Energy recovery from human faeces via gasification: A thermodynamic equilibrium modelling approach

T Onabanjo,K Patchigolla,S.T Wagland,B Fidalgo,A Kolios,E Mcadam,A Parker,L Williams,S Tyrrel,E Cartmell

doi:10.1016/j.enconman.2016.04.005

Abstract

Non-sewered sanitary systems (NSS) are emerging as one of the solutions to poor sanitation because of the limitations of the conventional flush toilet. These new sanitary systems are expected to safely treat faecal waste and operate without external connections to a sewer, water supply or energy source. The Nano Membrane Toilet (NMT) is a unique domestic-scale sanitary solution currently being developed to treat human waste on-site. This toilet will employ a small-scale gasifier to convert human faeces into products of high energy value. This study investigated the suitability of human faeces as a feedstock for gasification. It quantified the recoverable exergy potential from human faeces and explored the optimal routes for thermal conversion, using a thermodynamic equilibrium model. Fresh human faeces were found to have approximately 70–82wt.% moisture and 3–6wt.% ash. Product gas resulting from a typical dry human faeces (0wt.% moisture) had LHV and exergy values of 17.2MJ/kg and 24MJ/kg respectively at optimum equivalence ratio of 0.31, values that are comparable to wood biomass. For suitable conversion of moist faecal samples, near combustion operating conditions are required, if an external energy source is not supplied. This is however at 5% loss in the exergy value of the gas, provided both thermal heat and energy of the gas are recovered. This study shows that the maximum recoverable exergy potential from an average adult moist human faeces can be up to 15MJ/kg, when the gasifier is operated at optimum equivalence ratio of 0.57, excluding heat losses, distribution or other losses that result from operational activities.

Highlights

The development of sustainable energy systems is one of the top priorities for global development and the driving force for major political and societal reforms in the 21st century [13]
Considering a heat recovery efficiency of 80% as commonly reported for heat exchangers [4] and 40% thermal energy conversion of the heating value of the product gas for an indirect drying system [25], this study show that the recoverable exergy potential from an average samples of adult human faeces is within the range of 13–15 MJ/kg, provided the gasifier is operated at optimum conditions—Fig. 10
At the optimum carbon boundary point (CBP) gasification equivalence ratio (ER) (0.30–0.31), the resulting energy and exergy values of the product gas were within the range of $15–17 MJ/kg and 22– 24 MJ/kg respectively

Summary

Introduction

The development of sustainable energy systems is one of the top priorities for global development and the driving force for major political and societal reforms in the 21st century [13]. The flush toilet is a cost-, resource- and energy-intensive system It demands the use of at least 3 L of water per flush [10] that. ⇑ Corresponding author at: School of Energy, Environment Technology and can increase quickly up to 20 L, depending on toilet design and user’s behavioural pattern. It requires sewer connections for transportation, treatment and disposal of excreta, wastewater and grey water. All these processes present energy requirements and the linear use of resources including chemicals are financial and environmental burdens for both urban and rural communities. The development of sustainable sanitary solutions requires a shift in the use of resources and energy paradigm

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