Effect of feeding frequency and organic loading rate on biomethane production in the anaerobic digestion of rice straw

A.M Zealand,A.P Roskilly,D.W Graham

doi:10.1016/j.apenergy.2017.05.170

Abstract

World energy demand is growing and the negative effects of greenhouse gases (GHGs) and climate change are being felt more acutely. Accordingly, technologies that reduce GHG releases and produce renewable energy, such as anaerobic digestion (AD) with combined heat and power (CHP) systems, are potentially attractive for agricultural wastes, including rice straw (RS). Asia produces over 500 Mt of RS per year that is usually burned, wasting potential energy, causing air pollution and GHGs, and having negative health impacts. Therefore, making RS AD options more attractive is urgently needed. This paper shows biomethane (CH4) yields from infrequently fed RS AD units, which match better with RS harvest production cycles, can be very efficient at specific CH4 production without the need for co-digestion. Using Biomethane Potential (BMP) data to guide AD reactor conditions, five feeding frequencies (FFs) were operated for over 250days in bench-scale units, ranging from five feeds per seven days (5/7; frequent) to one feed per 21days (1/21; infrequent), using OLRs of 1gVS/L/d and 2gVS/L/d. Highest specific methane yields (148±6.3mL CH4/g VS/d) were observed at 1/21 FF and the lower OLR. In contrast, highest volumetric yields were seen for a FF of 1/7 at 2gVS/L/d (276±10.6mL CH4/L/d), although AD units failed at this OLR for FFs of 1/14 and 1/21 due to volatile fatty acids accumulation. This study shows RS AD is feasible without co-digestion, producing biogas that can be coupled with CHP technology to provide renewable energy. However, less frequent feeding regimes performed better than more frequent feeding regimes, suggesting infrequently-fed batch AD units may be a better option for biomethane production, especially for rural locations.

Highlights

World energy demand is forecast to increase by 48% by 2040 and, this demand will be shared worldwide, increased industrial growth of Asia, China and India, will account for more than half this increase [1]
To assess the impact of N addition, dairy manure (DM) was added at different C: N ratios and specific CH4 yields were quantified in the Biomethane Potential (BMP) assay (Fig. 1b)
Using 1 tonne RS per day in an AD unit of 1000 m3 volume, and CH4 yields from the OLR1 and from Mussoline et al [3] and Wu et al [59], RS AD/CHP could potentially generate between 400 and 500 kW h/d

Summary

Introduction

World energy demand is forecast to increase by 48% by 2040 and, this demand will be shared worldwide, increased industrial growth of Asia, China and India, will account for more than half this increase [1]. Renewable energy options that reduce GHGs and improve health outcomes are needed, including bioenergy production from agricultural waste streams, such as wa ste rice straw (RS). 620 Mt of rice was produced worldwide in 2009, equating to around 840 Mt of RS, and production levels are increasing [3]. Li et al [6] estimated rice cultivation accounts for up to 5.1 Mt of methane a year, approximately 10% of world emissions

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Discussion

Conclusion