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How To Realize High Value Utilization Of Biomass Energy?
- Jun 23, 2018 -

When it comes to renewable energy, people always think of solar energy and wind energy first.Nowadays, China has vigorously promoted the construction of ecological civilization, and the utilization of biomass has ushered in new development opportunities.Biomass energy is low-carbon and environment-friendly, which can fundamentally change the current situation of energy use and create great wealth.


1. Total biomass resource estimation

Biomass resources are mainly divided into 5 categories, as shown in table 1.



Table 1 total biomass resource estimation in China


The first kind of crop stalks (corn stalk, straw, straw, sorghum straw, chaff, etc.), beans, cotton stem, this kind of biomass resources, though more (7-900 million - t), but also take into account the crops straw in the back field, feed, and the application of as industrial raw materials, etc, the rest can be used in biomass energy and bio-based chemicals, 310 million t, equivalent to 150 million tce.


The second category is forestry waste, accounting for about 40 million t/a;


The third kind of livestock and poultry excrement is rich in biomass resources, reaching 500 million t in dry medium, and can produce 220 billion m3 of methane.


The fourth type is domestic sewage and industrial organic wastewater, which contains 25 million tons of solid waste residue.


The fifth category is municipal solid waste, which can be used for about 100 million tons.


Together, these five types constitute the distribution of major biomass resources in China, with the total amount reaching 800 million to 1 billion tons of standard coal.




The important components of biomass are carbohydrates, among which excessive oxygen content is one of the biggest characteristics of biomass (table 2).



Table 2 recorded Biomass (dry basis)


See from table 2, the product of photosynthesis, carbon compounds in proportion of oxygen in the water is as high as 40%, so the natural state of the carbohydrate energy density is low, the net thermal efficiency (LHV) only about 16%.Therefore, when designing efficient utilization of biomass resources, the following two issues must be given priority:


1. Biomass resources should be converted into materials with higher energy density (called hydrocarbon enrichment or deoxygenation process), which are easy to carry and transport;

2. When biomass changes from its natural state to a material form with high energy density, the conversion steps and loss are minimized.




The feasible way of high value utilization of biomass energy

At present, human development and utilization of biomass resources technique has six major categories, respectively is: direct combustion, anaerobic digestion (methane), sugar fermentation (produce ethanol), oil extraction and biodiesel, pyrolysis oil (produce), gasification (syngas).At present, there are three feasible ways to develop and utilize biomass energy with high value.


1. The production of biological methane (biogas) is realized through the biogas fermentation process, which is essentially the material metabolism and energy conversion process of microorganisms.In the process of decomposition and metabolism, biogas microbes obtain energy and materials to meet their own growth and reproduction, and most of them are converted to methane and carbon dioxide.


Scientific analysis shows that about 90% of organic matter is converted to biogas.10% is used by biogas microbes for their own consumption.The production of biogas from fermentation materials is actually achieved through a series of complex biochemical reactions.


To understand from the perspective of energy generation, think of it this way: the essence of biogas fermentation process, is the metabolic function of microbial community, through the rearrangement of molecules and atoms, the oxygen content as high as 40% or so of the biomass (via the formula is as follows: CH1.4 O0.6) to oxygen, carbon, hydrogen, two elements in the process of enrichment, the product is the energy density is much higher than the starting raw materials (biomass) hydrocarbon - methane.


Since nearly 90 percent of organic matter can be converted to methane, this is one of the few highly efficient processes.The theoretical value of the energy conversion efficiency of biogas fermentation is over 65%.This is one of the most important reasons why the technology has recently been promoted internationally.




Biomass generates electricity


Biomass power generation refers to the use of biomass raw materials to replace coal carbon, after combustion to drive the steam turbine units of thermal power plants for power generation.Biomass power is more feasible mainly because it can utilize the existing infrastructure of thermal power plants and reduce the total investment substantially.Biomass power generation in China mainly refers to biomass (such as straw, forest waste resources) burning power generation.In foreign countries, the raw materials for biomass power generation are divided into three parts:

1) biomass burns directly like firewood;

2) the bio-oil obtained from biomass "pyrolysis" can be transported to other places or can be burned on the spot to drive the steam turbine to generate electricity;

3) the syngas (hydrogen and carbon monoxide) produced by biomass "gasification" can be burned to drive turbines to generate electricity.


3. Low cost production of cellulosic ethanol


Before finding effective alternative to chemical in the coming decades, one of the biological liquid fuel ethanol is still the first choice, any discarded ethanol as liquid fuels in the current situation of idea is childish, or at least it is not realistic.Fuel ethanol is divided into the first and second generation.


The first generation mainly used starch (grain, potato and other human food) as raw materials.Because of food security, countries are now switching to second-generation fuel ethanol, which is made from cellulose.At present, the biggest problem of cellulosic ethanol is that the cost is too high. The cost per t in China is between 9,000 yuan and 12,000 yuan.There are three reasons for the high cost (excluding the price fluctuation of raw materials and the transportation cost of raw materials) :

1) high pretreatment cost of cellulose materials;

2) high cost of cellulase, up to 2000-3500 yuan;

3) low sugar content of cellulose hydrolysate and low liquor content of fermented mash liquid lead to increased distillation cost.


Iii. Problems to be solved in the high-value utilization of biomass energy


1. The control of energy conversion efficiency any form and use of energy shall be transformed from one form to another, and part of the energy shall be lost in each transformation step.This is especially true for biomass energy.The fundamental source of biomass energy is the capture of light energy by organisms.Therefore, one of the basic problems of biomass energy is conversion efficiency.


Biomass can be turned into energy or chemicals.The basic methods of transformation are two: chemical and biological.Whether chemical or biological, the core is the catalyst.Catalyst is the key to saving energy and improving efficiency.


For example, in the production of biological methane, the more active the methane bacteria population is in terms of enzymes, the less enzymes they use.In the production of cellulosic ethanol, cellulose gasification produces syngas (a mixture of hydrogen and carbon monoxide) that also relies on a highly efficient and dedicated catalyst to convert into liquid fuel.




2. Modification of plant lignocellulosic structure and component composition


The core problem of the structure and composition of lignocellulosic is how to improve cellulose content and reduce lignin content.This is especially beneficial for cellulosic ethanol production.Changes in relative content have a positive effect on both pretreatment and alcohol production rates.


conclusion

The exploitation and utilization of biomass energy is a new research field of interdisciplinary and fusion.The biomass energy industry, as a strategic emerging industry in China, has been vigorously promoted by the state, characterized by intensive technology, intensive capital, intensive labor, industrial chain factories and strong driving force.