Steam explosion is a thermo-chemical pretreatment widely used to disrupt the ultra-structure of the cell wall of the ligno-cellulosic fiber to improve the fractionation of the major ligno-cellulosic components of the biomass for biochemical conversion. In recent years, steam explosion pretreatment has been applied on the fibers for improving the pellet quality of woody and agricultural biomass for thermo-chemical conversion. The improved qualities include high bulk density, low equilibrium moisture content, higher heating value, mechanical strength and moisture resistance. All of these desirable properties allow the steam exploded pellets to be handled and stored outdoors safely, similar to coal. This also raises lots of interests in considering pellets as preferable feedstock for the thermal power plant or bio-refinery facilities. In this chapter, the state of art of research findings on the effect of steam explosion on size reduction and pellet quality of woody and agriculture biomass will be discussed.
TopIntroduction
The global climate change has resulted in increasing the occurrence of natural catastrophes in recent decade, e.g. prolong winter storm with extremely low temperature and frequent rain storm flooding in summer in north eastern part of North America. It is noticeable that the glaciers in North and South Pole are melting rapidly due to global warming. The major cause is the increasing concentration of greenhouse gases (GHGs) in the atmosphere (U.S. Environmental Protection Agency, 2014). The major composition of GHGs are carbon dioxide (CO2), methane (CH4), tropospheric ozone (O3), chlorofluorocarbon (CFC) and nitrous oxide (N2O). The GHGs warm the lower atmosphere and surface of the planet by absorbing and emitting infrared radiation from the gases. The global average surface temperature has increased over 20th century by about 0.6ºC (IPCC, 2001).
Among different energy production technologies, fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 30 years (IPCC, 2001). Estimates of global CO2 emissions in 2011 from fossil fuel combustion, including cement production and gas flaring, was 34.8 billion tonnes, an increase of 54% above emissions in 1990 (Le et al., 2011). Coal burning was responsible for 43% of the total emissions, oil 34%, gas 18%, cement 4.9% and gas flaring 0.7%. Therefore, it is of utmost importance to reduce the net emissions of GHGs caused by human activities by adopting renewable and sustainable energy.
Biomass is a promising coal replacement for combined heat and power generation. Unlike coal, biomass has a short life cycle and therefore considered as a carbon neutral fuel. In theory, biomass can be planted at a rate as they burned to produce emission. Biomass sequesters CO2 in the atmosphere and converts them into oxygen and carbohydrates by photosynthesis. As a result, this helps to maintain a net carbon cycle in the atmosphere. Biomass is dispatchable which can be burnt more to meet high power output demand in winter. However, one of the major disadvantage of using biomass as feedstock for power generation is their high logistical cost of transportation and storage due to their low bulk density (Mobini et al., 2013). For woody biomass, the bulk densities of wood chips and wood sawdust are around 200 and 300 kg/m3, respectively. It is even lower for the agricultural biomass. For example, the bulk density of switchgrass is between 60 – 250 kg/m3 depending on moisture content (Lam et al., 2008). In addition, the as-harvested biomass are wet and with rock or metals. Therefore, the biomass requires to be pre-processed with drying, size reduction, pelletization and engineered into pellet form and potentially install with pretreatment unit prior to drying (Figure 1). The as-received biomass can be in wood chips, sawdust or shavings.
Figure 1. A typical pellet plant with steam explosion pretreatment