Technological assessment of animal waste-based energy conversion systems

Abstract

Environmental concerns associated with the excessive application of animal waste on cropland, demands the development of alternative methods pertaining to its sustainable disposal. This project focuses on bioenergy production from poultry litter (PL), by investigating two thermochemical conversion technologies, namely combustion and gasification. Until recently, limited research has been conducted on the chemical characteristics of PL and its potential suitability as a fuel for energy generation in farm installations. Thus, the present study aims to provide useful insights with regard to the parameters that need to be considered prior to design and installation of combustion and gasification systems onsite. Firstly, experiments were conducted with a batch fixed bed lab-scale reactor to investigate the combustion behaviour of PL. Additionally, a blend of PL with wood chips (PL/WC) and softwood pellets (SP) on their own, were tested for comparison purposes. PL depicted the highest concentration in nitrogen (N) compared to the other fuels, and the performed evaluation tests suggested that it was mainly converted to ammonia (NH3) in the cases of PL and PL/WC combustion. On the contrary, N present in SP composition was mostly converted into hydrogen cyanide (HCN) during SP combustion. Furthermore, the findings revealed that the highest aerosol emissions occurred during PL combustion, whereas the corrosion risk was greatest in PL and PL/WC combustion, compared to SP. Overall, high estimated aerosol emissions, increased risk of corrosion and potential conversion of N into NOx emissions, reveal the main areas that need special attention before designing a combustion system based on PL. Gasification of PL, blend of PL with beech wood (PL/BW) and beech wood (BW) on its own were investigated experimentally using a lab-scale bubbling fluidised bed reactor. Experiments were carried out at different temperatures (700-750 °C) and air equivalence ratios (ER) ranging between 0.18-0.28. The findings revealed that an increase in operating temperature had a positive effect on both the lower calorific value (LCV) and carbon conversion efficiency (CCE), whereas in higher ERs, LCV decreased and CCE increased. PL generated lower amounts of tar compared to woody biomass. However, presence of alkali metals in PL ash, led to agglomeration and shut-down of the gasifier at 750 °C. The findings suggest that PL can be a suitable fuel for gasification, with lower gas cleaning requirements compared to woody biomass, due to the lower presence of tar. However, mitigation of agglomeration is crucial during PL gasification, since this phenomenon has a detrimental effect on the process performance. A modelling study of combined heat and power (CHP) production based on combustion of poultry litter, was also performed. Two different systems were investigated; i) a steam boiler coupled with a steam expander currently installed at an existing poultry farm and ii) a thermal oil boiler coupled with an Organic Rankine Cycle (ORC). The results suggested that for the same thermal input based on 0.1 kg/sec of PL, ORC outperformed the steam system by producing 157 kW of gross electrical power, compared to 110 kW. Moreover, heat generated in the condenser was ~1.25 MW for the steam system and ~1.15 MW for the ORC. Payback period (PBP) was found to be 4.4 years in the case of the steam expander system and 3.1 years in the case of the ORC system.