Article pubs.acs.org/EF
Life Cycle Environmental and Economic Tradeoffs of Using Fast Pyrolysis Products for Power Generation Ghasideh Pourhashem,† Sabrina Spatari,†,* Akwasi A. Boateng,‡ Andrew J. McAloon,‡ and Charles A. Mullen‡ †
Civil, Architectural and Environmental Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States USDA-ARS, Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States
‡
S Supporting Information *
ABSTRACT: Bio-oils produced from small-scale pyrolysis may have economic and environmental benefits for both densifying agricultural biomass and supplying local bioenergy markets with fossil energy alternatives to support state policies (e.g., Renewable Portfolio Standards). We analyze the life cycle greenhouse gas (GHG), energy, and cost tradeoffs for farm-scale biooil production via fast pyrolysis of corn stover feedstock and subsequent utilization for power generation in the state of Pennsylvania. We evaluate the life cycle ramifications of either cofiring the biochar coproduct with coal in existing power plants for energy generation, or using the biochar as a land amendment within the agricultural sector. The results show GHG emissions of 217 and 84 g CO2e per kWh of bio-oil electricity for coal cofiring and land amendment, respectively. Cofiring biochar with coal displaces more fossil energy than does land application. We discuss the potential for bio-oil and biochar penetrating nearterm electricity markets (c. 2015). Our analysis shows that the electricity produced from burning pyrolysis oil and biochar with variable operating costs of $93/MWh and $18/MWh, respectively, are competitive with the fuel oil and coal electricity markets in Pennsylvania within the vicinity of the agricultural sites supplying biomass in spite of the potentially higher NOx emissions due to nitrogen present in the fuel. Small scale pyrolysis bio-oil may be an economically viable and environmentally sustainable nearterm option for peak power production and for meeting the state’s Renewable Portfolio Standards. condensable gas (NCG).4 All pyrolysis products can be useful energy precursors for conversion to thermal energy, electrical energy, or transportation fuels. The permanent gases can be recycled and used as pyrolysis process energy.8,9 Although biooil contains about half of the heating value of petroleumderived heavy oil due to the presence of oxygenated compounds, it may contain more nitrogen, and only traces of metals or sulfur, which vary depending on the feedstock and the pyrolysis process.4,10,11 The carbon-rich and stable biochar coproduced from pyrolysis can be either used as a source of energy or as a soil amendment to enhance the sustainability of biomass harvesting by means of carbon sequestration.4,12−14 Owing to the adverse effects of harvesting crop residues for biofuel production on soil carbon,15 amending agricultural soils with biochar may be a sustainable approach to integrated agronomic-biomass-bioenergy systems. Since pyrolysis partitions most of the minerals and plant micronutrients, and about half of the N and S in the biomass feedstock into the biochar fraction,12 amending the biochar to soil returns most of the biomass nutrients to the soils from which they came. Renewable Portfolio Standards (RPS) that are being adopted by many U.S. states include some mixture of biomass for coal substitution in electric power generation.16 In certain regions of the U.S. biomass is a cost-effective replacement for heating oil (boiler fuel) with negative carbon abatement costs.17 Both
1. INTRODUCTION Advanced biofuels are currently supported in federal policy for development as domestic renewable alternatives to fossil-based liquid transportation fuels.1 If synthesized to specific refined biofuel products, such as green diesel, then they can serve as green “drop-in” replacements for the petroleum refining industry, thereby taking advantage of the existing refining and distribution infrastructure. Drop-in biofuels made from upgraded pyrolysis oil (also known as bio-oil or py-oil) have many technological advantages over other renewable fuels such as ethanol due to their compatibility with the existing refinery and delivery infrastructure. However, to convert bio-oil to market entry green diesel or gasoline products, it must be upgraded to hydrocarbon fuel intermediates by undergoing hydrotreating to remove oxygen and hydrocracked to gasoline and diesel molecules,2 the cost of which is high under current crude oil prices, which, on average, have ranged from $80 to $95 per barrel3 in recent years. As a result, other markets for pyrolysis oil such as electricity, combined heat and power, and space heating may be more economically attractive in the nearterm and cost-competitive toward mitigating greenhouse gas (GHG) emissions from those major emitting sectors. Pyrolysis, the thermal decomposition of organic material in the absence of oxygen, is one of many thermo-chemical processes that is effective at converting a variety of biomass feedstocks4−7 into liquid fuels and higher value products from biomass. Three primary products are generated from pyrolysis of biomass; a solid (char), a vapor that condenses at ambient temperature to form the dark brown, viscous liquid (pyrolysis oil; bio-oil when produced from biomass), and a non© 2013 American Chemical Society
Received: October 3, 2012 Revised: March 18, 2013 Published: March 19, 2013 2578
dx.doi.org/10.1021/ef3016206 | Energy Fuels 2013, 27, 2578−2587
Energy & Fuels
Article
electric power and thermal applications could employ stable bio-oil and biochar, derived from low cost pyrolysis technology, for base (cofired with coal) and peak (cofired with fuel oil) electricity supply, and potentially for heat and power in industrial boilers. Bio-oil and biochar offer a means of densifying the biomass, which has storage and transportation benefits prior to delivery for energy markets.18 Pyrolysis conversion reactors can be economically sized for small-scale (6 dry tons/day),19 on-farm application; they can be developed as on-farm “mobile” units or at slightly larger scales as central hubs that source biomass from surrounding farms. Environmental life cycle assessment (LCA) studies have been conducted for fast pyrolysis of woody biomass used in largescale (2000 dry tons/day) pyrolysis to produce transportation fuels,20 and at small scale (400 dry tons/day) to produce electricity,21 both studies assume that biochar is used for process energy. Gaunt and Lehmann22 compared biochar for electricity generation or for land amendment when produced in agricultural residue- and energy crop-fed slow pyrolysis systems, and concluded that greenhouse gas avoidance was greater for the land amendment case. To date, no study has used technoeconomic analysis along with LCA to evaluate the environmental and economic tradeoffs of small scale fast pyrolysis using agricultural residue feedstock, and alternative uses of biochar coproducts. Therefore, our objective is to comprehensively evaluate the engineering cost and life cycle GHG emissions tradeoffs of alternative uses of pyrolysis products produced at small-scale from corn stover in Pennsylvania. We examine agricultural residues (corn stover) within Pennsylvania due to (1) the availability of the biomass and the proximity of this biomass supply to a large electrical energy market prone to high levels of air pollution in the U.S. due to the dominance of coal in its electricity supply mix; (2) the economic viability of liquid fuel substitutes for fuel oil in peak electricity markets; and (3) the possible incremental cost and GHG emission benefits to cofiring the biochar with coal in base load power generation versus sequestering it in the agricultural soils.
Table 1. Typical Properties of Corn Stover Pyrolysis Bio-Oil and Heavy Fuel Oil property high heating value (MJ/kg) elemental analysis (wt %) carbon hydrogen nitrogen sulfur oxygen ash combustion products (kg/ kg of fuel) SO2 NOx
corn stover pyrolysis bio-oila 22.1
e
53.9 6.9 1.2
