Pump 'gas' from biomass: the journey from dream to reality (1)

INTRODUCTION

There are currently strong incentives and expectation for increased use of renewable fuels in the transport sector worldwide. BIOETHANOL and BIODISEL are the two major products.

It is not difficult to find that nowadays the most successful technological route for bioethanol production is through fermentation. And currently fuel supplement bioethanol (5-10%) is mainly derived from food-based crop such as corn starch and sucrose. From Ethanol Producer Magazine (2007), almost all of 162 bioethanol producer listed are corn-based plants.

Obviously, from a long term, change is needed for the feedstock resources i.e. utilization of non-food renewable feedstocks. It has been agreed that more efficient biofuel systems are those based on lignocellulosics and novel conversion technologies to attain high level of renewable fuels with great availability, potential lower cost, and the avoidance of the “food or fuel” argument.

However, unlike corn or sugar-based feedstocks, lignocellulosic biomass has heterogeneity in many aspects such as cell wall and ultra-microstructure, chemical components and distribution in cell wall, reaction rate, etc, which leads to the complexity of the treatment on biomass in technology, process, equipment as well as the cost.

The typical plant biomass that can be served as this purpose are:
–Agricultural residues: corn stover, bagasse, wheat and rice straws etc
–Waste wood and forest trimmings
–Energy crops: switchgrass, willow, poplar etc
–Waste paper, paper mill sludge

Native Plant Cell Wall Structure: Heterogeneity

Typically, plant cell walls have multi-layered structure that consist of the following three types of layers:

Middle lamella (ML): the first layer formed during cell division and makes up the outer wall of the cell and shared by adjacent cells. It is composed of pectic compounds and protein and has the highest lignin concentration

Primary wall (P): formed after the middle lamella and consists of a rigid skeleton of cellulose microfibrils embedded in a gel-like matrix composed of pectic compounds, hemicellulose, and glycoproteins.

The combined middle lamella and primary wall is also called compound middle lamella (CML)

Secondary wall (S): formed after cell enlargement is completed. The secondary wall is extremely rigid and provides compression strength but it is often layered. It is made of cellulose, hemicellulose and lignin and has the highest cellulose and hemicellulose concentration.