Influence of hydrothermal carbonization temperature on combustion characteristics of livestock and poultry manures

Abstract: With the rapid development of livestock and poultry breeding, China has become the largest producer of animal manure worldwide. If the animal manure was not effectively utilized, it could cause serious environmental pollution. Hydrothermal carbonization (HTC) is a promising pretreatment method in disposing high moisture content bio-wastes, involving the decomposition and carbonization of biomass material in water medium at desired temperature (generally 180- 250 ℃) and autogenous pressure (2-10 MPa). In this paper, 5 kinds of livestock and poultry manure, i.e. swine manure (SZ), dairy cattle manure (NN), beef cattle manure (RN), broiler manure (RJ), and layer chicken manure (DJ), were selected as samples. The effect of temperature on HTC of the livestock and poultry manures was studied. The hydrochar was produced in a laboratory scale semi-batch 1L Parr autoclave reactor (Model 4523, Parr Instrument Co., USA). Approximately 30 g manure (in dry basis; solid-to-liquid ratio was 1:12) was loaded into the reactor vessel, which was then sealed and purged with nitrogen 5 times to displace headspace gases, and the autoclave was heated to desired temperature (180, 210, 240 ℃).The uniformity of treatment throughout the manure sample was ensured by a rotor, which rotated inside the reactor at a constant speed of 100 rpm. The reactor was held at final temperature for 60 min and then quickly cooled down to room temperature. The hydrochar was recovered as solid residue by vacuum filtration and dried in an oven at 105 ℃ for 24 h. The produced hydrochars were characterized and their fuel qualities were evaluated, including proximate analysis, elemental analysis and combustion behaviors evaluation. The differences between different livestock and poultry manures were compared. The char yields were decreased with the increasing of reaction temperature, from 67.34%±11.2%, 64.55%±1.93%, 65.9%±4.38%, 54.61%±16.93% and 58.05%±10.41% at 180 ℃ to 54.30%±5.70%, 49.33%±2.78%, 50.70%±3.63%, 45.83%±19.32%, and 49.69%±9.77% at 240 ℃ for SZ, NN, RN, RJ and DJ, respectively. The hydrochar of NN and RN, got the maximum energy yields at 180 ℃, which were 71.07%±1.23% and 71.93%±13.71%, respectively. The hydrochar of SZ, RJ and DJ, got the maximum energy yields at 210 ℃, which were 65.45%±3.22%, 60.72%±10.77% and 64.43%±10.42%, respectively. It was found that HTC can improve the content of carbon (C), fixed carbon (FC) and higher heating value (HHV), and decrease the molar ratios of oxygen and hydrogen to carbon (H/C and O/C) and the ratios of volatile matter to fixed carbon (VM/FC). In the van Krevelen diagram, the resulted hydrchars are similar to lignite. The thermogravimetric experiment found that the peak temperature at volatile devolatilization and combustion phase moves to higher temperature range, from 284-308 ℃ for manures to 312-320 ℃ for hydrochars. On the contrary, the peak temperature at FC combustion phase moves to lower temperature range, from 435- 483 ℃ for manures to 398-435 ℃ for hydrochars. The HTC can narrow the difference between different livestock and poultry manures and make the hydrochars more similar. The hydrochars from cow and beef cattle manures have higher char yield, energy yield and composite combustion characteristic index compared with other manures.