Comparative techno-economic and life cycle assessments of single and intercropped-mixed feedstocks-based biorefineries for profitable and cleaner production of energy

Abstract

The role of intercropped mixed feedstocks (IMF) in advancing biorefineries is rarely studied. However, as opposed to conventional single feedstocks (SF) from monocropping systems (MCS), IMFs could potentially enhance the economic profitability of biorefineries by reducing the cost of feedstock transportation and overall operations due to high feedstock yield/ha and fewer inputs e.g., diesel. In addition, IMFs could improve environmental performance by minimizing emissions associated with feedstock farming inputs (e.g., fertilizer), transportation (e.g., diesel, and tear & wear), and overall operational requirements. Furthermore, intercropping systems (ICS) could mitigate feedstock yield uncertainties and ensure stable feedstock supply because legumes (e.g., pigeon peas) are drought-resistant and inhibit pests & diseases when intercropped with maize. Nonetheless, the actual benefits of IMFs must be quantified. Therefore, techno-economic and life cycle assessments were done, using the Malawian economic context, to evaluate IMFs of maize stover (MS) and pigeon pea stalks and leaves (PPSL) for ethanol, electricity, and biogas production in varying biorefinery capacities of 156 000 and 2184 000 mt/y. Four scenarios were investigated; MS from MCS (S1), MS from ICS (S2), MS and PPSL from ICS (S3), where PPSL was dedicated for combined heat & power (CHP), and MS and PPSL from ICS where MS was dedicated to CHP (S4). The biorefinery systems were simulated in Aspen Plus ® to obtain mass and energy balances (MEB) which were used in assessing economic performance via biorefinery gate feedstock cost (BGFC), net present value (NPV), and minimum product selling price (MPSP). BGFC was evaluated by aggregating the cost of buying feedstocks, labor, milling, baling, and transportation. Transportation cost was computed via summation of the cost of fuel, driving labor, depreciation, and maintenance influenced by transportation distance and feedstock yield/ha. MEBs were also used as input and output data in SimaPro software to estimate emissions in terms of fossil resource depletion (FRD), global warming potential (GWP), terrestrial acidification (TA), and freshwater eutrophication (FWE). The IMFs improve biorefineries' economic performance by reducing feedstock logistical cost and MPSP, and environmental performance by reducing FRD, GWP, TA, and FWE. For example, BGFC, MPSP & FRD for IMF's S3 were $228 million/t, $2.28/L & 135 kg oil eq/mt, respectively, compared to $258 million/t, $2.37/L, and 146 kg oil eq/mt respectively for MSF's S1 at 2184 000 mt/y. Thus, IMF improved BGFC, MPSP, and FRD by 12, 5, and 8%, respectively. Therefore, utilization of the IMFs can boost the economic and environmental performances of biorefineries.