Catalytic steam reforming of biomass tar: Prospects and challenges

Tar is unavoidable by-product during biomass gasification process. Catalytic steam reforming of tar to syngas is a promising way for the removal of tar from the gas products. However, the key issue for this way is catalyst development. To date, the developed catalysts always have advantages and disadvantages: nickel-based catalysts have high activity, but they are easily deactivated by coking; noble metal based catalysts have high catalytic activity, long-term stability and high carbon deposition resistance, but they are expensive; other transition metal catalysts such as Fe, Co and Cu exhibit a good performance, but they are also deactivated easily by carbon deposition in the case of high heavy-tar content in the tar; alkali metal catalysts also have high catalytic activity for tar reforming, but they are easy to be evaporated with the generated gases; natural catalysts have been widely applied for the steam reforming of tar due to its inexpensive, abundant and disposable, but their catalytic activities are lower than those man-made ones, and especially have low mechanical strength, making them not suitable to be used in fluidized bed reactor; zeolite is suggested to be a good catalyst support due to its high thermal/hydrothermal stability, high resistance to sulfur compounds, and easy to be regenerated; biomass char has been used as the catalyst or catalyst support in the steam reforming of tar due to its low cost and its natural production inside the biomass gasifier; even biomass ash now is considered to be a good catalyst for tar removal. In this review, to get better understanding of the mechanism of catalytic steam reforming of tar derived from biomass, tar formation, tar properties and catalytic reaction mechanism are also introduced, and prospects and challenges are summarized.