First-generation biofuels like ethanol and biodiesel are made from food crops. Ethanol is typically made from sugar cane, corn or wheat. The manufacturing processes are fairly straightforward. In the case of sugar cane, the cane is milled, the sugar juice is fermented and then distilled into ethanol. Making ethanol from corn and wheat is only slightly more complicated. Corn and wheat are milled into starch, which is then liquefied and cooked with enzymes that convert the starch to sugar. Then the sugar is fermented and distilled into ethanol. Biodiesel is another first-generation biofuel. In Canada, biodiesel is typically made from canola; again, a food crop. Canola oil is converted to biodiesel by a process known as transesterification, which is the reaction of a triglyceride (fat/oil) with an alcohol to form esters and glycerol.
第一代生物燃料存在严重的缺点：只有植物的一小部分用于制造燃料 - 即糖、淀粉或油 - 而其余部分被浪费。 使用粮食作物来制造燃料是有争议的；温室气体减排不足；没有足够的耕地来生产足够数量的第一代生物燃料，不利于替代化石汽油和柴油消耗。这是促使第二代或先进生物燃料发展的原因。先进的生物燃料使用非食品原料生产。使用整个植物，而不仅仅是糖，淀粉和油。先进的生物燃料通常比第一代生物燃料更可持续，并产生更大的温室气体效益。
There are serious disadvantages to first generation biofuels: only a small part of the plant is used to make fuel – i.e. the sugar, starch or oil – while the rest is wasted; using food crops to make fuels is controversial; the greenhouse gas savings are insufficient; and there isn’t enough arable land to produce a sufficient volume of first-generation biofuels to make a meaningful impact on displacing fossil gasoline and diesel consumption. This is what has prompted the development of second generation or advanced biofuels. Advanced biofuels are produced using non-food feedstocks. The entire plant is used rather than just the sugars, starches, and oils. Advanced biofuels are typically more sustainable than first-generation biofuels and yield greater greenhouse gas benefits.
Lignocellulosic biofuels are one family of advanced biofuels. Lignocellulosic biofuels, as the name suggests, are made from lignin, cellulose, and hemicellulose. All plant matter contains lignin, cellulose and hemicellulose. Typical feedstock for lignocellulosic biofuels might include bagasse and straw from sugar cane production, corncobs and corn stover, grasses, fast-growing forest crops like willows and poplars, and forestry residues from logging and sawmill production.
将木质纤维素原料转化为生物燃料是很好理解的。然而，影响转化所需的工程过程仍在开发中，预计在2020年之前不会广泛商业化。一般来说，木质纤维素生物燃料是使用生物化学或热化学转化方法制备的。生化转化用于制造纤维素乙醇。 这个过程比转化淀粉和糖的第一代过程要困难得多。 使用木质纤维素原料，植物细胞壁由复杂聚合物组成，复杂聚合物必须经历第一阶段酸解，然后固液分离，并且在发酵和蒸馏成乙醇之前进行第二阶段的酸解。这种复杂性将使其超过第一代乙醇的生产成本。
The chemistry related to converting lignocellulosic feedstocks into biofuels is well understood. However, the engineering processes required to affect the conversions are still being developed and are not expected to be widely commercialized before 2020. Generally speaking, lignocellulosic biofuels are made using biochemical or thermochemical conversion processes. Biochemical conversion is used to make cellulosic ethanol. This process is much more difficult than the first-generation processes of converting starches and sugars. With lignocellulosic feedstocks, the plant cell walls are composed of complex polymers, which must undergo a first stage of acid hydrolysis, then separation of liquids and solids, and a second stage of acid hydrolysis before the fermentation and distillation of sugars into ethanol. This complexity will add significant cost over first generation ethanol production.
Thermochemical processes include pyrolysis and gasification. The main product of pyrolysis is bio-oil, which can be refined into transportation fuels and other chemicals. Unfortunately, bio-oil is acidic, has high water content, and is unstable. Engineering processes to cope with these challenges and to purify bio-oil are still being developed. The main product of gasification is syngas, which can be refined into ammonia, methanol, synthetic natural gas, other chemical outputs, and even jet fuel. The processes to produce biofuels through gasification are also still under development. Thus, it is unlikely that second-generation biofuels will be widely deployed before 2030.