which, CO, SO2, NOx and particulate matters are responsible for respiratory inorganics impact (see Table 3). A further breakdown analysis among these four emission species shows that NOx is by far the largest contributor with a share of 62% to the gross impact (calculation not shown here). This relatively high contribution is explained by the amount of NOx emissions from the combustion of straw in comparison with CO, SO2, and particulate matters (see Table 2) despite a conversion to PM2.5 eq by multiplying with an equivalent factor of 0.13. In comparison with fossil fuels, the results at the aggregate level show that the environmental performance of straw as an energy source is much better than that of coal (0.05 versus 0.1 EUR/ kW h) but a bit worse than natural gas (0.05 versus 0.04 EUR/ kW h). As can be seen in Table 4, much of the inferior performance of straw compared to natural gas is accounted for by the impact on respiratory inorganics (0.018 versus 0.004 EUR/kW h). 3.3. Improvement options for straw-to-energy conversion system In order for straw to be a viable alternative fuel source especially compared to natural gas, the question is whether and how a reduction in the single score per kW h of electricity produced from straw is feasible. We consider two possible approaches. First, based on the results and discussion in the preceding section, we suggest that a reduction in emissions of NOx is likely to offer a reduction in the impact on respiratory inorganics and thus a reduction in the total single score. Second, based on the discussion in sub-section ‘Straw conversion to energy’ of Section 2.1, we suppose that an increase in power generation efficiency would result in a straightforward reduction in resource consumption and emissions and hence a lower impact score per kW h. For evaluating these two approaches, we perform a scenario analysis. For the first approach (ALT1), we assume an 80% reduction in NOx emissions in the strawfired power plant as a result of implementing SCR technology, data for which are available from Liang et al. [23]. For the second approach (ALT2), we simulate the baseline scenario by keeping the following variables constant: (1) inputs, (2) outputs of heat and straw ash, and (3) emissions per tonne of straw input to the power

plant, while scaling up electrical efficiency to a level comparable to that achieved in another straw-fired power plant in Denmark, 29% [5]. The results of the scenario analysis performed for straw-fired electricity production versus the reference ‘‘natural gas’’ are presented in Fig. 3. As it shows in the first four columns of the chart, by implementing SCR technology for NOx reduction (ALT1), the impact score of the system process ‘‘straw combustion’’ is reduced by 50% resulting in a reduction in the total single score of 30% compared to the baseline scenario (0.035 versus 0.050 EUR/kW h). This approach makes straw a competitive alternative energy source to natural gas (0.035 versus 0.041 EUR/kW h). With the second approach (ALT2) – improving electrical efficiency, i.e., achieving more kW h per tonne of straw burned – one can observe a consistent reduction in the impact score of all system processes considered, leading to a reduction in the total single score of 15% compared to the baseline (0.042 versus 0.050 EUR/kW h). A comparison with the total single score of electricity production from natural gas shows that this approach also effectively enhances the competitiveness of straw as an alternative energy source (0.042 versus 0.041 EUR/kW h), though less attractive than the approach of NOx reduction. 3.4. Impacts of straw removal on soil property, not included in the analysis As discussed in the previous section, removing straw for energy purposes, instead of returning to soils, has the potential to offer environmental benefits, despite the inclusion of the environmental costs of soil carbon sequestration and nutrient loss. However, there are opportunity costs of other foregone benefits if straw or other crop residues are returned to soils. For instance, the return leads to improved soil texture and properties e.g. soil bulk density, soil porosity, soil moisture holding capacity, etc. [41]. The ecological functions of crop residues thus play an indispensable part in sustainable agricultural production. The question of balancing between crop residues removed for energy purposes and crop residues left on the field for soil quality has been addressed in literature. It is believed that a sustainable diversion of crop residues to energy use can be made possible by removing only the amount of residues that is beyond that needed to replenish soil organic

640

T.L.T. Nguyen et al. / Applied Energy 104 (2013) 633–641

matter and applying appropriate measures to conserve soil quality and maintain soil productivity e.g. reduced tillage, cover crops, etc. [2,4,41–43]. The maximum amounts of crop residue removal for soil properties have been suggested in different studies. In cereal production, for example, studies in the US indicate that about 34% of available corn stover and 50% of wheat straw can be removed for biofuel use [44]. As another example, in sugar cane production, studies in Puerto Rico, Jamaica, Hawaii and Thailand show that up to 50% of the total crop residues (cane trash) can be taken away [45]. Apart from the above examples, Danish studies [46–48] suggest that not all straw produced in different areas of the country can always be removed without degrading soil quality, as far as the soil organic carbon content relative to clay is concerned. The reasoning behind is that as soil organic carbon content decreases with the removal of crop residues, the formation of free clay particles appears to increase and thus causes problems for soil structure and physical behavior [46]. It is agreed that a clay/organic carbon ratio of 10 would serve as a threshold for clay dispersibility [46,47]. This threshold places a restriction on the removal of straw from approx. 5% of the Danish cereal area where the ratio of clay to organic carbon content is found to be above 10 [48]. 4. Conclusions Based on the results of the study, the main conclusions are drawn as follows. The midpoint impact assessment shows that substitution of straw for fossil fuels as an energy source in power plants would yield both environmental benefits and costs depending on impact category. The substitution reduces the impact on global warming, non-renewable energy, human toxicity and ecotoxicity, but increases eutrophication, respiratory inorganics, acidification and photochemical ozone. As an explanation of the inferior performance of straw compared to fossil fuels in the four listed impact categories, straw removal is an important contributor to ‘Eutrophication, aquatic’, whereas straw combustion contributes a relatively large share to ‘‘Respiratory inorganics’’, ‘Acidification’, ‘Photochemical ozone’, and ‘Eutrophication, terrestrial’. When all midpoint impact scores are weighted and summed into a single score, it shows that the use of straw biomass for conversion to energy scores much better than coal but a bit worse than natural gas. So, the question is whether and how a reduction in the aggregated environmental impact score (single score) per kW h of electricity produced from straw is feasible so that straw can be realized as a viable fuel source to substitute natural gas. We perform a scenario analysis where we consider two possible approaches. First, a significant reduction in emissions of NOx would effectively reduce the impact on respiratory inorganics and thus significantly reducing the total single score. Second, a potential increase in power generation efficiency would result in a lower impact score per kW h. The results of the scenario analysis show that both approaches, ‘‘reducing NOx emissions’’ and ‘‘increasing efficiency’’, are effective in enhancing the competitiveness of straw as an energy source, with the first being somewhat better. Last but not least, it should be stressed that the results of our study are limited to the condition that the removal of straw for energy use takes place at a sustainable rate and with appropriate measures in place ensuring that soil productivity is maintained. Acknowledgments This study was financially supported by the Department of Agroecology, Aarhus University, Denmark. The authors are also grateful for comments from reviewers and editors on the manuscript.

References [1] Lal R. World crop residues production and implications of its use as a biofuel. Environ Int 2005;31:575–84. [2] Ekman A, Wallberg O, Joelsson E, Börjesson P. Possibilities for sustainable biorefineries based on agricultural residues – a case study of potential strawbased ethanol production in Sweden. Appl Energy 2012. . [3] Gabrielle B, Gagnaire N. Life-cycle assessment of straw use in bio-ethanol production: a case-study based on deterministic modelling. Biomass Bioenergy 2008;32:431–41. [4] Powlson DS, Riche AB, Coleman K, Glendining MJ, Whitmore AP. Carbon sequestration in European soils through straw incorporation: limitations and alternatives. Waste Manage 2008;28:741–6. [5] Skøtt T, Hansen MT. Danish biomass solutions – reliable and efficient. Denmark: Centre for Biomass Technology, dk-Teknik Energy & Environment; 2000. [6] Tonini D, Astrup T. LCA of biomass-based energy systems: a case study for Denmark. Appl Energy 2012;99:234–46. [7] Pels JR, De Nie DS, Kiel JHA. Utilization of ashes from biomass combustion and gasification. In: Proceedings of 14th European biomass conference & exhibition, Paris, France, 17–21 October 2005, ECN-RX-05-182. [8] European Commission. Reference document on best available techniques for large combustion plants. Formally adopted BAT reference document (BREF). European IPPC Bureau, Seville, Spain; 2006. . [9] Hansen HK, Pedersen AJ, Ottosen LM, Villumsen A. Speciation and mobility of cadmium in straw and wood combustion fly ash. Chemosphere 2001;45:123–8. [10] Petersen BM, Knudsen MT. Consequences of straw removal for soil carbon sequestration of agricultural fields. Using soil carbon in a time frame perspective. Aarhus (Denmark): Faculty of Agricultural Sciences, Aarhus University; 2010. [11] IPCC. Guidelines for national greenhouse gas inventories. In: Eggleston HS, Buendia L, Miwa K, Ngara T, Tanabe K, editors. Prepared by the national greenhouse gas inventories programme. Japan: IGES; 2006. . [12] Petersen BM, Berntsen J. The turnover of soil organic matter on different farm types. DARCOFenews (3). Denmark: Danish Research Centre for Organic Farming; 2003. . [13] Petersen BM. Personal communication. Denmark: Aarhus University; 2011. [14] Wiersma JJ. The value of straw. Minnesota, USA: AgBuzz.com in cooperation with Minnesota Farm guide and University of Minnesota; 2008. , . [15] Nemecek T, Kägi T. Life cycle inventories of agricultural production systems. Final report ecoinvent v2.0 No. 15, Duebendorf, Switzerland: Swiss Centre for Life Cycle Inventories; 2007. [16] Nguyen TLT, Hermansen JE, Mogensen L. Fossil energy and GHG saving potentials of pig farming in the EU. Energy Policy 2010;38:2561–71. [17] Overgaard P, Sander B, Junker H, Friborg K, Larsen OH. Two years’ operational experience and further development of full-scale co-firing of straw. In: 2nd World conf. on biomass for energy, Industry and Climate Protection, Rome, Italy; 2004. [18] Møller J, Thøgersen R, Kjeldsen AM, Weisbjer MR, Søegaard K, Hvelplund T, Børsting CF. Fodermiddeltabel. Sammensætning og foderværdi af fodermidler til kvæg. Rapport nr. 91. Landbrugets Rådgivningscenter; 2005. [19] Miao Z, Shastri Y, Grift TE, Hansen AC, Ting KC. Lignocellulosic biomass feedstock transportation alternatives, logistics, equipment configuration and modeling. Biofuels Bioprod Bioref 2012;6:351–62. [20] Miao Z, Grift TE, Hansen AC, Ting KC. Energy requirement for comminution of biomass in relation to particle physical properties. Ind Crop Prod 2011;33:504–13. [21] CER. Treatment of small, renewable and low carbon generators outside the group processing approach. The Commission for Energy Regulation, the Exchange, Belgard Square North, Tallaght, Dublin 24, Ireland; 2009. [22] Euroheat & Power. Ecoheatcool project: recommendations and project reports. Euroheat & Power, Avenue de Tervuren 300, 1150 Brussels, Belgium; 2006. . [23] Liang Z, Ma X, Lin H, Tang Y. The energy consumption and environmental impacts of SCR technology in China. Appl Energy 2011;88(4):1120–9. [24] Beér JM. Higher efficiency power generation reduces emissions. Issue paper. Massachusetts, USA: National Coal Council, Massachusetts Institute of Technology; 2009. . [25] Galinato SP, Yoder JK, Granatstein D. The economic value of biochar in crop production and carbon sequestration. Energy Policy 2011;39:6344–50. [26] Matovic D. Biochar as a viable carbon sequestration option: global and Canadian perspective. Energy 2011;36:2011–6. [27] Laird DA, Brown RC, Amonette JE, Lehmann J. Review of the pyrolysis platform for producing bio-oil and biochar. Biofuels Bioprod Bioref 2009;3:547–62. [28] Songa W, Guob M. Quality variations of poultry litter biochar generated at different pyrolysis temperatures. J Anal Appl Pyrol 2012;94:138–45.

T.L.T. Nguyen et al. / Applied Energy 104 (2013) 633–641 [29] Nielsen PH, Nielsen AM, Weidema BP, Dalgaard R, Halberg N. LCA food data base. Denmark; 2003. . [30] DBDH. News from DBDH, Journal number 1/2004, Danish Board of District Heating, Taastrup, Denmark; 2004. . [31] Ecoinvent Centre. Ecoinvent data v2.2. Dübendorf (Switzerland): Swiss Centre for Life Cycle Inventories; 2010. . [32] Dalgaard T, Dalgaard R, Nielsen AH. Energiforbrug på økologiske og konventionelle landbrug. Grøn Viden Markbrug nr. 260, Danmarks Jordbrugs Forskning; 2002. [33] Anonymous. Halmasker, kemisk sammensætning (videblad nr. 146). Videncenter for halm og flisfyring, Copenhagen, Denmark; 1999. [34] Nielsen M, Illerup JB. Emissionsfaktorer og emissionsopgørelse for decentral kraftvarme. Eltra PSO projekt 3141. Kortlægning af emissioner fra decentrale kraftvarmeværker. Delrapport 6. Faglig rapport fra DMU, nr. 442, Danmarks Miljøundersøgelser; 2003. . [35] IEA. Energy efficiency indicators for public electricity production from fossil fuels. IEA information paper. Paris, France: Organisation for Economic Cooperation/International Energy Agency; 2008. [36] Weidema BP. Using the budget constraint to monetarise impact assessment results. Ecol Econ 2009;68:1591–8. [37] Jolliet O, Margni M, Charles R, Humbert S, Payet J, Rebitzer G, et al. IMPACT 2002+: a new life cycle impact assessment methodology. Int J LCA 2003;8: 324–30. [38] Hauschild M, Potting J. Spatial differentiation in life cycle assessment. The EDIP2003 methodology. Report no. 80, Danish Ministry of the Environment. Environmental Protection Agency, Copenhagen, Denmark; 2005. [39] Potting J, Hauschild M. Background for spatial differentiation in LCA impact assessment – the EDIP 2003 methodology. Environmental project no. 996,

[40]

[41] [42]

[43]

[44]

[45] [46]

[47]

[48]

641

2005. Copenhagen, Denmark: Danish Environmental Protection Agency; 2005. Weidema BP, Wesnæs M, Hermansen J, Kristensen T, Halberg N. Environmental improvement potentials of meat and dairy products. JRC Scientific and Technical Reports, EUR 23491 EN-2008. Blanco-Canqui H, Lal R. Crop residue removal impacts on soil productivity and environmental quality. Crc Rev Plant Sci 2009;28:139–63. Wilhelm WW. Impact of crop residue removal on future productivity and soil quality. In: 26th Symposium on biotechnology for fuel and chemicals. Abstract no. 1A–07; 2004. Lemke R, VandenBygaart A, Campbell C, Lafond G, Grant B. Crop residue removal and fertilizer N: effects on soil organic carbon in a long-term crop rotation experiment on a Udic Boroll. Agri Ecosys Environ 2009;135:42–51. Biomass Research and Development Board. Increasing feedstock production for biofuels, economic drivers, environmental implications, and the role of research. Washington, DC; March 2009. Gabra M. Sugarcane residual fuels – a viable substitute for fossil fuels in the tanzanian sugar industry, vol. 8(2). Renewable Energy for Development; 1995. Schjønning P, de Jonge LW, Munkholm LJ, Moldrup P, Christensen BT, Olesen JE. Clay dispersibility and soil friability—testing the soil clay-to-carbon saturation concept. Vadose Zone J 2012;11. http://dx.doi.org/10.2136/ vzj2011.006. Dexter AR, Richard D, Arrouays D, Czyz EA, Jolivet C, Duval O. Complexed organic matter controls soil physical properties. Geoderma 2008;144: 620–7. Schjønning P, Heckrath G, Christensen BT. Threats to soil quality in Denmark: a review of existing knowledge in the context of the EU soil thematic strategy. DJF report plant science 143. Aarhus (Denmark): Aarhus University; 2009.