Our society currently faces three challenges, including resource depletion, waste accumulation and environmental degradation, leading to rapidly escalating raw material costs and increasingly expensive and restrictive waste disposal legislation. This work aims to produce clean solid biofuel from high moisture content waste biomass (bio-waste) with high nitrogen (N)/chlorine (Cl) content by mild hydrothermal (HT) conversion processes. The newest results are summarized and discussed in terms of the mechanical dewatering and upgrading, dechlorination, denitrification and coalification resulting from the HT pretreatment. Moreover, both the mono-combustion and co-combustion characteristics of the solid fuel are reviewed by concentrating on the pollutants emission control, especially the NO emission properties. In addition, the feasibility of this HT solid biofuel production process is also discussed in terms of "Energy Balance and economic viability". As an alternative to dry combustion/dry pyrolysis/co-combustion, the HT process, combining the dehydration and decarboxylation of a biomass to raise its carbon content aiming to achieve a higher calorific value, opens up the field of potential feedstock for lignite-like solid biofuel production from a wide range of nontraditional renewable and plentiful wet agricultural residues, sludge and municipal wastes. It would contribute to a wider application of HT pretreatment bio-wastes for safe disposal and energy recycling.

The hydrothermal (HT) conversion has been proposed to produce nitrogen, chlorine free solid biofuel or liquid fertilizer from high moisture and nitrogen content bio-wastes, such as municipal solid waste (MSW), mycelial waste, sewage sludge and paper sludge. However, the energy and economic efficiency of this process has not been fully investigated yet. This work focuses on energy recycling from sewage sludge by producing solid biofuel with HT carbonization, in order to optimize the operating parameters and evaluate the energy efficiency of this fuel production process. The effect of the HT temperature and holding time on the biofuel recovering ratio, calorific value and energy recovery rate was investigated. This evaluation fully considered the effect of the HT conditions, mechanical dewatering, thermal drying, and biofuel recovery ratio. Moreover, the energy consumption of sludge thermal drying was introduced to illustrate the economic efficiency of the HT biofuel production process more intuitively. The results show that the HT biofuel production process was more cost-effective than the conventional thermal drying. The HT temperature was the most important parameter to affect the biofuel properties. The carbon content of solid biofuel kept increasing both with HT temperature and holding time, resulting in an increase in the calorific value of biofuel; whereas, the biofuel recovering ratio α, defined as the mass ratio of solid biofuel to raw sludge, also dropped causing a reduction in the energy recovery rate. After the HT temperature was above 200 °C, the energy recovery rate was around 40%. A moderate condition-HT temperature of 200 °C and holding time of 30 min was suggested to produce solid biofuel from

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