1. Introduction
The World faces an energy crisis and increasing environmental pressures as a result of the development of global industrialization and extensive use of fossil resources. Due to its abundance and renewability, biomass has become one of the most important sustainable energy resource alternatives. As the demand for this kind of energy is growing, the utilisation of solid wood as Fuel has changed from its traditional role as a locally utilized form of energy into an internationally traded energy commodity. This has led to trade flows not only within Europe, but also in other parts of the World, including Mozambique.
Mozambique is an African country with vast forestry resources, including 120 tropical wood species [
1], some of them with internationally recognized high commercial value. Thus, exportation of timber is a commercial option of considerable value for the country. However, evidences of the selective logging practices can be found. According to Marzoli [
2], only three wood species (
Afzelia quanzensis Welwn,
Milletia stuhlmannii Taub, and
Pterocarpus angolensis DC), locally known respectively as chanFuta, jambire and umbila, represented 78% of the total wood exploited in Mozambique in 2004. Moreover, a considerable biomass material such as bagasse, rice husks, sawdust, coconut husks and shells, cashew nut shell and lump charcoal waste is available in the country as residues. The potential biomass residues from forest logging and timber processing were estimated at about 2.7 PJ [
3]; however, its utilization as a Fuel is marginal. In this context, an integral and efficient use of timber including residues would contribute to reduce the depletion of this species and therefore contribute to a certain extend to the protection of the forest.
Although biomass is an attractive renewable source of energy, in general it is difficult to handle, transport, store and use [
4,
5,
6], due to low homogeneity, energy density, and presence of non-combustible inorganic constituents which leads to different problems in energy conversion units like deposition, sintering, agglomeration, fouling and corrosion [
7]. It is important to the production of an affordable Fuel alternative which satisfies requirements for biochemical or thermochemical conversion processes, such as fermentation, combustion, gasification, pyrolysis
etc. requires physical and/or chemical pretreatment.
The hydrothermal carbonization also known as wet torrefaction or wet pyrolysis process is a spontaneous, exothermal and relatively low temperature (175–350 °C) process which, under pressures and subcritical water converts lignocellulosic biomass in order to obtain a carbon-rich solid fraction, so called biochar, biocoal, hydrochar or HTC-char [
8,
9,
10,
11]. It has shown that hemicellulose, which is the most susceptible to thermal decomposition of three main components of the lignocellulosic biomass [
12,
13], is decomposed first, while most of the cellulose is retained and lignin fraction, produces apart from char, non-condensable gases such as H
2, CO, CO
2, CH
4 and a large variety of water-soluble organic compounds. However, due to the formation of a multitude of Furan-type dehydrated intermediates from carbohydrates and the complexity of the chemistry, the formation process and the final material structures are rather complicated and a clear scheme has not been reported [
14].
Even though, the chemistry of the process for converting biomass into biochar is not yet completely understood [
15], it is known that the HTC-coal formation results from hydrolysis, dehydration, decarboxylation, polymerization and aromatization reactions, which in essence are respectively cleavage of ester and ether bonds, removal of biomass water, elimination of carboxyl groups and condensation [
9,
15].
The main advantage of this process is that it can convert wet input materials, which include animal manures, human waste, sewage sludge, as well as aquaculture and algal residues into carbonaceous solids at relatively high yields without the need for an energy-intensive drying before or during the process [
16].
Research on HTC has been carried out over the last few years by several researchers [
8,
9,
10,
11,
17], mainly focusing on the effect of HTC of woody and herbaceous biomass feedstock. In spite of its importance, it is noteworthy that there is only a small amount of research published on the subject of mass and energy balance of HTC process specifically for longer treatment time with or without the use of catalysts. In the present study, a hydrothermal carbonization process was conducted to pretreat umbila. The objective of this work was not only to investigate the impact of a mild subcritical HTC on pretreatment of biomass but also the mass and energy balance of the process by characterizing and quantifying the solid, liquid and gaseous products.